Need Help! 400EX

[RiderTRX400ex]

Hey, I am rebuilding my friends 00 400ex for him.. I installed a wiseco 416 piston, put it all back together and ran into problems... the engine turns over, but doesnt sound right.. his engine sounds like it makes compression half as much as any other 400ex.. my EX engine makes 2 compression strokes in the same time his engine makes one. It fires, but will not run. Fires every time.. Plentiful spark, fuel is good... I have triple check timing.. everything.. I thought I had it 180 out but i reset it and nothing. I just tore the head out and check all the valves to make sure they are seating good and they are.. Im stumped.. the engine acts like the timing is off.. it is crazy.

This is the first problem i have ever had and I have built/rebuilt many 400ex Engines.. never have had a problem....


Any input or ideas will be appreciated!!

Thanks guys

[m36indiana]

check decompression lever on cam, could be stuck

[GPracer2500]

Stock cam? If it has an auto decompression device (stock cam does) make sure the little spring and plunger that sits in the small hole in the head is there. If it's missing the decomp won't work right.

[RiderTRX400ex]

i know for a fact the decrompression spring and plunger is NOT in the head.. we are still using the stock cam..

[RiderTRX400ex]

Late night BUMP

[GPracer2500]

Originally posted by RiderTRX400ex
i know for a fact the decrompression spring and plunger is NOT in the head.. we are still using the stock cam..

I'm confused. If you're using the stock cam (and the decomp mechanism wasn't removed from it) and you know the decomp spring and plunger are not in place----->that's the problem.