How to tweak to peek revs ?


Active Member
Local time
12:37 PM
Jan 31, 2008
South Australia
My first build seems ready now after runin to start a series of 2-stroke do-able tweaks.
I am no expert on 2-stroke in a practical sense, mostly lawnmowers.

So if could ask the experts on 2 stroke HT engines, I have a 50cc, 2 weeks old and most interim bugs are ironed out.
The clutch works, the motor sits on frame stable and everything sits right and tight. Minimal vibration and starts OK.
Spark plug is NGK gapped at 25.
Needle is set at 2nd from top. Runs better at this setting than the settings it was packed with.
Exhaust gasket hole has been enlarged to equal outlet and the inlet gasket has been resized so there are no obvious air/gasket restrictions.

1.On the straight is goes well, but loses power on a steep rise.
2.If I rev the engine out to full throttle, it seems as if this engine is
It won't "scream". It will hold it's revs at full throttle with no obvious fear of blowing up. It feels like it wants to rev more, but something is holding it back.

Where do I start step by step to tweek this engine to reach peek revs ?

Thanks in advance.

I ran into the same problem on one of my builds. I did all that you did then started trying a few other things. Plug gap I set to 30, this makes for a hotter spark ran alittle better.Took off intake manifold and polished inside with porting dremel, ran alittle better. Finally I took off the exhaust pipe end and ran the bike it was loud as **** but ran all the way through the powerband, that was when I decided to make two outlets on pipe. I cut the out tube in half drilled a hole in the base far enough to be able to weld the other tip in place reinstalled on to muffler bike ran fine. Hope this helps.
I wouldnt suggest running a exhasut without SOME backpressure. It might run good, but it wont for long. 2 stroke engines require backpressure. You might want to do some searching on expansion chambers, or tuned pipes, and that would explain better than I can.. Here is a good link explaining the exhaust pulse, and why having it IMPROVES engine performance....Bolts you can start by drilling the jet out one size and see what that does, since jet is the metering device for WOT. You might have to play with the needle and clip to adjust 1/4- 3/4 throttle, and if it doesnt work or you dont like the changes you can solder the jet closed and redrill to factory size I also have the 50cc Roundhead Grubee engine and just recently put a pocketbike pipe on it which improved performance but am still working on rejetting, cause like you mine seems like it has more at the top end, and I believe just needs more gas....
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Taking pipe end off was only to see how the engine responded. Yes you don't want to ride around for long it's way to loud and could cause problems. I played with a siik mfg. chamber and found it to work well on another build I did, it did require some cutting and re-welding and fit very nice on bike. When I figure out how to up load pictures will do so.I am also going have have siik make these pipes for me, I have previous customers who want them for their bikes. They will be adjustable to a certain degree to fit most bikes.
Description of errors in students' English writing

Yeah,granted, I've heard about Chinese students having to study Greek, latin and French in order to understand our English, in particular the medics.
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