expansion pipe problem - motorized bicycle

The problem - the pipe wasn't designed for your engine and isn't tuned. Some members have been successful adapting other pipes to the HT. Do a search. Download a copy of Two-stroke Tuner's Handbook from Gordon Jennings.

Good Luck,

Chris
AKA: BigBlue

I say remove that restrictive so called silencer and use the one made for that pipe. See what happens then....

One more thing... Did you port or clean up the intake and exhaust port? Trim the piston skirt? Break the motor in with a couple tanks of gas and do the things I mentioned. Hope it helps

Looks like the pipe is just flat too big to me. Not enough backpressure.

Ok...I will step back on what to do...cuz I have become a Four Stroke Tuner...

A reed valve, '3 port'-port job & a better intake & carb will make a lotta difference.
(& get rid of that stock muffler like CCC said)!!

& while you're at it, get some good front brakes!!!

for speed, go with arrow and stuff your crank case, also get a bigger, tunable carb (Dellorto SHA clone, AKA Runtong or DaxRT, isn't a great carb tbh, I didn't notice anything but better idling). a 19mm-20mm Mikuni or Dellorto PHBG carb are fully tunable, and will provide the flow the motor is demanding. Also because that pipe is rather short before the expansion chamber, odds are you may not see "coming on the pipe" until higher RPM's, maybe around 6000. Reed port the piston, boost port the intake, raise the exhaust 1mm, and widen it to 30mm (ROUND THOSE EDGES! you could break a ring on the first rev if you dont!). That should net you the power and revs to get on the pipe effectively. If you find you've lost some of your low end, get an offset intake (or buy some 3/4" ID silicon tube, its like $2 for a foot), and add around 3-6" to your intake length to raise the power band at the low end a bit. You will need to make some kind of adapter to go from the 3/4" stock intake to the almost 1" throat of the carb.