is it the main jet? and if so what size for airbox mods like whole lid off I have k&n with outerware and it rains allot around here latley. no pipe if any in the future would just be slip on or the internals out of a 94 or older xr400 they flow better I have heard. please assist thanks.

 

The main Jet circuit is the place to start with rejetting and then test the other circuits. Right at the start, the key words here are:TEST & READ.Agood rule of thumb in re-jetting the main is to go one size fatter for each air flow alteration, in your case at present, this would be two. Then test & read your plug for color. You didn't say what your elevation is & that will have a bearing on this. To test, ride the bike at 3/4 to full throttle for about a mile in whatever gear is practical. STOP WHERE YOUR ARE! and pull the plug. The insulator & electrode color will indicate whether the engine is rich, lean, or correct. Correct color is a tan or light brown
dark or black is rich
gray or white is lean (bad news)
Depending on elev & other factors two sizes up on the main could be slightly rich, & you could try the next size down & re-test. It's always best to err on the side of rich rather than lean & go down if necessary.
It's important in testing to make sure you do it at 3/4 to full throttle as this is the working area of the main jet.
Finally, regardless of what any one tells you, including me, the only one who really knows what YOUR engine wants is YOUR engine itself!
I have a file photo of a plug from a correctly jetted engine which I'll email to you if your email address is correct here.

 

stock main is a 150, u can go to 155 with lid and 160 without lid and with K&N, put on a pipe and 165, i run 170 but mine is a 453 with cam and port and polishing, ride red!!!!!!