Question about 1978 Yamaha XS 650 SE

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My Yamaha is in excellent condition with new plugs, rebuilt carbs, new gaskets, filters, etc. Compression is above spec, valves are adjusted, timing is dead on at idle and advance. The only problem is that it has a flat spot on the low speed mixture when I just crack the throttle. it runs perfect at idle and at high rpm. All the symptoms seem to point to a lean condition that i should be able to remedy by fattening up the primary jets- I've already tried adjusting the needle. I've gone one size up, but that didn't really help. I could always go bigger... any suggestions would be awesome. By the way, i'm ASE certified, I just don't work on bikes that much and I'm young enough that 95% of my experience is with fuel injection so don't worry about confusing me. Thanks a million Dustin

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  • ladysingsthe Jul 28, 2009

    The exhaust appears to be original, unless its a replica. The thought has crossed my mind... that it be more free flowing due to this possibility. I have never repacked a muffler though...



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Does you bike have Headers? if so yes keep playing with the jets. If it is org. exhaust system then you may have to repack or restuff the muffler. If you have headers I would start with a more open intake filter like K and N's . have you gone to >< or >< or >< There is lots of info out there on these great bikes. 

Posted on Jul 28, 2009

  • John Fourteen Six Jul 28, 2009

    If you think you need to repack, it is an easy thing to do. You can get stuffing from most motorcycle parts supply here on the net. Cheapest is the best. all it is is fiberglass in small squares. find your rivet on your muffler close to the end cut/drill that rivet(s) and **** out the baffle with a pair of pliers. rewrap the baffle (instructions come with the new stuffing). But how does it do without the air filter? do you get the same flat spot. I drove mine for years with that same thing, just got used to it and rode it to death. If I had that bike back I would go for different carbs. I found a one carb manifold on the net. I would try to find me one and get a Mikuni at about 32 mm. But thats me. Have you tryed to see if the carbs are flowing equal. They need to be SET together, both floats need to be close as they can to the same. and the metering jets moveing at the same time. You can use an old flow meter on this bike. clear plastic with a tube and a bb to ck air flow and adjust till both carbs are the same.



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I have a 92 yamaha 350 warrior rebuilt the carb new stater and pick up coil I have it in time and plenty of spark and compression but still wont start.....

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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 your carbs have an accelerator pump or enrichment mechinism.

Jul 28, 2009 | Yamaha XS 650 F Motorcycles

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So let's start from the beginning I bought the

Your information is far too vague to even start to diagnose why it runs poorly. But if its been rebuilt check obvious things like timing, spark plugs, wires, distributor, basically all ignition parts. If you don't find anything there you've got fuel delivery to look at, maybe check the compression, do the valves need adjusting, vacuum leaks, cracked intake manifold, and the list goes on. It would really help to know what the vehicle is, what engine, year, etc so we know if it's injected, carbed, hydraulic or mechanical valves, what kind of ignition, etc. Good Luck!

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Fuel starvation - 'lean fuel' condition
Basic things to check:
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Nov 09, 2009 | Echo Garden

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Hi,i`ve got a yamaha fz750 1988..and it won`t seem to run on all four cylinders..i took the air box off and while it was running put my hand over each carb one at once and found that on carb 4 it stopped...

Hi JIm,
If it stopped @ 4 tht means thts the only cylinder firing .
Remove the spark plugs,earth @ on the cylinder head, crank start and check for sparks...if its sparking tht means the coils etc. are ok.
As the spark plugs are out exam them to their conditions visit this site for more details :-

From here you would know if its carburator or probably it may need a complete tune up....carburator cleaning, float level..adjustment, valve clearance.. new plugs (gap to specs.), check air filter, replace or fit 'inline' fuel filter thn balancing of the carbs.
Hope this helps!

Oct 29, 2009 | 1987 Yamaha FZ 750 MG (Europe-US)

1 Answer

Flat spot on 79 yamaha

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.

Aug 01, 2009 | 1979 Yamaha XS 650 F

1 Answer

How do i begin adjusting the carbs?

1st-clean air filter and oil,fresh plugs,good fuel,valves adjusted correctly,check for tears or cracks in intake boots,fire in all cylininders;minimum compression & w/in10% of each other in all cylinders, and no exhaust gasket leaks. once these components are w/in specs. and operable the carburators can be adjusted, as long as they are internally clean and operating as designed.1st- set high idle, then set idle rpm, now synchronize vacuum on carburator set,now readjust idle rpm.

Feb 19, 2009 | 1980 Yamaha XS 400

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