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.
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.
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.
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.
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.
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).
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.
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.
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...
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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...
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.
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.