66cc motor WOT problem

[Bgnlb]

My 2nd bike started dying at full throttle,I removed filter in tank and swapped out carbs. I had a expansion pipe on it. I put the stock pipe back on,no more cutting out at hi rpm. Has anyone had same problem,is it a back pressure problem?

 

[Fabian]

More than likely it's a carburation issue/air leaks issue.

The only effective method that gives absolutely reliable carburation is a Walbro style (diaphragm) carburettor, as used on chainsaws.
They don't leak (fuel or air) and have external jetting adjustment to zone in on the perfect air/fuel ratio; keeping the jetting setting locked in place once set.

 

[Bgnlb]

I think it's a back pressure problem. When I had expansion pipe on it would cut out at hi rpm. I put the stock pipe on and no problem at hi rpm. In the 70's 2stroke road race motorcycles had 8-16 inch stingers

 

[Fabian]

It's sounds like a jetting issue. Once the air/fuel ratio is optimised for your setup the engine will run correctly.

 

[butre]

there's not really such a thing as a back pressure problem. There's overly restrictive and there's poor scavenging, neither of which have anything to do with pressure.

 

[Bgnlb]

You're rite,when I had expansion pipe on,it would run good when I fiddled with petcock