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Old 06-12-2019, 03:32 PM
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Usflag 04 YFZ 450 no compression

Back story:
So I started leaking some oil and decided to get a gasket kit. I have an Athena 98mm stroker kit fyi. Almost everything went back together nicely at first. When I initially started it the compression was too high to turn it over enough to start it. I assumed this was a timing issue not allowing that exhaust cam to auto decompress while starting. Then things went wrong apparently. I did break a bolt on the cam cover that I had to have drilled out (had to remove head) and the decompression plug fell out and got lost but I replaced that. When I started it back up after getting it together there was no compression at all. It just turns over. I flipped the cams just in case they were 180 off but no change. I rotated the crank and the lobes are compressing valves as they should. I know it's timed right per spec (exhaust marks at 9 and 11 and intake 1 and 3). I even took out the spark plug and made sure piston was TDC and lined up with timing mark. Still no compression. When I turn it over and stick my hand over exhaust pipe there is an alternating suck/blow corresponding with a complete cycle. Does this sound like a stuck valve to any of you? Or something else? Before I pull the head back off I thought I'd seek some advise. Thanks in advance.
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Old 06-12-2019, 03:57 PM
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Time to pull the head. You may have a stuck or bent valve.
But first, be 100% sure the cam timing is spot on...Not close, but spot on.
Also be sure the valve clearances are correct.

Paw Paw
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Old 06-13-2019, 09:56 AM
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I hate to say it but... you done messed up A-A-ron.
Something is definitely out of whack, I would suggest going back over the entire process very carefully and thoroughly. Are you using a manual to ensure you are getting the cams timed properly? If memory serves me correctly the YFZ quad uses the same timing as the WR bikes, NOT the YZ's which is one tooth retarded on the exhaust cam I believe.
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Old 06-13-2019, 10:20 AM
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Something is definitely off. I would:
1. Find the pdf of the service manual. The broken bolt tells me you're likely "winging" it and the torque values, particularly of the head, cam caps, etc. are critical.
2. I'd pull the valve cover and check the valve clearances. I had a "rebuilt" '05 recently that the PO had installed two valves in the wrong position. The center intake valve should be the longest. If placed anywhere else, it will be impossible to shim that center valve into spec and it will leak all compression out.
3. I'd check the cam chain tensioner
4. I'd do a leak down test to see where the compression is leaking out through.
5. I'd order AT LEAST a top end gasket kit expecting that it's likely the head (and at that point why not pull the cylinder as well?) will need to come off to have a closer look at what's going on. You'll want to refer back to the manual for proper torque sequence. I believe it's 20something ftlbs on lubed fasteners, back them out, retorque to 14 ftlbs, turn an additional 90 deg, and then a final 90 deg. Don't quote me on it though. FIND THAT MANUAL.
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