Exhaust Jag pipe on 66cc

Stock intake port duration is too little. Increase it to 120 degrees open.
After that if you still want more speed then raise the exhaust port 1mm at a time till it's where you want.
Make sure your main jet is optimized for speed. Too big or too little will limit speed. What color is the plug?
Picture of the plug. But it isn't too accurate as it is not from a WOT plug chop but just some messing around and maybe even idling.
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Also looks like I have some "nice" blowby
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nice color plug.
it's hard to get these fat rings to seat well, even if you use the right break-in method. (using more oil and taking it easy is the wrong method)
 
Haha.. Measured the porting on the black jug and almost pissed my pants.
The measurements are not 100% accurate but I guess close enough.
Deck .7mm
Exhaust 27.66 which gives 152deg
1st transfer 30.78 which gives 132deg
2nd transfer 32.43 which gives 120deg
Piston 44,5 and intake 56.2 which gives 110deg

Thats the stock porting for this jug, exhaust might be rised .5mm or so as I gave it few strokes with a file at some point when I had the engine apart the previous time.
 
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Anyway had some fun with da Dremel and some files so now it should be 160 ex, 120 in and didn't touch the transfers as I have to still figure out which is worse. Unequal transfers or too high transfers.
 
Naaah.. shiet. Top speed 45 and no noticeable difference in acceleration. Hits 40-42 quite fast and then like a wall. I think the header might be a bit long but would be hard to shorten as the pipe would be lower and hitting the ground soon. Also I have a offset "key" on magneto side so it retards timing and on top of that ignition that retards timing so might be too retarded as a result. Was too lazy to change it after fitting the custom CDI. Will flip the key around to advance or try a stock key. WOT plug chop added.
 

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45 is enough.
yes, that wall is due to pipe design.
now think about putting a good carb on it.
 
45 is enough.
yes, that wall is due to pipe design.
now think about putting a good carb on it.
Thanks for your input. I might manage to upgrade the carb later this month.

As for 45kph top speed, I think I'm not going to settle for that. I did some calculations and if they were not way off then 2mm rings in this engine limit it to 8500rpm so with a small safety factor I'd like to go up to 8000rpm. Before that I'll try to balance the crank a bit more and maybe if I find the equipment try to true it better than it is atm. I guess I'll have to start looking for a piston with 1mm rings soon. Quite hard task, maybe fitting a scooter jug with its piston might be the easier route. Has anyone found a piston that would fit and rings that wouldn't destroy the chinasil coating?
 
look on my site for info on alternate piston and rings.
45 is about the limit for moderately modified china girl engines. and if you port it to go faster then it sucks at low RPM.

what did you use to calculate the max speed due to the thick rings?
 
45 seems pretty slow , all I did was make a jag style pipe on my 48 cc and it goes 42 on flat everything else is stock except sparkplug wire and plug
 
maybe your speedo isn't calibrated right. It normally takes more than my pipe to get that kinda speed.

and 45mph is the safe limit for a motorized bicycle. Any more should only be used on a drag strip or closed race course.
 
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