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39cc pocket dirt bike "all out" tuning experiments

14K views 85 replies 10 participants last post by  jusubbi 
#1 ·
Hello. New to this forum.
A few months ago I started working on this 39cc "pocket dirt bike" that had been lying in backyard under a tree for a few years. A few years back my friend got this bike and we worked on it and rode it back then, but then it broke up and we abandoned it.. but now I wanted to get some fun tinkering so I decided to revive it back from the dead and see how much power I can get out of it. As a background I have played with twostroke moped engines for almost 10 years now, I have a lot of experience in porting cylinders and making exhausts etc. So I found this forum when I was googling what the internet has to offer for these types of bikes. I thought to share my experiences with it. For a grown man it's a pretty ridiculous machine to ride but still quite fun... at first I considered it a complete child's thing but actually it's surprisingly capable and enjoyable to ride.

Here you can see the bike about 2 months back when I first got it running. Seat and foot pegs not attached here. I have made the exhaust pipe about 8 years ago to a different bike and it was modified for this bike about 5 years ago... The front fork tubes are very rusted and other tube is completely jammed so there is no front suspension.

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When I worked on this last time 5 years ago, I found that the stock ignition system is quite poor. The spark was weak when starting and also I suspected that the ignition will hit a limit at around 11000rpm, because even though I had made a tuned exhaust and ported the cylinder, I couldn't get it to rev past 11000rpm. So now the first thing I did I made a battery-operated CDI ignition box which creates the spark primary voltage from battery, so I can get full power spark at low revs too and also the ignition is programmable. The system is my own design and uses a microprocessor similar to an Arduino board. I can fully adjust ignition timing curve and guaranteed no rev limiter, tested at over 15 000rpm in other engines.

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The stock ignition coil is still retained, I use that to give the CDI trigger pulse. It is done by using the original kill switch wire from the stock coil. The kill switch wire is attached to the primary ignition coil, and gives an AC signal which can be used for triggering a CDI (with proper filtration).

Originally I did not disassemble the engine because I wanted to do this just for laughs and wanted to see if it would run well after just upgrading the ignition and some basic carb cleaning. However I found that the flywheel keyway was messed up and it caused the flywheel to slip losing ignition timing. So I needed to fix the keyway and taper by remachining them in a lathe, so I had to disassemble it. Then I found that the bearings were rusted frozen and the crank was actually slipping in the bearing races. Overall the bike was in very bad shape from lying outside for many years.

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I still didn't want to buy new parts other than absolutely necessary, so I just threw all parts in vinegar for rust removal and then ultrasonic cleaner. The bearings spun perfectly well after that and I assembled the engine again. I even reconditioned the old crank seals by swapping the rusted internal springs from some seals that I had lying around...
I glued the crankshaft to the bearing races with thread locker to help prevent slipping.

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Then I made a new reed valve block by casting it from aluminium and machining with my lathe and dremel. I have recently learned the metal casting process, it's a great way to make small engine parts in your own garage and very well doable after some reading and learning from youtube metal casters.
The reed valve is a minarelli scooter reed valve from aliexpress, cheapest I could find. But it is still a ton better than the original reed valve which has very poor flow path.

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Installed reed block. I also upgraded the stock carburetor to a chinese 19mm dellorto copy. The stock carburetor is a piece of garbage. It can work in a completely stock engine but as soon as you start modifying you will run into problems. You cannot find larger mainjets for the stock carb. First I tried to modify the stock mainjet to an adapter which enabled to use 5mm dellorto mainjets. But then I ran into a problem that the float bowl ran dry because the engine started to drink so much fuel after these modifications. The needle valve and petcock is badly restricted in the stock carb. Immediately after switching to the dellorto copy carb the engine gained huge power and driveability (not because of the larger size, but because of better fuel delivery at part throttle and adjustability).

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The stock clutch is in a bad need for some stiffer springs. I modified some piaggio scooter tuning springs to fit in it (have to bend the spring end 90* and elongate the mounting pin in a lathe slightly). Also drilled the shoes. With this modification the clutch engages at a whopping 9000rpm and the bike starts to have some respectable acceleration.


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I have also ported the cylinder but not much. Exhaust port is widened and raised and I ground a boost transfer port but the timings were still quite conservative. At this point the bike ran reasonably well and the power is good, I estimate it accelerates to 45km/h (28mph) in about 5-6 seconds with a 85kg rider (187 lbs) and could easily smoke the back tire flat under a minute (doing few second burnouts is a good and fun power test but I want to preserve the tire). I don't have any numbers about stock performance, but I think it has easily double or triple the stock power.

But despite all these modifications it still refuses to rev past 11000rpm or so. When driving it is limited to about 10500 and 45km/h is the top speed with current gearing. It could go faster with taller gearing but I don't want to do it because it will hurt the acceleration. I know that it is possible to get a lot more revs out of it but need to find out what is restricting it.

Latest modification is to raise the cylinder by 1mm with a spacer plate and change the cylinder head.
This brought the timings to 190 exhaust and 133 transfer.

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I tested it with the spacer plate and it only brought the power down and no help at top revs!
Power loss was due to the combustion chamber volume going up. Then I knew that I need to make a new cylinder head.
I machined the stock cylinder head off from the cylinder. Then I cast some scrap aluminium into a tin can and machined a new cylinder head out of it.

Here is the aluminium block that I cast and the piece of stock cylinder head that I sawed off the cylinder...

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New head machined. Now I can get a proper combustion chamber and smaller 10mm thread spark plug as a further optimization.
The volume is very small. I want to experiment with a ridiculously high compression ratio and see what is the limit. I calculated that with current ~1mm squish gap the comp ratio should be between 16 and 17:1. The stock piston has a relatively tall dome and influences the volume a lot. The squish gap is still large, but I have found out that the crank bearings are so worn out that the gap will almost close at high rpm. The piston had already touched the stock head 😀 It seems that it is necessary to opt for some new bearings if I want to continue tuning.

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I have made a series of videos about it. I added english subtitles for first 4 videos, will try to add for the rest too.



My goal is to tune it so that the peak power is around 12000rpm and it can rev to 14-15000 when driving. I want to get a ridiculous acceleration and sound appearance. The 45km/h top speed is already scary but I want to improve that too.

Let me know if you found this interesting.
 
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#2 ·
Finished the cylinder head and bolted it on.

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It was hard to accurately measure the volume but I believe it is around 2.5cc with the piston dome in account, and with 0.8mm squish when measuring. But as installed the squish is now 1mm because I wanted to test the fitment and starting first before fine tuning it. So the compression ratio should be around 16:1 I think, with the 0.8mm squish it would be 17:1...

When starting it, there is now much more resistance from compression and it also kicks back. I dropped the ignition advance to only 5 degrees when starting, that reduced the kickback. That already tells me the burn speed is at another level compared to the stock head. Then I dropped the advance at powerband from 32 to 20 degrees.
There is a huge difference in power. The head has now restored power that was lost when the compression was dropped by the cylinder spacer. Compared to stock cylinder height, there is noticeably more oomph. Basically the port timings and combustion chamber should be now in order..except I still have room to tighten the squish gap but I have to replace the crank bearings first. There seems to be no detonation but I have run it pretty rich and only short rides.

Max rpm is still pretty much the same. Compared to stock head and cylinder height, there is now maybe 300rpm more. If you wonder how I can determine rpm values, I do it by calculating the engine noise frequency from video playback on computer with an analyzer program(audacity). I think it has topped at 11400rpm when cold. When it gets hot it starts to misfire at top and cannot get over 11k any more. I think it might be due to running rich. I changed a bit smaller main jet and it improved the misfiring, but might be still too rich.
My list to try to improve the rpm has still some things to try:
-carbon reed petals
-smaller main jet
-less ignition advance at high rpm
-more thorough porting on cylinder (reshape transfers)

If these cannot bring it over 12000, I think I need to start cutting the cases or crankshaft :unsure:
I have a theory that the crankshaft rotation blocks the reed inflow at high rpm. At low rpm the intake charge has sufficient time to "hop" between the crank cutouts as it rotates, but as rpm increases the crank webs have rotated in front of the intake before the flow has properly started. I thought I could try to grind "side passages" to the cases or grind the crank webs so there is more space to flow over the crank. Although it starts to get a bit too serious and time consuming (like the cylinder head and cdi box wasn't already)..

Then I accidentally found out the reason for the seemingly low top speed. The clutch is actually slipping at speed. During a test ride one of the clutch springs snapped. When I was experimenting to find a way to install the extremely stiff clutch springs, I had ground one clutch spring a bit thinner from the hook end, so it snapped. Just before it snapped(probably the spring giving way and clutch gaining grip), I felt the bike accelerated easily past the previous 28mph even though the rpm did not change. (didn't have gps on so no new top speed, and I hit the brakes because I was frightened as the front wobbles at that speed). So I have to install a bit less stiff clutch springs.
 
#5 ·
Finished the cylinder head and bolted it on.

View attachment 157026


It was hard to accurately measure the volume but I believe it is around 2.5cc with the piston dome in account, and with 0.8mm squish when measuring. But as installed the squish is now 1mm because I wanted to test the fitment and starting first before fine tuning it. So the compression ratio should be around 16:1 I think, with the 0.8mm squish it would be 17:1...

When starting it, there is now much more resistance from compression and it also kicks back. I dropped the ignition advance to only 5 degrees when starting, that reduced the kickback. That already tells me the burn speed is at another level compared to the stock head. Then I dropped the advance at powerband from 32 to 20 degrees.
There is a huge difference in power. The head has now restored power that was lost when the compression was dropped by the cylinder spacer. Compared to stock cylinder height, there is noticeably more oomph. Basically the port timings and combustion chamber should be now in order..except I still have room to tighten the squish gap but I have to replace the crank bearings first. There seems to be no detonation but I have run it pretty rich and only short rides.

Max rpm is still pretty much the same. Compared to stock head and cylinder height, there is now maybe 300rpm more. If you wonder how I can determine rpm values, I do it by calculating the engine noise frequency from video playback on computer with an analyzer program(audacity). I think it has topped at 11400rpm when cold. When it gets hot it starts to misfire at top and cannot get over 11k any more. I think it might be due to running rich. I changed a bit smaller main jet and it improved the misfiring, but might be still too rich.
My list to try to improve the rpm has still some things to try:
-carbon reed petals
-smaller main jet
-less ignition advance at high rpm
-more thorough porting on cylinder (reshape transfers)

If these cannot bring it over 12000, I think I need to start cutting the cases or crankshaft :unsure:
I have a theory that the crankshaft rotation blocks the reed inflow at high rpm. At low rpm the intake charge has sufficient time to "hop" between the crank cutouts as it rotates, but as rpm increases the crank webs have rotated in front of the intake before the flow has properly started. I thought I could try to grind "side passages" to the cases or grind the crank webs so there is more space to flow over the crank. Although it starts to get a bit too serious and time consuming (like the cylinder head and cdi box wasn't already)..

Then I accidentally found out the reason for the seemingly low top speed. The clutch is actually slipping at speed. During a test ride one of the clutch springs snapped. When I was experimenting to find a way to install the extremely stiff clutch springs, I had ground one clutch spring a bit thinner from the hook end, so it snapped. Just before it snapped(probably the spring giving way and clutch gaining grip), I felt the bike accelerated easily past the previous 28mph even though the rpm did not change. (didn't have gps on so no new top speed, and I hit the brakes because I was frightened as the front wobbles at that speed). So I have to install a bit less stiff clutch springs.
Looks like you could use a billet case instead. They are cag cases cnc made to be more designed like a proper reed port engine with the reed intake to be closer to the top rather then the back. Here is one on ebay in Europe. I think it would be a fun build. There was a member here that has been working with one but he hasn't been progressing much with it. I think you could really benefit from it.

 
#6 ·
Superb work. Make it look easy mate, not seen anyone casting their own parts before, impressive I'll be keeping a close eye on this thread. Great to see someone really pushing these little engines again. Seems like a lot of the OG builders have gone quite on here lately so great to see new blood taking up the mantle. I've just started building a new engine myself, I'm a good few notches behind you on the knowledge and skill scale but I'm learning. They are very forgiving engines to work with. If I'm honest I couldn't give a toss about max rpm etc. The track we race on is so small and tight its all about acceleration rather than top end. Nice work on the clutch I need to do something similar myself.
Just a suggestion get yourself a 15mm pumper carb set up mate. They've worked brilliantly for me and for on the fly or continues tuning which you will likely be doing they are perfect. Can get the complete set up for less than £20. Keep going bud, great work.

Max
 
#11 ·
Really good website. Yeah be a killer combo. Im pretty sure looking at it most of the performance stuff is the same as ebay so you can find it cheaper on there if your prepared to order direct and wait. But loads of other good bits like quality crank bearings, FCC and this little beauty!
Not seen this before and 70cc can that be right? I see one of these going on the Christmas list and straight into the offroad miniquad. Its even got the reed plate on the top like the polini series 2. Thats quite interesting. For 200quid brand new. I'm going to have to get one and find out. Top work mate.

Max
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#12 ·
I'm thinking they used that 39.7mm stroke crankshaft and a air-cooled scooter cylinder. Bore is 47mm så it would be ≈68cc




 
#13 ·
Before you buy all of those parts to make the CNC billet case work!

And try to jam the billet aluminum case on to the Cagllari Daytona and realize that the carburetor is about where the seat is supposed to be and realize that the exhaust won't fit.

and not being able to purchase a clutch that can handle the power and constantly disassembling the engine to replace the wrist pin bearing .

You can get a used BZM or Polini engine and build off of a proven power plant!!!

The CAG engine is fun to build off of but when if starts costing $$$ to get the power that other engines have stock is it honestly worth it ???
 
#14 ·
So that 70cc engine is meant for a scooter. That case was not mean with a pocket bike in mind. I would want a cag case where the intake reed port is on the top like that case but more flat like a polini series 1 case then the carb and intake manifold might fit with an exhaust. I would love to cast my own cag case to but i don't have the right tools to do so. The cag case is so inefficient it bothers me.
 
#16 ·
This case is made for cag bike. It has the same fittment as the standard cag engine, you will only need an exhaust like on the elite, what is under the engine.
The biggest crank has 39,7mm stroke and every cylinder for minarelli engine fit on the crankcase. But it isn't worth, buy a polini 6.2 engine with a 5 port cylinder and you got the same power. And it's easyer to fit the polini into the cag than get a exhaust that fit into the cag frame. Also it's at the end of the day cheaper to buy a complete euro bike and you know that you buy quality.
 
#15 · (Edited)
If someone wants to do the design work I will print the crankcase out for test fitting then we just have to send it to China or something for it to be cnc'd

It would be a lot of work but in the end it would definitely be worth it.


Notice in the picture of that way overpriced engine you can see the crankcase halves don't fit together properly this is the problem that other members have reported that have purchased those casings the other thing that members have reported is the bearing wells are not deep enough meaning the crankcase needs additional machine work after you get it to make it functional.
 
#17 · (Edited)
Good discussion here. Glad that my project brings some interest.

Buying new cases is definitely not an option because I want to do this on low budget and humor basis :D
The more power I can get out of it with original or diy parts the funnier it is.
I agree that the stock cases are poor and the reed valve flow path is probably a bad restriction. I have thought about casting new cases, I think it would be quite possible to do it and machine them with just my lathe and drill press. The cases are so small and simple. It's just a lot of work and time consuming, I have too much other projects too. But I'm definitely thinking about it...
Other possibility is to try to figure out a new creative way to attach the reed valve block on top of the cases close to the cylinder but that's not very easy to do either.
But first I'm going to try "blueprinting" the cases as it is mentioned quite a lot on this forum. My view is that it is not going to help much.
Haven't done much for the bike lately. I tried a smaller main jet and retarded ignition to 15 degrees but it still cannot rev past 11500. Next I'm going to buy carbon reed petals and see what they do.
Here is my newest video, still without english subtitles though. But at 2:24 and 16:40 you can see a little driving and burnout. It is super fun to burn the rear tire with it, but there is so little rubber on it that I don't want to do it for long, otherwise I would need to replace it all the time...



I also noticed that the cylinder plating is a bit worn at some spots. I guess the chinese did not use a proper nicasil coating for it. It looks more like a chrome coating to me. Due to this issue, I wanted to test what would happen if I removed the lower piston ring. This will reduce friction and preserve the cylinder a bit longer. I was expecting that reduced compression and leak from transferports through the piston ring groove would cause a noticeable power loss, but I didn't feel any difference in power. Maybe even the opposite, I could almost swear that it had actually more compression when I tried to start it with only the top piston ring, but I don't have any measurements. But I'm pretty sure it doesn't wear the cylinder as fast now.
 
#18 ·
Sorry, didn't mean to derail things, just hadn't seen that engine before and thought it was interesting. Thsnks for the info chrisi, good to know.
I'm all about the budget philosophy, that's kind of the whole point with these things, the chinese stuff anyway, cheap fun. And trying to make power cheaply.
If by blueprinting you mean smoothing and flowing the inlet through the case and the head I did that faintly for the first time to my last build that engine is very strong. I'm doing the same to the next one, this is a totally stock case and head and piston, so we will see how it runs. I would say its definitely worth doing. Every little helps and it all adds up.
Yours seems to pull nicely, acceleration is good. I've played around with the rocket keys, advancing seemed fatser to me but the track we race on is very tight with only two shortish straights so its all acceleration, the advance seemed to help with that. I'm no expert that's just my take.
 
#20 ·
I got the blue tuning springs with a scooter that I bought years ago. I believe they are some general tuning springs for a Piaggio or Suzuki scooter. They are slightly longer than Minarelli scoot tuning springs and thus easier to fit. The Minarelli springs are very common and easy to get but they are very short and need to be stretched a lot. Actually I show that process in the video from 9:20. Also I needed to turn the pegs, where the springs hook onto, in my lathe a bit but this can also be done with an angle grinder, just needs more time and patience.
I just remember that that spring I fit in the video, is a CPI/Keeway minarelli clone spring. I recall they are also slightly longer than original Yamaha minarelli springs.

The blue Piaggio tuning springs that I fitted in first place, I think they are too stiff. The clutch engages at 9000rpm, it launches well but it seems to hurt top speed because the clutch is still slipping a bit at speed. I think the best option would be to try stock Piaggio springs. If those can't bring rpm high enough, then try stock CPI/Keeway clone Minarelli springs. All of these should be found from a general moped/scooter tuning shop.
 
#21 ·
Pocket bike madness gets worse.


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I have made a Pocket Bike Dynamometer.
It's a 15 inch car tire filled with concrete, weighing about 70kg or 150lbs. It's mounted on a peugeot car wheel hub which is bolted and welded to a frame.
There is a speed sensor in the wheel hub which can be used to input a signal to my laptop and then I can use a dyno program to get a power curve.

I'm going to use it to test some common cag engine mods found on this forum and provide some cold hard numbers instead of making claims and debating.
I want to try to disprove some theories that I think are questionable and promote my own tuning advice instead...we will see what I can get out of it ha ha.

A word about using a concrete filled car tire as a dynamometer roller. It can be extremely dangerous. Since tensile strength of concrete is poor, it will disintegrate if accelerated too much. I have made some rough calculations that the critical speed for this tire would be in range of 120-150km/h. I have actually made it 2 years ago when I wanted to dyno my other moped (70cc am6 engine). But it's too powerful for this. If I want to get sufficient load for the engine, the speed will be over 100km/h and it frightened me too much so I stopped using it...but I figured I could well use it for this pocket bike since this cannot do over 50km/h (except if I change gearing). So I just want to say that if you want to make something like this, be careful.

It's basically ready for pulls, except I'm missing a cable plug that is used to connect the roller sensor to my laptop. I need to get that done and then can start experimenting...
 
#22 ·
Today I had some fun time trying it. I just did a base measurement to see what it's got and then a bit of carb and ignition tuning.
Didn't take any picture but testing scene looks just like above except a laptop is there.

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Blue curve is the first, a whopping 3.2 HP. I think the dyno might be on conservative side but I guess it is better to show too little than too much.
On the green curve you can see a later run but with cold engine. The difference is massive when cold, it can rev much more (but still sputtering). Just 2 pulls later it dies a little over 10000 and sputters.

I tried to drop the main jet size from 82 to as small as 76 and to my surprise it still seems not lean and there is a hefty gain on midrange, but no help for top end. Except when the engine is cold, there the main jet made a big gain. Usually I have seen that top end getting worse when hot is due to running too rich but seems it's not the case.
I'm quite surprised how it can run with such a small main jet 76 and the carb is a 19mm dellorto. Usually it requires around 90 main jet on gear moped engines. I think that this engine has a high crankcase compression ratio, so it may create a strong vacuum which pulls a lot of fuel from the carb. Goes to show how large the main jet can be and the bike still seems to run "ok", but would waste a lot of fuel. When I first put the 19mm carb there it had something like 88 main jet and that worked too.

Last I tried to advance the timing at high revs. I had a curve where under 7000rpm it was 25deg, 7-10k 20deg and at 10k drops to 10deg. I thought the 10deg drop might be the issue and put it back to 20 but no, there was no change. Actually from that I got the best run 3.45HP but that may also be due to engine temperature difference since it doesn't even really hit the switch point.

Next time I think I'm going to compare the stock reed valve if my custom one is worth anything.
Any suggestions what is causing it to lose topend power when hot? I have a few suggestions but I want to see what others think...
 
#25 ·
I'm going to post a video where you can hear it screaming in the dyno soon. I think I'm not going to do a dyno build video because I kinda feel like I don't want to show people detailed instructions how to make something that could potentially be very dangerous if used improperly, lol. But let's just say that it has a drum brake car wheel hub that is mounted from the wishbone bushing holes onto a welded support that I made from box section pipe. The frame has to be able to carry the 150lbs tire + weight of the moped and some more.

The dyno electronics are very very simple. There is an original ABS sensor of the car in the wheel hub which has just 2 wires. The wires are connected to a voltage divider (2 resistors) which drop the voltage a bit, and then output is wired to a stereo plug which goes to the microphone jack of the computer.

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There is no separate input for engine RPM. The software can calculate it from the gear ratio. Actually because a pocket bike has a centrifugal clutch, it is slipping at first so the power readings at low rpm are not correct(higher than should be), but at about 6000rpm the slipping starts to stop and the power reading becomes correct versus the rpm. And I have swapped the stock clutch springs back to make it engage at low rpm for dyno tests.

Today I spent whole day trying to debug it. First I tried to reduce the main jet from 76 to 72 as that was the next smallest. Didn't make much difference. Then I tried the next smallest that I had, 62. With that, the first pull was good and it sounded really strong, but then started to bog down so that was too small. So the main jet seems not solving the problem.
Then I started to think what if the issue is that the fuel starts foaming inside the carb because of the crazy vibration and it makes it to go lean/too rich. There was a slight evidence of this because when it gets hot and starts misfiring, there is a little bit of fuel dripping out of the overflow tubes(but not much).

Then I wasted couple of hours making a new intake manifold, which would allow to mount the carb with a rubber pipe which could hopefully reduce the vibration at the carb.

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I didn't find a better diameter pipe so I was forced to make one that reduced the intake diameter from 19 to 15mm. I was not very happy with it, also the carb hits the frame which doesn't help with the vibrations. Then I tried it and a bummer, it didn't fix the problem and actually lost over 0.5hp everywhere! That was a total waste of time... With this manifold, the jet sizes were affected and 76 became way too small, it started to bog. I swapped a 82 jet and the power was way down, it was probably still lean but I didn't try to test it further. I don't believe the diameter reduction caused it because 15mm should still be plenty for this size engine, but it was probably the jetting.

Then I finally found some larger pipe and remade the manifold and now it's 19mm inside diameter.

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Because the rubber piece is so short, it is still pretty stiff and probably doesn't dampen the vibration by much..so I cannot even fully verify this theory. At least there is no change. This manifold now works well and performance is same as before.

The chinese had done a terrible job of finishing the reed valve...

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I went and cut excess rubber off and did a little bit of smoothing to it, but couldn't do much because on the other side the rubber was actually half detached and I was afraid that it could detach completely if I tried to port it further. Very bad quality part.

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Then I had one more try on the ignition timing. This time I gave it 5 degrees more advance for high rpm so it is 25 degrees at low revs and also at powerband. Hoping to solve the sputtering problem. Well turns out that it doesn't help either. Here is some curves that I got.

Hot engine. Green is with the new intake manifold, filed reed valve and 5deg more advance. Blue is previous best with 20 degrees. Seems a little less peak power, either it is temperature difference or 25 degrees is too much at that rpm. But on low end there is a good gain.

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Cold engine. Red is today's modifications, 25 degrees advance. Blue is previous best with 20 degrees.

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And here is again the difference between hot and cold engine, both curves are with today's mods.
Now there is no loss on lowend torque any more but at high rpm it is still the same.
When cold it sounds really strong except right at 10000rpm there is slight stuttering but it still pulls over 11k. I'm sure it could do very close to 4hp if it didn't have this symptom.

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Now I'm starting to believe the problem is the steel reeds. I think at 10000rpm the reeds start to flutter and that causes reverse flow which pulls extra fuel from the carb and then later that gets sucked in the engine, so in effect it has a rich mixture surge at that rpm. It kinda would make sense because generally too rich mixture causes worse performance when hot. But on other hand it doesn't because the behavior is the same with all main jets that I have tried and hot engine performance stays totally same. Also I cannot see any cloud of fuel coming out of the carb. I have seen that before on other engines when steel reeds cannot keep up. Strange.
 

Attachments

#26 ·
Here is the video about first dyno tests, english subtitles available:


Yesterday I spent again the evening tuning it. Not much point but it's a hobby ha ha.

I bought a sheet of carbon reed petal material to cut new reed petals from. But before I did that, I wanted to test a theory that by reducing the reed petal stopper distance, it could help the reeds to not flutter. The reason a reed petal flutters is because it has too much mass and moves too slowly to keep up and cannot close in time. By reducing the stopper distance the reed cannot move so far, so it has more time to return to closed position.

Here was the original stopper distance, about 7mm

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I reduced it by about 2mm

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This was the result. Red curve is previous best with 7mm opening and blue curves with 5mm opening. It seemed to run now rich and now the midrange got worse when it heated up.
But as you can see, from 6000rpm there is practically no difference, actually the smaller opening seems to do a slightly better job at higher rpm.
But..it does not fix the problem. Still dies at 10000 when hot.

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Then I cut new petals from the carbon sheet.

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I was pretty sure that the carbon petals would fix the problem but I was again wrong...


Blue and purple is with the carbon petals. On low end there seems to be a big loss compared to steels, but from 6000rpm it is better. Now it roars out 3.6hp.
Blue curve is with cold engine. There is a large dip at 10000rpm, at that rpm there is something strange going on in the engine.

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So the issue is not the reed fluttering. Then the only possibility I can think of, is that the spark has not enough power to ignite at peak torque rpm. The pipe is working hard at 10000 and pushing mixture back into the cylinder, and that causes the cylinder pressure to be so high at ignition time that the spark cannot fire. A cure to that which sometimes works, is to make the spark plug gap smaller. So I changed it to pretty minimal 0.3mm. And that too didn't help, but it did give some power improvement. The best hot engine run is now 3.7hp!!

Then I swapped the stock reed valve in to do the comparison, whether my custom reed valve made with great effort is worth it.
Except it is not fully stock, there is a malossi carbon reed which I have put on there many years ago.

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This is how the intake manifold looks with the stock valve. Actually it is not that bad, the carburetor is already at a good angle for the flow to go through the reeds.

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And here is comparing to the best 3.7hp curve.
It ran really lean with the stock reed valve, I had to keep the choke on and it was still lean. So this is not really a fair comparison.
But the stock reed valve is pretty close. It does just as well or even better at low rpm. At high there is not nearly as much power, but I suppose if I had changed a larger main jet, it would have got really close to the minarelli reed valve.

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Stock reed valve pull was on cold engine so it again revs more. When it got hot, it started to bog down so badly that I couldn't make a pull. Two strokes are pretty strange machines. After startup and bit of idling, the first pull is partly very rich due to residual mixture in crankcase and at high rpm it goes leaner. When the crankcase clears from extra fuel, it will go too lean if the main jet is too small, and that's what was happening with the stock valve. Then when it heats up it will go richer again.

In conclusion, the stock reed valve is not that bad of a restriction as I would have thought. Would be interesting to adjust the carburetor properly for the stock valve, and then try porting the valve, but it is a lot of work to fiddle with it and swap parts back and forth so I didn't want to do it now.

There seems to be this myth of cag engine sensitivity to crankcase volume, that you should not enlarge it. I think this pretty well busts it. My custom reed valve block clearly adds volume, but it still works just as well and makes more power at least in this test. In my view, you can do porting on the crankcase totally freely and don't have to worry about the volume at all. If using a tuned pipe, a large crankcase will only help it, it will provide more mixture that the pipe resonance can suck into the cylinder.

Here is comparison to when I made the first dyno pull. It was 3.26hp and now is 3.68. Actually the record is almost 3.8, that was one pull with cold engine but forgot to picture that.

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Pretty good improvement so far. It felt already powerful, must feel now even more insane. Basically it is smaller main jet, carbon reeds and more ignition timing. There is probably more to be gained from the ignition timing, so far I have just made some rough adjustments just trying to improve the hot engine misfire problem.

I'm a bit frustrated because of the problem. If it cannot do more than 10000rpm it's not going to have any kind of totally insane power that I hope for. I still have one thing left to try which is to improve the ignition box. I can improve the circuitry to produce more spark voltage. Although the small spark gap did not cure it, it improved slightly and I have a feeling that it might still require increasing the voltage from the ignition box. If that doesn't fix it, then I have to admit that I do not know how to tune toolbased engines as CAM2 has suggested, lol.
 

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#28 ·
Amazing work! And thank you for the english subtitles very much appreciated.

Have you thought about testing stock ignition for comparison?

I have had the same thought of crankcase volumes. And just as you pointed with a tuned pipe it's even a benefit for making power with larger volume.

Keep up the good work and keep us updated 🙂
 
#29 ·
And I just watched a YouTube video about building a voltage doubler for ignition systems the circuit was really simple and just consisted of some capacitors and diodes unfortunately I'm unable to find it in my history.

I have also seen members convert these engines to capacitor discharge ignition using Hall sensors and other typical motorcycle parts with the benefit of being able to change the timing by moving the hall sensor Rcexl Universal type Ignition Hall sensor KIT for Engine Ignition | eBay

There is a guy on YouTube his channel is called 2 stroke stuffing he builds crazy high-reving high horse power 50cc engines in the 14K rpm range you might want to talk to him and ask him about his ignition systems

One more option would be to try to adapt a coil off of a BZM or Polini engine Chrisi ( a member here ) would know more about the possibilities on doing that I believe those engines hit 10 to 12 K stock maybe more.

These are just brainstorming ideas that I'm just throwing out there.
 
#30 ·
Thanks for your comments. Seems my thread is now pinned, that's cool.

Basically my ignition box should be capable enough. Just by looking at the spark I can tell it is stronger than most stock systems on any moped I have seen. But maybe the high compression ratio I'm running combined with the pipe requires a really hard spark. Although I have now come up with some other possibility what could be wrong. Will keep posting.
 
#31 ·
I didn't modify the ignition box yet, as I started to suspect that the issue might also be related to so called "squish velocity" inside the engines combustion chamber and how the worn out crank bearings might cause too high squish velocity.
Basically it means that when the piston comes up and squeezes the mixture on the very small squish gap, the mixture will be pushed out with x velocity. The smaller the squish gap is and the larger the squish area is, the higher the velocity. Then I speculated that if the velocity is too high, it might have undesirable effects on combustion. Maybe it would somehow "blow out" the spark. As my crank bearings were worn out, it would cause the squish gap to get smaller at high rpm, because the piston would pull on the bearings and reach higher. There is evidence of this, marks on the piston crown hitting the stock head back when it still had it. With the new head there was no new marks.

So I decided to change the bearings. I again went with low budget and bought some chinese bearings from hardware store and just ripped the seals out of them. Might be a mistake to use general chinese bearings here ha ha..

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At least the new bearings felt good. There is a lot of play on the old ones and new ones can feel absolutely no play.

When I assembled the engine and tried to start it, I immediately noticed that the engine now turns a lot smoother and with less noise. I always wondered why the engine is so noisy and seems to have this high pitched rattle when running. I thought that it is coming from the pull start, but it was actually the bearings...

It also seemed to start and run better. I had high hopes but unfortunately the dyno said different.

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The red curve is with new bearings. If anything, there is about 0.2hp loss on peak power and no change on the problem.
On green curve I got the most power with cold engine so far, 4hp. That was after I tried to clean the spark plug.

Speaking of the spark plug, that was the next idea to try. Maybe it would be that simple? The spark plug was years old and not very clean. First I tried to just clean the old plug by heating it with a gas torch burning old carbon away. There was a noticeable improvement at first(the 4hp curve), but then it seemed to reduce back to old level. Then I bought a new spark plug to confirm it. That didn't make a difference either.

Maybe too high compression ratio, or still too high squish velocity.
I took the head off and slightly enlarged the combustion chamber, making squish area smaller.

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No change.

Then I started to try various adjustments. It feels like it's still a bit rich with the 76 main jet, but I'm missing a 74 jet, 72 was too small. I leaned out the needle and mixture screw if that could make it slightly leaner. Well, it only lost power as a result. Then I adjusted ignition timing again. It was 25 degrees constant curve, first I tried to increase it to 30. That didn't make much difference, there was maybe slightly more low end torque but more misfiring at high. Then I tried 22 and 20 degrees and with those, it started to act even stranger. Cold engine power slightly reduced and with hot engine, a huge power loss (about 0.5hp) at all rpms... it just didn't make much sense. I also tried a larger 78 main jet with the 22 degree timing, because it felt like the timing changed it so that it went lean when hot, but that was also a wrong assumption, it got only worse.

Then I sighed oh well, and thought that maybe I just continue with the original tuning plan improving the parts and see how it changes. Sometimes these things are so random that it's impossible to predict what change makes it better or worse so I might as well change something that shouldn't relate to the problem. So the next thing to improve was the squish clearance. It was around 1mm, so I took the cylinder off and mounted it in my lathe and machined 0.5mm off the top.

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Turns out that I had measured the 1mm squish clearance in a hurry. It was actually less and now the piston was hitting the head lol.
To fix it, I slightly machined the head, but that was not bad since the squish band angle was already wrong, I could fix that at same time.
After that I measured that the squish clearance is now about 0.32-0.35mm. That might be too little. A rule of thumb for minimum squish clearance is 1.5% of stroke, and since the stroke is 32mm, this is now around 1%. But it was getting late so I left it. Might be a bad mistake, the piston may hit the head at high revs, I have to check for that.

Then, testing the new squish clearance.
Obviously the compression is now higher. Bike sounds much more snappy at around 2000-5000rpm. But again a bummer, after I dynoed it, it became clear that it is again a backwards step.. :ROFLMAO:

sorry about the bad image, had to take it with a gopro

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The highest curve is the best overall hot engine curve from last week(3.7hp).
Pink curve is with cold engine with the 0.32mm squish gap. Barely 3hp. If I remember right, it still had 3.6hp just before the change.
Then when hot it just gets worse. This thing is annoying. It seems that I could easily spend all my spare time tuning it if I want to get great results.
Although these curves are still with the 22 degree ignition timing. I'm pretty sure that the worst drop in performance came from changing it from 25 to 22. That's when hot engine power dropped under 3hp.
There is a slight improvement in the high rpm region though. Now it continued to rev over 11000 even after couple of pulls although still with some misfiring. But it might just be that it didn't get so hot, because it was making less power overall.

I have to put the timing back to 25, and also test settings between 25 and 30. Although I'm a bit afraid that there might be something else going on. The cylinder is also pretty worn, I think that may also contribute to power loss when hot and makes it harder to tune.

Here's a couple of pics of the cylinder. The exhaust port is raised(cannot remember how much in mm), I have ground a boost port and slightly filled the transfer corners with JB weld, but that was a poor job and it really needs much more filler. Planning to redo it at some point. I also plan to do some proper porting on the transfers, but I would like to get the engine running well and fix the misfiring problem first, to really see the effects of porting.

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I have to say I had much higher expectations at first about a cag engine power capability with tuning. Seems not easy at all. I'm still keen to improve it and solve the issues, but it seems to steal so much time that I may need to take a break..:rolleyes:
 
#32 ·
Interesting results.

Have you considered using e85 and some oil that mix with alcohol?
Will help with pre detonation too.
Might cool the head?

Do you still have the stock intake, reeds and carburetor to try if the problem is the reed and reed cage?

You will find the problem/problems.
🙂
 
#33 ·
E85 test would definitely be interesting. It would surely cool the engine a lot and help with the issue. But it will bring more problems with tuning. I read that it is especially hard to get the low throttle settings in tune with ethanol. I think I want to get it working with normal fuel first.

I have the stock intake parts. I have already tried it (posted about it). It was badly out of tune, but it seemed like the behavior was totally same regarding the misfiring problem. But I may give it another try if I cannot figure it out otherwise. Didn't try the stock carburetor, because it leaks. I think that is the worst carburetor I have ever seen. It was always leaking and very hard to start :D
 
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