400EX wont start after engine re-build

[free_rider20]

Alright, so I havent ridden in over 3 months because ive been rebuiding. I had it bored out to a 426 with stock compression. We have it put back together and it wont start. The plug is firing. It isnt backfiring or anything. I am almost 100% sure that it is timed correctly. What could my problem be? Is it typical to have this problem after re-building? Could it be because of the larger pison? Any help would be MUCH appreciated. Thanks

[mojoe2878]

try spraying a little wd-40 , or some gas in the plug hole put the plug back in and try it. dont use starting fluid for this, it has a tendancy to burn pistons and valves. if it cranks and runs for a second or 2 then its in the carb. check to make sure its gettin fuel thru the gas line and that the float isnt stuck, if doesnt do anything then you need to recheck the timing.

[nakomis0]

Did you put the decompression plunger back in the cam correctly?

Fresh gas? New plug? Gas is turned on? Spark plug wires on?

Are you getting spark?

How about gas, is the plug wet after trying to start it?

[free_rider20]

It is getting spark and it is getting fuel. We have trie starting fluid in it and it still wont fire. It seems so much like it is in the timing but we have taken it apart and checked it three times and it should be right. Would it not start if the decompressor assembly wasnt installed correctly?

[R3Concepts]

Are we talkin stock cam here? If not then the decompressor does not go back on the cam. Sounds like the cam is upside down to me. Lobes down, Allen head and the top, cam marks on sprocket should line up on both sides, on TDC.

[mojoe2878]

yeah check the timing, if its stock cam make sure the decompressor isnt stuck. when you bring it up on TDC make sure its on compression stroke and the timing mark is on the T then make sure the cam lobes are facing down.. the two lines on the cam should be lined up with the head and the center mark by the cam sprocket bolt should be facin up. if ya got fuel and compression then you just arent gettin the spark at the right time, sounds like its 180 degrees out of time.

[free_rider20]

this doesnt make any sense to me. It is on the t mark. The two lines line up with the cylinder head and a line straight up. The lobes are facing down. It is on the compression stroke.

[xX-400racer-Xx]

did you rejetted it after the rebuild ?

[mojoe2878]

check the valves and see if you have any clearance. if there is no clearance then it wont start because it wont build compression.

[free_rider20]

It definately has compression and the plug is wet after trying to fire. When it blew up, it backfired fairly loud. Could the flywheel have sheared the key? This would cause it to be out of time even though it seems to be on time by the markings, correct?

[mojoe2878]

yes if the key broke it would make it either not fire or fire out of time.

[jerry-89-250R]

are you using a aftermarket cdi, i re-built one and it wouldn't crank after trying everything listed above i put the stock cdi on and it started right up.

[free_rider20]

Yeah, I have the stock CDI and the stock cam as well.

[44oEX]

the flywheel pin is probably not your problem it won't just brake like that for no reason.

I did have this problem before.it's just for the first time the engien does not fire. as soon as you get it started it won't do it again.

All I did was hook it up to a car batery cause it just didn't turn fast enough for the fist time. I know it sound kind of dumd but I would give it a try you have nothing to loose.

[JDiablo]

I had the same problem on my 300 after a month and a half for my rebuild,I just needed my uncle come and help me push start it,it worked after that,so idk give that a try,only thing i could think of

[Chanman420q]

i had the same problem, except i bent a valve

[free_rider20]

Well, thanks for all the help. I tried jumping it off my truck and the started ran faster but it still wouldnt start. We put it completely back together and pulled it behind the truck and after about 20 yards of being in gear it fired up. It was revving much fater than normal but nothing adjusting the throttle cable couldnt fix. It runs great. I just wanna open it up but i gotta break it in right so i dont have to do this again for a while. I do believe that if all you had to do was swap out the piston, and you had all the parts 2 people could do it in one nights work. Again, thanks for all the help.

[44oEX]

I think I speek for every body when I say glade we could help. and good luck with the new motor.