Question about 1979 Yamaha XS 650 F
My 79 yamaha is in excellent condition with new plugs, gaskets, filters, rebuilt carbs, petcocks etc. the only issue is that it has a flat spot when I crack the throttle just above idle. Its bad enough that you cant take off. the bike idles great, and runs great at WOT... except for the stumble in the middle. All the symptoms seem to suggest a lean mixture which would normally be corrected by richening the pilot jets, but I've played with the jets a bit to no avail, so I was thinking that maybe the mufflers need to be repacked, but they are welded together... so I'm not sure if its worth cutting them apart. If anyone has dealt with this before, please help!! thanks a million Dustin
Hi and welcome to FixYa,
Your posted symptoms is as you described, running lean which can be verified by the colors of the plugs, brown, light brown to almost gray / white. While the jets and the exhaust could be worked on, the simplest try of a fix is to restrict the air intake; the lower the RPM of the flat spot, the more air restrictions should be added. Experiment by temporarily placing upholstery foam on the outside portion of the air filter. You may have to try different thickness / porosity.
Good luck and thank you for asking FixYa.
Posted on Aug 01, 2009
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Posted on Jan 02, 2017
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