I just "completed" a rebuild on a 400EX, and I can't for the life of me get it started.

It was running before, no issues. A 440 big bore kit, and and aftermarket airbox which enclosed the battery and relay within the airbox was purchased.

The build went ok, didn't run into any real issues. The electrical system was a pain to cram into the aftermarket airbox, but it eventually went together ok.

Now that it is all together, I cannot get it to start. As it turns over, it seems to fire on every other stroke. I hear combustion, but it doesnt seem to be enough to start the engine. It enough combustion to heat the exhaust pipe, so I am not sure why it wouldnt start. I thought maybe the timing was off, but I just finished pulling the rocker assembly off and went through the timing mess again. While it is still a half tooth off, it appears to be close enough from what I have read in other threads.

I haven't touched the carb other than cleaning it out to ensure that wasnt the issue.

Any suggestions? I am wits end with this thing, and I need to get it going asap. This is a build I was doing for a brother, and I have my own toys that need work. Any and all help is greatly appreciated.

The setup is as follows:

440 Big Bore kit, Namura 10:1 piston
FMF exhaust, power bomb, and white bros slip on
Uni filter
160 Main Jet
38 Pilot Jet
3 1/8 turns out

 

Did you do any value work? If so what did you replace or modified?

 

If ran before build carb should be fine..Did you replace spark plug?

 

The only thing replaced was the cylinder, piston, and rings.

I think I may have a timing issue. While I lined up the cam notches with the lobes down, and the T mark on the magneto, I think I did on the wrong stroke, making the timing essentially 360 degrees out.

If I had set the timing (lining up cam notches, T mark) on the exhaust stroke and not the compression stroke, the timing (I think) could be 360 degrees out (360 degrees at the crank, 180 degrees at the cam). This would make sense as to why I am getting combustion to occur, but not enough to start the engine, as the combustion is off time. I think it also explains why the header pipe is able to get hot without even starting. Each time fuel is igniting, its shooting down the exhaust, since that valve is open. Thinking of the 4 stroke cycle, it should be firing on 2 (compression), but I think it may be firing on 4 (exhaust).

Its a thought, anyway. This evening I will line up the T mark with the cam lobes facing up, remove the chain and let the cam spin around to so that the lobes are facing down. This should correct that issue if it is the problem. I will then check valve lash again, followed by a compression test once all back together.

Hopefully this thing fires up!

 

yes you will have to have correct stroke.

 

When you have your cam off and your magneto on the T that's the ready to fire position because when our remove the cam you reset everything.....putting the cam lobes down and sitting on the start of the T should be perfectly in time....be sure to make sure the spring and plunder are still in the head for the compression release or it will burn your fuel with the right side valve open and it won't fire and turn over once you get it running it breaks away and lobes work as usual....I nothing else your looking at plugged pilot jet or maybe mixture issue try "bump starting" if it runs and runs hard and strong it isn't anything in the engine....

 

What ta he11 are you talking about McDude?