carb help PLEASE

[400_ex_man1987]

I am having trouble with my carb. abviously. I have exhaust K&N on it and I resently put a 6inch vent in the airbox lid. but now when u crack the throttle wide open it sputters and bogs. I figure its the main jet thats the problem but I do not know if it is adjustable or if you juss have to replace it. if its best to just replace it then I will do that. If its not the main jet then what is it. Can someone please help me out.
THANX

[Bill Fuller]

If it ran fine before you put the vent in the lid than I would guess it's the amount of air you are getting,probably need to adjust the air fuel mixture.Best thing to do is a plug check and adjust from there.