My sons 90 is running rough. if he give it gas slow it doesn't break up. If he stabs the gas it breaks up but as the rpm come up it clears up. I have tried smaller jets, bigger jets, moving the needle up and down. I can make the stubling worse but can't get it to go away. I have cleaned the carb well a few times.
These are the recent changes:
- new big bore kit
- high comp. head
- clamp on filter
- reed valves

He's getting frustrated because his brothers 90 has same mods, runs good and beats him. Any help would be greatly appreciated.

 

have you tried ajusting the air it sounds like it is might be to rich on bottom end also change plug they can do the same thing what carb are you running?

 

I have adjusted the air screw and I have tried jets from 100 to 110 with no luck. I am starting to think it is a electrical problem, I will get a new plug today and see if that does anything. I don't know the brand of carb on the mashine but it is stock for the bike.

 

does it pop and crack wosre after it warms up

 

No it gets a little better after its warm. I will get new plug today. Could it be pre detonation due to low octane and high comp. head?