On the 400ex if you turn the motor over backwards when you go to set the valve adjustments by turning the crank it will engage the compression reliese I had this happen a few years ago on a buddies and it was a nightmare to figure out it would not start at all just spit and backfire once I figured it out it started rite up and ran fine. I just don't recall what I had to do to the comp reliese to reset it but I do remember I had to take the cam back out to do it.

 

This one is not backfireing at all. I know that if the timing was off just a little that it would still backfire on me some. This one doesn't spit or anything. It just spins over and over and over with no signs of life.

 

<div class="FTQUOTE"><begin quote>The piston is not hitting the valves. I've checked that already. </end quote></div>
How did you do this?
Where is the compression going then, unless the decompressor is taking the compression down to 60psi?

 

I tore the engine back down and checked the valves and pistons. I even put a different head on it. I'm not sure where the compression is going but it's pulling a hard vacume on the exhaust and blowing out thru the carb.

 

Unless the thing is cranking backwards, the only thing I could imagine that could cause strong vacuum at the exhaust and blowing out the intake would be the cam being nearly 90 degrees out (or nearly 180 degrees out on the crank)...

 

I think so as well. I'll check the timing probably Monday and get back with you guys. I know the decompression valve can cause the low compression problem but I don't think it would cause the suction on the exhaust?