Fuel Starvation?

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    You seem to have covered most possibilities, the petcock, and float bowls.
    It’s possible you have a sticking needle valve or a bit of dirt in the valve which slows down the filling of the carb and causes fuel starvation.
    Also check the float levels of both carbs, as if they are too low, you may be running out of fuel in the bowls with the sustained throttle opening.

    One other thing is the fuel pipes themselves, I have known a fuel line which had a flap of rubber fall away inside slowing the flow of fuel.


    Hi all, Steve from Australia.

    I finally had the chance to do some longer K’s on my bike today, but whenever I rode at 100 km/hr for any longer than about 5-7 minutes, the power dropped right off and engine kinda spluttered (felt like I had choke all the way on). After stopping (or backing off to 80km/hr) it was OK again, but then had the same problem a short time later.

    I was fortunate to be meeting a group of old bike nuts, so we had a look at the petcock filter (some crap, but not much) and drained ther float bowls (same story).

    The plugs were blackish around the outer rim and the electrode was clean or if anything a bit whitish.

    The body of the motor felt much hotter than usual (subjective, but there you go…)

    The concensus was the bike was running lean after riding at high speeds (not noticed before as I live nowhere near any roads I can do any sustained speed on)

    Here’s the question, what could be causing the starvation?

    I will have a fiddle with all suggestions, but will need to ride for 1/2 hour at least to hit a high speed road for testing…

    All suggestions, comments greatly appreciated.



    1977 KZ750 Twin



    White? I’d be worried about white plug electrodes. That’s too lean.

    If you’re certain that there’s not clogging at the petcock, then check before and after it. The worm-hole air-hole in the gas cap (all mine were blocked with corrosion or swollen fiber gaskets when I got them), and float level, although I think float level could cause running lean, I don’t think it would cause the decrease in performace oat higher speed…

    If you can, try riding with a lower amount of fuel in the tank, with the cap un-latched, sitting gently closed. That will let more air in, and tell you if its the worm-hole at fault.


    that’s a real good looking bike steve !
    my only suggestion would be the check the position of the air screws ,
    about a turn and a half open for sea level i seem to recall …
    mine had a tendancy to vibrate open until i changed the springs !


    The air screws usually only make a difference up to around 1/4 throttle, I may be wrong on these carbs as I haven’t looked into it, but that’s a general rule of thumb.
    I still think you’ll be looking at float levels or fuel lines for the symptoms described. Lack of fuel will also cause the whitening of the plugs, although it’s more difficult to be sure with todays low octane fuels as that also causes a weaker mixture and leaves a white residue.


    If you run the bike with the cap open and it still dies I’d take a look at the fuel tap itself, if the diaphragm gets crap in it or sticks it can restrict the fuel flow too.


    I cleaned out the gauze filter in the bottom of the petcock, and it was quite blocked, but with a fairly clear gum, which I did not see when it was in place.

    A few hours in kerosine and it was like new.


    Took the bike for a spin, and problem seems to have gone away. There was a strange knock/clicking noise from the rear end though.

    I cut the ride short and came straight home to find one of the bearings has shagged itself.

    Might as well do all three if I’m doing one…

    Back to Kawasaki again tomorrow then!



    Thanks for letting us know what you found Steve, bad luck about the bearings though. At least it’s easily fixed this time.

    Steve_Chambers wrote:
    …A few hours in kerosine and it was like new….

    …one of the bearings has shagged itself. …
    Might as well do all three if I’m doing one…

    Great news!
    About the bearings, I love the kits from allballsracing.com. Absolutely do all three, you won’t regret it. I’ve done all the wheel bearings on my bikes….

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