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Wow did not thought the chopped pipe would do that bad, that's insane.
All power disappeared.

I have been thinking of try another cylinder than stock on the cag engine.
This one. It got real ports and either jb weld the intake on the cylinder and making a boost port and keep the location of stock intake.
Or plug the stock intake and stuff it a bit and then use the intake on the cylinder, then use the piston port intake or chop the piston and grind the intake and put a reed valve on the cylinder intake.


They are a bit expensive though and require some work to get it rolling.
 

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Discussion Starter #42
I have continued troubleshooting the bike.
After changing crank bearings, drilling the carburetor air jet hole and shortening the exhaust were all failures, next suspect was leaky crankcase seals.
I read from a twostroke guru forum that a case leak actually makes a motor run rich, not lean. It is pretty hard to explain and I cannot explain it fully but one thing to consider is that if there is a case leak, it will also blow out air(and fuel) the same way as it will suck air in, so it cannot be directly said that a case leak makes it to go lean. But if it really goes rich, it could explain the sputtering and spark plug always being dark/black.

So I went out and changed new crank seals...
After that, it again seemed to start and run noticeably better. I had to disassemble the dynamometer temporarily so I had to test by driving. Seemed to have a lot of power, no signs of the <2hp that the dyno showed after I shortened the exhaust. Strange.
Then after just a short drive it started to bog down. It was clearly lean now. At first I thought that the carb must be running out of fuel, so then I went to drill out the carburetor needle valve...

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Basically I drilled two side holes on the needle valve channel, so the fuel has a shorter path around the needle. This improved fuel flow from 170cc/min to 200cc/min. But didn't fix the problem.

Then I realized that the main jet must be too small now. Either it is because the crank seal leaked and really made the motor to run rich and the 76 jet was ok, or then the carb air jet hole that I drilled on previous page made it go lean and I didn't realize it when running on the dyno. This would also explain the power losses, so the shorter exhaust didn't really cause a power loss. It just didn't do the bogging on dyno, so I didn't see that it was lean.
So..then changing a larger main jet. Now I put there a 80, too lean, 85 too lean, then 92, much better and then 90 was the best.

Then I also made a new exhaust mounting flange that uses springs to hold the exhaust, so the exhaust got about 20mm length back. At this point, the bike now had the previous power level back, and I could get 3.8hp on cold and 3.6hp on hot engine, but still the problem that it loses about 1000rpm of revs when hot.
Then there was the ignition box. I made a new CDI with about 20% more spark power and thoroughly tested it in test bench that it should be able to provide full power to around 15000rpm. Didn't help. Still one thing to check with the ignition is the actual timing. I guess it's possible that the trigger coil goes haywire at 10k, and throws the timing off.

Ok. I again scratched my head and tried to come up with new theories. I again came up with a new idea which seemed that it could be likely. Maybe the problem is that the exhaust is way too short, and actually does not even get to powerband, but it only gives a power dip at 10000rpm. The "power dip" is a symptom that occurs in all two stroke tuned pipes, that at 2/3 of peak power rpm, there is a torque dip, due to exhaust pressure wave returning to cylinder at a wrong time and pushing mixture back to crankcase. So, if the pipe length would be suited to around 14000rpm, then there should be a power dip starting at around 9500rpm. If you look at some of my power curves, with the cold engine curve there is always a dip at 10500rpm and then it gains power back and continues to rev over 11000. Maybe when the engine is hot, this dip is too much for it and it cannot rev past it.
Now there was just a question of is the pipe really that short for this to match? I have usually seen that a pipe length of around 750mm will produce a power peak at around 12000rpm. The pipe on this pocket bike was 725mm in length, that would mean 12500-13000rpm. So it doesn't quite match... but I figured I still give it a try.

So...then I took some old car exhaust pipe, chopped the exhaust in half and welded a generous 100mm elongation to it.

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Now the pipe extends way out..looks pretty funny. I thought that if it works, I will remake the pipe with bends to fit it better inside the bike.

It was again very exciting. If the calculations match, this pipe having now a tuned length of around 820mm, should produce a power peak at 9000rpm. So the engine should get a crazy boost.

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Again it turned out that this engine just doesn't behave the way I expect. The red curve is with the short 720mm pipe and blue curve elongated 820mm pipe (graph description says 800mm).
Almost identical peak power and practically no change in peak power RPM! I was completely amazed at this..I was sure that there should be a significant difference at least on peak rpm...
Then the green curve is cold engine power with the short pipe. Cold engine power with long pipe is less and it cannot rev over 11000. At least now the heatup problem is "fixed", because now it doesn't rev as much when cold. So clearly the exhaust plays some part in it, I just don't get it right. It seems like the exhaust pipe is not doing much at all. I might as well make a test with no pipe at all and see how the curve looks like.

Not going well. I was hoping that it could be doing well over 4hp at this point. But at least I have learned something.
I still believe there may be something wrong in it which causes problems at higher revs, because it still sounds it is misfiring. Next I'm going to check the ignition timing with a strobe light.

Then, the next tuning modification. I have read a lot about porting the cases on this forum. I gave that a go...

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Basically I rounded out the edge on the cases, ground out a "channel" around the bearing and then cut the cylinder skirts, smoothing the path to transfer ducts.
To my eye, there is not much that is improved by grinding. There is not really so much flow obstructions. Maybe the exhaust side cylinder skirt corners were a bit on the way.
Then I cleaned it up and put back together. Then a test..this was again very exciting. According to a member on this forum, the case flow porting will gain hugely. Maybe I could get over 4hp now.

The result, hot engine (blue: stock cases, 820mm exh.; purple: ported cases, 820mm exh.; red: stock cases, old shorter pipe) :

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Ha! No difference! This was again quite interesting, as the peak power is identical which is remarkable because usually even the slightest change may show up on the dyno...
There seems to be some gain on the lower end, and I think it might have sounded a bit better on takeoff, but in conclusion it is really not worth much to grind the cases...
Then another point, the crankcase volume.. some guys seem to worry about the effect of diminishing crankcase volume. Well there is some reference for you.

Next plans to get to over 4hp and over 11000rpm power band...
Unless there is some problem with the ignition timing, it seems like the engine is not going to give any serious power beyond 10000rpm.
I believe the back reed valve is a bad restriction, with the turning crank webs acting like a rotary valve blocking the inlet flow.
I have designed a new reed block:

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By grinding out the crank halve locating pin boss, there is just enough space (I think) to mount a reed valve block on with a circular flange matched to the crankcase shape. It will require a lot of grinding and sanding to seal it, but I think it can be done. I 3D printed a first model, but it is too large, have to shrink it further.

Then some failures.. On a test ride, the freewheeling clutch broke and it didn't engage any more. I fixed it by welding it solid on the wheel hub.

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Then pretty soon after that the clutch bell holding nut snapped off (just as I did the test for the case mod) and the engine locked up. I thought that it seized and was pretty annoyed, but fortunately it didn't. I tried to fix it by welding the bell back on, but I didn't screw it back on fully before welding so it is now binding on the clutch and have to grind the weld out and redo it...

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Pocket bike tuning continues...
 

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Checking the timing is probably a very good idea.
I would also suggest you do a leak down test to be sure there is no leaks.

It looks like you did the path on the case on the wrong side? Considering which way the crank rotates.

I have also been thinking of put the reed valve there. Then fill the cases where the stock were.
And use a full circle crank. Would make it to rev easier in theory.

There is a member that got the cag engine rev to 14 000 rpm with the reed valve on stock location. He did however use a morini ac cylinder. He also used a full circle crank. Search for rcfish90 or Fish's big block project.


Do you still got a stock flywheel and ignition coil?
For test purpose.
To see how stock ignition performs on dyno.

Strange about the exhaust, will you test a straight pipe?
Without an exhaust you will suck in fresh air in the cylinder making go lean.

Really likes your updates.
Fun reading.

I did a test ride on my 44mm cylinder.
But haven't machined it yet so it was with very low duration and a very tight squish. I think the exhaust timing is 155 😂
 

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Discussion Starter #44
Thanks for your comment.

It looks like you did the path on the case on the wrong side? Considering which way the crank rotates.

I have also been thinking of put the reed valve there. Then fill the cases where the stock were.
And use a full circle crank. Would make it to rev easier in theory.

There is a member that got the cag engine rev to 14 000 rpm with the reed valve on stock location. He did however use a morini ac cylinder. He also used a full circle crank. Search for rcfish90 or Fish's big block project.
I figured that the "path" should be made on lower side, because it has slightly more space there, and also the stock reed valve is oriented that way to make the mixture go to the lower side. Although since I'm using the minarelli reed valve which is oriented straight, it probably does not make a difference. But I also think that the lower side might be the better side, because the intake flow does not start immediately after piston starts to rise. I read that there is quite a large time delay in a reed valve engine intake flow and the piston is going well past middle stroke before the flow has fully started from carburetor. This means that the crank webs start clearing the lower side at this point. When the piston is at TDC and slightly past it, there may be still mixture flowing from the reeds because of inertia, and at this point the crank webs have cleared the lower side and there is more room for the flow.
But after all, I think the grinded "path" may be not worth anything. It will just be a small space between crank webs and the cases. Not sure why I even made it, I thought that I had seen someone grind that and wanted to see what it does. But I saw that it does not make almost any difference.

I read the Fish's big block project. Was pretty cool tuning there. Didn't find any proof of 14000rpm, but in one burnout video I could record from the sound track that it did 13000. And 12500 according to tachometer. Highest I have got so far is 12500 and that is for just a few seconds with stone cold engine, then it cannot do it any more.
As a side note about Fish's big block engine, I think there is not much point putting a 70cc minarelli cylinder into cag cases and 32mm crank stroke, because the cylinder might reach maybe 50% of its potential.. It is because the port timings will be way off with the 32mm stroke, and the crankcase mouth is so small that the transfer ports will be choked.

I have seen some 50cc minarelli AC cylinders from china that have so small ports that they would probably work well with a 32mm stroke. It would just need a spacer plate and machining the cylinder top a lot.
 

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Discussion Starter #45
Double post.

I have now tried replacing the trigger coil. I use the stock cag ignition coil as a trigger coil to give the CDI a signal of crank rotation. I had a doubt that the cag coil would start to give erratic signal at high revs and throw the timing off. First I wanted to check if this was actually the case. I have an oscilloscope which is a device which can be used to probe electrical signals and see what is happening inside those wires and parts. Here is a plot of 2 signals when the thing was running full throttle, heated up and rpm about 11000.

Yellow line is the output of the cag ignition coil from the kill switch wire. Purple line is a filtered output of the same signal, that goes to the CDI "brain". When the yellow line goes high enough, the purple line goes low and the CDI has detected one crank rotation. Then there is that spike in the purple line, that is noise coming from spark plug when it fires.

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Now the problem could be that the yellow line goes erratic and does not trigger the CDI, or then it would trigger it slightly later or earlier. And you can actually see there that the second yellow pulse indeed has some problem, the initial pulse does not go as high and there is some oscillation. But looking at the purple line, the CDI is still triggered, and there seems to be no obvious problem.
I also recorded a lot of the noise spikes from spark plug, trying to see if sparks are missed. There was no missing sparks.

But I was going to replace the trigger coil after all to be sure of it. I took it off and used a hacksaw to completely rip out the black box around the metal core. Then I put some electrical tape on the bare metal core and winded some 400 turns of copper wire on it, then put electrical tape on top and installed the coil. Pretty high tech.

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Other side of winding goes to ground and other to CDI. Couldn't be any simpler than that. Now there is a clean signal, and should not be any changing of the signal shape at any rpm.
Tested it and guess what. No difference whatsoever. Not a surprise though, as the oscilloscope showed there was no evidence of a trigger signal problem.

I wonder what I'm going to try next. I think I'm not going to chase the "problem" any more, it just seems like a huge time consumer.
Still haven't got the new cylinder. When it arrives, I will test that. I'm pretty sure it should give more power because the old cylinder and piston are obviously worn.
Also need to start working on the new reed valve block, that should be quite interesting.
 

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Nice work. Always fun to read your updates!
You are probably right about the path and there is some delay with the reeds so that speaks for your hypothesis to be right.

I only have a small handheld oscilloscope 😉

It seems like you have eliminated most of the things that could restraining the rpm.
Do you still have missfire's at high rpm hot engine?

Have you thought about testing a full circle crank? They need to be drilled/lightened on top to make the balance factor a bit better.
They are cheap if order from china.

I have thought about order a full circle crank, but have not decided yet if I'm going to.

I think those open transfer and boost ports are pretty bad for making power.

I also have thought about the Chinese minarelli vertical ac cylinder.
Cylinder are very cheap but needs reed valve, intake and carburetor too, so for me it gets expensive.
The morini ac engine have a shorter stroke of 37,4mm, i think cag full circle crank are 33mm.
So either a 5.4mm or 4.4mm stroke difference with the morini cylinder.
Minarelli 39.2mm stroke. 7.2mm or 6.2mm difference.

But changing cylinders are a bit extreme on those engines maybe.
Those clutches are very bad on the cag engine.


When your new cylinder and piston arrives, will you make a boost port on that too? Or will you try without first?
Seems like on those engines the boost port reduces the rpm quite a lot.


Keep up the good work! 🙂
 

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Discussion Starter #47
It seems like you have eliminated most of the things that could restraining the rpm.
Do you still have missfire's at high rpm hot engine?
I think usually it sounds like it is misfiring, like a rev limiter. But sometimes it just sounds "normal", it just cannot take more rpm. And when the engine is stone cold and it has oomph to take over 12000rpm, even then it sounds it is misfiring and stuttering, like there is a limiter at also that rpm. Partly the power loss is explained by that the intake charge gets smaller as it heats up and this naturally reduces power, but usually the misfiring should not occur, and it should be fixed by a smaller main jet, but in this case it just doesn't work out.

About the full circle crank, I think it may reduce power since it will make the cases flow through even worse. I have no idea about the balance factor. It might be better with full circle crank or then it might be worse. I have thought about checking the balance on the crank assembly. I would like to reduce the vibration as much as possible.

When your new cylinder and piston arrives, will you make a boost port on that too? Or will you try without first?
Seems like on those engines the boost port reduces the rpm quite a lot.
I can do a test without a boost port first.
It is possible that the boost port could reduce revs. If it directs mixture too much into exhaust, it will lower the exhaust pipe temperature and make the tuned length effectively longer, lowering peak power rpm.
But this requires that a tuned pipe is used. When using a normal sh**ty cag stock pipe, there should be no effect like this. I just wonder how many people saw this effect when adding a boost port and were they using a cag stock pipe?
 

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Discussion Starter #48
Bad news.

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I found a problem from the CDI's program code which caused the spark advance to jump at least 4 degrees back and forth. I could see this from the oscilloscope pictures, but didn't realize it at first. Actually there may have been even larger jump but I don't know why yet. I fixed the program bug and went out to test it. Then the small end bearing disintegrated... That was really unfortunate, if it had lasted for 2 minutes more, I would have known if the CDI fix made any difference...:D

Of course the cylinder was ruined even though just barely. I had ordered a new cylinder from china long ago, but it still hasn't arrived and looks like it never will. I ordered a second one, let's hope that gets here.

I also noticed that the engine had a bad leak on the base gasket, it was dripping fuel. This is due to my habit of testing and doing everything, but doing everything in a hurry. When the new cylinder arrives, I will make proper gaskets this time to avoid leaks. Then I will also find a better quality needle bearing. Those china needle bearings are notorious for ruining engines. Does anyone know the dimensions for cag needle bearing? I have found some mixed information that the outer diameter is 13.8mm or 14mm. Inner is 10mm. A minarelli scooter needle bearing would fit if it is 14mm, and you can get some quality ones.
 

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Get a BZM or a Polini smallend bearing, that is high quality and has the same dimensions!
 
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