I am working on a new expansion chamber....
looking to break 150kph (93.2 MPH)...
future plan for the summer is to install this..(turbo charger).
An expansion chamber works by forcing unburnt fuel in the exhaust back into the cylinder via a resonance wave.
Properly tuned it makes these little 2-stroke engines extremely efficient.
How you might be able to tune forcing more intake in as well from the feeble exhaust flow via a turbo charger is a mystery to me but not what really got my attention...
It is you 'looking to break 90 MPH' on that knobby tired used ~$200 mountain bike that really cracked me up...
Heck, that bike would self destruct before you even got to half that speed and blow you off it before if it didn't.
Do have any clue how fast 90 MPH really is?
90 mile per hour wind is hurricane force.
Heck, 90 in a nice car is fast, ever rode a real motorcycle 90?
Put you hand out the window in a car 90 and see how it feels.
So back to reality.
A shift kit helps as does an expansion chamber exhaust and good spark plug, but shoot for high 30 MPH speeds at most is my advice.