Hi,I have a 2000 400EX.It has the following mods,K&N filter/Outerwear(with lid),FMF MegaMax2 silencer.Now my question is that I'm getting my head ported,10.5:1 piston(stock bore),and an all around cam.I would just like a "guesstimate"of what size main jet I should go with,so I don't buy too many more than I need.Thanks.

 

Just a guesstimate, I would say not far from what you got.

 

The reason for bigger jet is more intake air. If you run the stock lid then you can't get more air. You have to drill holes or remove it completely. I think stock jet is 148 If you drill holes run a 155.