Question about 2004 Yamaha YZ 450 F

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Yz450f that quit running

I got a yz450f that quit running about half way up an FMX ramp, Crash was a little painful. Anyway, My bike is getting fire and fuel and attempts to start but will not. I noticed that if I kick it slow it seems to have compression, but if i kick it normal to hard it seems to have no compression. Is there an automatic compression release and could this be the problem?

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Yes, it does have an automatic decompression mechanism. It is a centrifugally operated mechanism and it is EXTREMELY unlikely that is causing your problems.
The two highest probabilities are:
1. Fouled spark plug. Forget what it looks like, put a BRAND NEW plug in. It is the cheapest form of diagnosis you can do. These bikes are terrible about fouling spark plugs and a fouled plug produces the exact symptoms you described.
2. Inspect your valve clearance. Tight valves will also make the bike hard/impossible to start with the kick starter. In some cases, it can be push started, but still won't kick. With tight valves, the bike, although hard starting, will run pretty good, for awhile. Eventually it will burn the valves and quit running though.
If your bike isn't kick starting fairly easily, something is wrong and needs to be addressed. The only other issue is making sure the idle speed is set per the service manual. These bikes have a fairly high idle, some as high as 1700 rpms. If it is set too low, it makes them hard to start too.

Posted on Oct 05, 2009

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Check the clutch free play at the lever end--should be about 3/4 inch. A compression release would be effective no matter how hard you kicked it. Why the engine quit--maybe the ignition timing shifted because of loose mounting screws. If you have access to a timing light, point it at the flywheel mark while you kick it through.

Posted on Sep 11, 2009


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Hi, Ssfichter1 and the usual suspects are:
1. Severely discharged or a damaged battery, should have 12.5 volts or better and be able to pass a load test if necessary.
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16. Vacuum line from intake manifold to petcock broken, cracked, or not attached, carburetor vent line plugged.
17. Needle and seat stuck closed in the float bowl.
For more information about your issue and valuable "FREE" downloads that you need please click on the links below. Good luck and have a nice day.
2006 YZ450f backfires then dies YZ 400 426 450
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