i just got a engine a 400 yz put it on a 400ex frame its got 12.1 copression, ported and polished, but cant get the carb right . tryed every thing i could think of. it dies if you hit the gas hard but if slow revs good . any help would be greatly appreached

 

My cousin is having the same problem with his, what year is the engine from?

thanks,

-hondaman0045-