Question about Yamaha XS 650 F Motorcycles

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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 except for a flat spot when I first crack the throttle. Idle is perfect, and once you push it past the flat spot it again runs perfectly. The flat spot is bad enough that you can't take off. All symptoms seem to point to a lean condition on the fuel mixture controlled by the pilot jets. I've already tried adjusting the needles, and there are no air or vacuum leaks. I've already tried going one step up on the pilot jets, but that didn't help much... I could always go bigger, but I wouldn't mind a second opinion. By the way, I'm ASE certified, i just don't work on bikes that much, and i'm also young enough that 95%+ of my experience is with fuel injection, so you need not worry about confusing me, but I could definitely use some help. Thanks a million Dustin

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Since you did 99% of the troubleshooting already so I can only put in few items that someone may overlook on a motorcycle.

Check the CDI module contacts and cable connections.

Grounding between CDI box to frame.

While you at it,why not continue ground this wire to the stator and frame.

Have you check the condition of stator laminated magnetic wire?
Check for hot spot(s) near stator and CDI box.

Do a tap/wiggle test. Use a small screw drive and tap/wiggle all wire harness on the bike.

Take the air filter out for a short test ride.....this will rule out the air restriction.
I hope you are not using a stock air cleaner (not K&N oil type filter)

Got fresh gasoline?

If the engine is modified for higher compression then I will try fuel with one higher RON octane rating.

You did not post the year. If it's a 80's + model. You may have few electronic solenoids to look into.

The last thing.

Open it up and RIDE!

Posted on Jul 28, 2009

  • Vincent_G
    Vincent_G Jul 28, 2009

    Depending on the year of your bike,it may be benficial to use the stock plug.

    If all else fail then use copper plugs.

    Have you check the coil resistance on the primary and secondary?

    Check the ohm readings again the factory specification.

    If it's just a little off then try to gap just a hair closer to get a stronger spark.


    When was the last time you ran this bike? If the bike was in a long term storage or transport for a long's possible you need a float adjustment.


    Please reply via the comment window/page.

    I will follow up on it later.

  • Vincent_G
    Vincent_G Jul 30, 2009

    Is this happen any temperature?

    Are you taking this bike for a high elevation road trip?

    Is the tank (FULL) when you have this off the line acceleration problem?

    Can you blow out he fuel line?

    Make sure there is not air pocket on the line.
    No vacuum/suction problem in the tank.

    Is the tank clean and free from algae?

    Please post your founds on the comment page / windpw.

    I will follow up on it.



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Makle sure fuel is getting to carbs, by loosesing the carb drain screw - fuel should flow out.
if your getting fuel, tighten the drain screw back and turn the motor over.

make sure key switch is on and on/off switch is on.

if motor doesn't start, spray a very little of carb cleaner spray into air filter box.

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