I completed a rebuild of our 99 400EX because it smoked like a 2 stroke. The rebuild consisted of a new wiesco piston and rings (std bore), redoing the valves with new seals, and the having the cylinder honed.

I put the engine back together and at 1st it would not run. The valves were way out of adjustment. I adjusted the valves and she started right up.

Now for the problem... after its running usually while idling, , it will backfire through the carb. It did not do this prior to the rebuild. It backfires causing the engine to die. Hit the started and she fires right up. My guess is the valves (exhaust valves) are still not adjusted correctly, but I'm just guessing

Any suggestions?

 

Mabey camshaft timming, but check the exhaust valves first and let us know.

 

Could be a few things.

#1. Double check the valves and adjust them properly.
#2. Check for a vacuum leak. You might be lean for some reason. (Carb boot crack, not on correctly ect)

 

I plan to check the timing just to make sure its spot on, and I will check for a vacum leak. I do have 2 valve cover bolts that I need to address, the are stripped and will not tighten.

I'm leaning towards exhaust valves that need adjusted. I hope to get a chance to get to it tomorrow.

 

yeah tight valves will cause exactly what your descrive

 

Valve timing all the way!

 

Backfire thru the carb is a lean condition

Backfire thru the muffler is a rich condition

as the other guys said....probably some type of timing issue.

 

I had a little time today. My son and I pulled the valve cover off, and rechecked the timing, it was dead on... We then checked the valves and the exhaust valves were a little too tight. I adjusted them a little. The bike will now idle for several minutes, and then a slight backfire through the carb. Sometimes it’s enough to kill the engine, other times its not. If it does kill the engine, just hit the button and it fires right up.

As my son put it, we are so close. So I think I still need to adjust the valves some more but boy we are so close.... We just need some more time to dial it in.

 

1/4 to a 1/2 a turn out on the idle fuel jet. IOW give it just a tad more fuel at idle.

 

BTW, good job at doing your own work!!!! AND showing your son how to do it!!!!
Most kids don't even know what end of the plug to put in the hole.

Thanks again for teaching him.