Ive got a 400ex that wont start. It wants to. Itll run for a couple seconds at a time but thats all. I just rebuilt the top end and changed cam chain and tensioner. Im pretty positive i set my timing right. I just dont wanna take it down again and waste a gasket to re check. But i did clean the carb and same thing. Something i did notice though was that once i refilled my oil i didnt have any oil coming out of the tube on the top of the head. Im guessing the oil pump isn't working. But that wouldnt make it not start would it? Any advice?

Oh and I've read about the decompression mechanism, if anyone thinks that could be the problem?

 

if you didn't put the decompression pin and spring back in thats your problem,it could be a number of things,fouled spark plug,dirty car,dirty petcock,valves set to tight,out of time,weak spark ect

 

^ X2
What did you bore it to?
Make sure you rejet.