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Old 01-04-2016, 08:33 PM
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sbest sbest is offline
Motorized Bicycle Elite Member
Join Date: Nov 2015
Location: Nova Scotia
Posts: 249
Default Re: Engine won't go past 30MPH

Originally Posted by Keef B View Post
Thanks Guys much needed info. I was racing my friend over the weekend and his bike did 45mph. He was giving me head starts and was still whooping me on the top end. Any Ideas on how I could increase acceleration, I have a basic arrow motors sort of modified exhaust but it has leaks.
Mine is stock with a stock carb and stock exhaust on regular fuel. I can reach 35mph to 37mph on the flat with the engine "tuned up" lightly.
1) Carb jetting set up by looking at the soot-up of a new plug insulator.
2) Cylinder head squish area reshaped with sandpaper to match the piston dome.
3) Piston to head gap set to around 1mm by sanding the cylinder and/or head, and/or shimming the basegasket height.
4) Aligning the drive chain to run straight
5) matching the exhaust pipe flange and gasket to match the exhaust port.

This is where I am right now. I am using this engine as a testbed to test whether "known" 2 stroke tricks really work. I have been doing them one at a time so that I am sure what works and what doesn't.

So far one that didn't work was port matching the intake.
The stock mismatched intake (too small compared to port) works better!
The reason is likely due to reversion pulses. Also runs leaner this way.

Next steps, which I hope will push me over 40mph, are going to be:
6) File the casting flaws off the port edges of the intake and exhaust.
7) file the gross ridges off the transfer ports
8) trying a longer smoother intake and a trumpet like tapered intake to the carb.
9) Drastic step, pull the engine apart and stuff the crankcase with epoxy.
I will be doing this first (am doing it now actually) to an older 66cc engine that will not rev as high as my new engine due to crank balance issues. So I will drill a couple balance holes, plug them with foam and epoxy, then fill the case with several more tubes of epoxy to reduce case volume and primary compression. I am doing this all before a tuned exhaust on purpose. When the engine is maximized first, it is easier to tune in the pipe, so:
10) Tuned pipe.

I see many on here do 4 or 5 mods at the same time, never learning the true effect of any of them. I see failures from ill conceived "improvements". Try things one at a time on these engines. They are so simple I can have the crankshaft out of the motor in 1 hour.


Mr Happy Piston...

Last edited by sbest; 01-05-2016 at 04:00 PM. Reason: spelling
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