2012 yz450f won't start after rebuild

Hi guys, just did top end on my yz, new genuine Pistons rings & gaskets and new stainless valves with Springs to match. My problem is the thing won't start.

It's getting fuel (indicated by wet plug)

It's getting spark (tried 2 different new plugs both have great spark when on frame)

The timing is spot on, have checked twice now and the new valves are set perfectly.

I have gone through the manual and tried the proper starting procedure, I have tried fuel straight in the spark plug hole (should mention that I also drained the take and filled with fresh fuel)

I've tried bump starting but don't have a lot of room to really get a good run up.

This thing pre rebuild would start after 2 or 3 kicks with shot valves and low compression, I really don't know what it's problem is now, please help :(

Have you done a compression test?

No, one piece of gear I don't have handy. I know it's hardly an accurate measure, but it feels like it had decent compression when turning it over. Certainly has lots more when kicking than it did before the rebuild.

No, one piece of gear I don't have handy. I know it's hardly an accurate measure, but it feels like it had decent compression when turning it over. Certainly has lots more when kicking than it did before the rebuild.

Well unfortunately that is the only thing it seems you haven't checked and I suspect is the culprit

Something to do with the decompressor.

Are any of your valves adjusted on the tight side of tolerance?

Edited by Bass Mechanic

The valves are supposed to be on the tight side of spec'd clearance, ideally, and the only thing the decompression system can do wrong is not decompress.  If that was happening, you'd know.

 

Conventional compression tests on big single cylinder engines are almost impossible since you can't really turn them over without decompression, and compression testing on engines with decompression systems is about as useless as a thing gets.  Leak down testing is the only accurate way of going about it.

 

I'd suggest connecting a tuner or a tech scan tool and check the system for any fault codes to begin, then check for fuel pressure, nozzle discharge, etc.  There have been several people have trouble getting the flywheels back on these things for some reason; you may want to double-check verify that the flywheel is properly keyed and seated on the taper. 

Having the same  exact  issues with my 2010 did you ever find out what the issue was?????

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