rebuilt 400 ex wont start

[jpojoe]

03 400 ex new piston,rings {stock bore ring gap miced out}, wrist pins , new tittlewhite valves. cylinder honed crossed hatched, timing perfect,
wont start, pops back through intake, new cdi mod, actually in shop right now and shop cant make it run i know it sounds like a timing issue but i know its not PLEASE SOMEONE OFFER ME SOME HELP, was running perfect when pulled down little slapping noise from top end due to galled piston

[John451]

my guess would be the cdi or the carb's bein wierd

[jpojoe]

rebuilt carb replaced cdi mod with a known good one, i cant figure it out

[matts27]

Valves clossing? Check to make sure they're fully closed on the compression stroke. Maybe too tight, I'd loosen the rockers up and try it again, or the carb got screwed up somehow. Just guessing gotta see it or hear it really, but it does sound like a timing issue... but you said you checked that. Good luck, let us know what you find

[gojk]

Are you sure you aren't a tooth off on cam installation? That happens more often than you would think.

[speedfreek]

Do a compression check to see if the valves are seating.

4 things needed to make an engine run, air, fuel, compression and spark.

Double check timing? Don't look at the timing marks. Find TDC by sticking a wire into cylinder to see when the piston is at the top then look at the flywheel mark to see if this is correct. Then look at the cam placement. The valves should be closed. Then check the marks on the gear just to see if they are correct.

What I am trying to say is don't rely on the "markings". If you are familiar with building engines you don't need any marks to get one right.

Hope this helps.

[GPracer2500]

Stock cam? If so it sounds like the decomp spring and plunger are missing.

[400ex quad race]

^^^what he said. if its lost i think you can just take the decompression assembly off you cam. dont really need it anyway.