I have an 02 400ex. I was having the problem before shortly after it was bored out. It shut off on my one day and since then hasen't started. It was blowing back through the carb so we checked the valves, both intake vavles were bent so we replaced them, reset the timing and adjusted the vavles. At first it seemed like it wanted to start and the carb was only sucking. Now it started blowing back again and the valves arent bent this time. What else could cause this??  

when you bored it it out did you do the rebuild yourself?

if so are you sure that you lined the ring ends up with the pins?

if not the rings will break and you'll lose compression.

another possible problem, your timing could be off a little, causing the valves to open and close a little off, this could most definately cause it to spit back through the carburetor.
and could be the reason it has re-occurred.

did you determine the reason the valves bent the first time?  

Well since you got to take it apart again you need to check the piston to valve clearence.  take a piece of silly putty and put it on top of the piston, bolt the head on with the cam chain attached, roll the motor over.  take the head back off cut the putty in half and measure with a set of calipers.  if you have a aftermarket piston the dome could be wrong.  find out from the piston manufacturer and cam manufacturer what the piston to valve clearence is and voila.  

I did rebuild it my self and the rings are lined up and it has good compression. The timing has been beeing relly fussy like we'll set it perfect try and start it and then when we take it apart itll be off a little bit, could this be caused by slack in the timing chain? And I believe the valves bent the first time due the the timing being off. It was a notch advanced I believe.  

Also another possibility for the bent valves is the valves adjustment was off by a substantial amount when it happened  

have you checked the timing chain tensioner?
if it's loose then it's just going to jump time whenever you try to start it.  

Yea, its sticking a little bit, I thought that might have been the problem too but I don't believe its jumping time because after trying to start it we ripped it back apart and the time was still set. I think I might be something in the chain possibly being stretched? And someone else told me you have to seat the valves, how exactly would I go about doing so?  

the valve seat is the chamfer in the head that the valve touches when it closes.

Some valve seats are ground to angle of 44 degree's and the valve face (the angled area on the valve that touches the head.) is ground to 45 degree's.
The 1 degree variation produces a hairline contact that gives fast initial seating.


but you'll definately need to check with your dealer to see what angles you need for your valves.

you can buy a valve seat cutter to do this, but honestly it would be alot better if you paid a pro to cut your seats for you.  that way you'll know the valve stems are ground properly.

hope this helps.  

I know this may sound really corny, but I had a similar problem and my fuel petcock was plugged. There wasn't enough fuel to fire normally but would backfire thru the carb. Try using reserve and see if it starts.  

not corny...

sometimes it is the simplest things that cause the problem.

an exmaple would be a yamaha blaster i rebuilt for a customer.

I got it all back together test rode it for 3 minutes and it died.
long story short i checked everything even dissassembled the engine and put it back together again.

i finally changed out the cheap spark plug the customer insisted on using and put in an NGK and it started right up.

sometimes it's the simplest thing, even though all of the symptoms point elsewhere.