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2003 Yamah F90TLRB Sneezing/Backfiring Through 1 Carb

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  • 2003 Yamah F90TLRB Sneezing/Backfiring Through 1 Carb

    Hello! first post here. Got a 2003 Yamaha F90tlrb.
    Idles fine with little issues.
    Compression numbers are good i believe (187-192)
    When giving it throttle, the bottom carb occasionally sneezes fuel/air back into the intake. I think I even saw it ignited once.

    I have changed spark plugs, checked compression and removed the bottom carb, cleaned and reinstalled with no avail. The carb didn't seam dirty at all, no clogs or anything. Just the slight yellow (old gas?) tint to it.
    Gas is new, i am running a small portable tank with new gas and seafoam in it.

    Not sure what to look at next? I have also tried to adjust the air/fuel mixture screw and it seemed to help a tad, but not really.
    Would my timing be off? But if that was the case, wouldn't the top most cylinder be off also? Wouldn't timing also not show good compression?

    Sorry for the inexperience, 25 year old newbie here.

    Thanks in advance!
    Last edited by ruslanyzhuk; 4 weeks ago.

  • #2
    to back fire into the carb on a 4 stroke , something is allowing burning fuel to get thru the intake valve on that cylinder.
    spark happening while intake valve is open, sticking valve, burnt valve.
    if timing belt is off all cylinders would be the same