Ok, the problem I am having is crank, spark, compression, no start. The quad is a 98 Yamaha Warrior. I am just finishing up a top end rebuild and am really having a difficult time getting it to run. The reason it was rebuilt was a valve guide in the head broke and caused a smoking condition. I bought a used head and checked it out throughly. The valve seals were replaced as well as a piston kit and gaskets. The motor is all together but it seems the timing is way off. The first time I tried to start it it backfired a few times loudly. I removed the cam cover and plug to confirm the timing, which was off. Reset the timing and reassembled. Second try motor fired and ran well without smoke. While at idle I adjusted the air screw slightly and the quad ran for about ten minutes at idle. After that I gave it a touch of throttle and it went blah and quit. Thinking the plug was loaded up I replaced it and tried to start the quad to no avail. It had the same symptoms as the first time, horrible backfires with no start. I took it apart again and now with everything lined up ( the T on the flywheel and the mark on the head and cam sprocket) the piston is actually at the bottom of the stroke. The TDC mark no longer matches actual TDC. Next move, I make a mark of my own on the flywheel by using the screw driver feel method and time the marks on the head and cam sprocket to that. Still to no avail, backfires are less often but occur, still no start. Anyone have any suggestions on where to go with this next?

 

It sounds like the flywheel key broke. Pull the flywheen off and check the key and the alinement of the flywheel to the crank. I had the key break on my warrior once.

 

Thank you for the info. I've been trying to figure this out for two days. This is my first four stroke rebuild. Maybe a dumb question, but the flywheel key is some sort of shear pin behind the flywheel? Theres no dumb questions only dumb people....

 

Also check your compression again to make sure you didn't bend a valve.

 

The flywheel key goes on the crank shaft and the flywheel goes over it. it alines the crank and the flywheel.