I recently adjusted the valves on my 400ex. I followed the D-I-Y article provided on EXrider.com. I thought that things went well. I also cleaned the air filter. I took the ex out for a short 10 minute ride and everything seemed to be working good. I then took the ex to a dice run in my area a couple days later. Prior to the start of the ride, I started the wheeler and noticed that it would not idle very well. I could start it and it would idle for a few seconds then stall. I did not know what to think. I then went on the ride. I was really hammering on it and every time that I would let off the throttle, it would backfire really bad. The one time I had to look over my shoulder to make sure no one was shooting at me.

I don't know if I screwed something up when I adjusted the valves. Was I supposed to adjust the timing? ????????????? I don't want to ruin my wheeler so any assistance would be greately appreciated. Thanks inadvance.

 

Re-check the valve clearance again. Sounds like maybe you did not get the set nut tight and the valve is not adjusted properly now. If its backfiring it is probably going to be on the intake valve and this is why it will not idle anymore. You should not have to adjust the timing unless you messed with it before. Please post back if this does not correct the problem.
REX

 

Would it be adjusted too tight or too loose? Thanks for the info.

 

Exhaust too tight.