Question about 2004 Yamaha XT 660 R
The three required ingredients for combustion are: fuel, compression, and a spark (at the right time). So, if it's got fuel and spark, I would check the compression. With the throttle held wide open, you should see at least 140 psi of compression. Anything less than that - and it isn't going anywhere (under its own power).
If the compression is low, check and adjust the valve tappet clearance. Insufficient tappet clearance is the most common cause of low compression in 4-cycle engines.
If the compression is okay, and it still won't run, then check for plugged carburetor jets. Especially if the bike has ever sat for more than a few months. Gasoline, when it evaporates, leaves a gooey mess behind.
Posted on Dec 12, 2009
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Posted on Jan 02, 2017
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