Helping a friend, he bored his 400ex to 440, new valves, no other mods, got it back together, set timing per spec by service manual. but can seem to get it to start, any ideas?

 

I would start with the obvious. Fuel, spark, and air. If he has all those then I would do a compression test to make sure it has adequate compression.

 

we have fuel, we have spark, have not put the breather box on yet so carb is sucking air freely. when it turns over it will kinda pop like it wants to start but doenst

 

Since you have the air box off,try blocking the rear of the carb with your palm to see if it can get a good charge of fuel into the combustion chamber. May help get the engine kicked off. Or simply put the air box and filter back on. Last resort you can give a small shot of ether to get it going on it's on if timing and everything else is ok. Good luck.

 

did the hand thing, it seem to make it pop a lil more but still a no go. have had a few people tell me to up the jet in the carb from stock to about 3 sizes over, any thoughts on that?