New member and need help

[yeller04]

Howdy folks, im new to this forum but also love workin on my own chit so always come to this site for answers to questions but never joined. So i joined to get better advice and help others in anyway i can. I just picked uo a yellow 400 and it seams to bog at times after sitting and some while riding. It has stock jets with a drilled airbox and fmf slipon, so my question is what jets i need to help this problem

[Steve86trx250R]

I'd try duct tape over the airbox holes and see what happens if it doesn't seem right do a plug check run it wide open after warmed up and shut it off while riding and pull plug and check it that's best way to figure jetting I'd say two sizes up from stock on main jet would be a good start