Question about 1979 Yamaha XS 650 F

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Flat spot on 79 yamaha

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

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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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2 Answers

My bike is in excellent shape with new plugs, filters, gaskets, etc. Carbs have been rebuilt, compression is above specs, valves, are adjusted, timing is dead on at idle and advance, everything is great...


the color of your plugs will give u an indication of the rich/lean ratio. try disabling the advance, if possible, it might be too quick or too far.do your carbs have an accelerator pump or enrichment mechinism.

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1 Answer

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4. Fuel supply valve/petcock turned off.
5. Fouled spark plugs.
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7. Vacuum hose to the fuel supply valve/petcock disconnected, broken, cracked, or pinched.
8. Fuel valve/petcock or filter clogged.
9. Fuel line to carburetor or throttle body pinched, kinked or blocked.
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11. Fuel injectors clogged.
12. Fuel injectors stuck open.
13. A severely discharged or damaged battery, you need 12.5 volts or better and be able to pass a load test if necessary.
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15. Loose or corroded wire connection at the coil or plug between ignition sensor and ECM module.
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18. Faulty neutral, clutch lever, or kickstand safety switch.
19. Faulty fuel pump or fuse or relay.
20. A stuck bent or burnt valve.
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My issue with my bike is constant fuel overflow, even after the bike is shut off fuel keeps running out. A little back history. The bike has sat for 26 years in a shed. I took the carbs apart and cleaned...


Your petcock needs to be rebuilt (if parts available), or replaced, if it does not shut off the fuel. It also may not be saving reserve fuel , if gas is bypassing the ON position. You may also need a new fuel filter .Maybe add an additional in line filter. If you have replaced the float needles and the carbs are still overflowing you probably have a problem with rust in the tank. Have the inside of the tank cleaned or use a rust removal kit on the tank and see if this does not solve the problem. Try to avoid fuel tank liners if possible , they can cause more problems than they solve.

Jul 02, 2010 | 1980 Suzuki Gs 550 L

1 Answer

Yamaha xs 650 I am having problems with flat spots during acceleration. I tried new spark plugs, but have not gotten any results. I believe it is an improper jet setting. I live in kansas city and need...


Hello unless some one has changed the jets the original jets are standard the thing to do i suggest first check for intake leaks at the carb joints between the head and carbs run the bike and use WD40 spray arounf the joints if the rpm's change you have a intake leak.
Also a carb cleaning and carefully inspect the carb slide diaphragms for cracks and pin holes.
replace as needed carb joints is hardened or cracked gaskets Etc.

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