Carby Fuel Overflow

Discussion in '2-Stroke Engines' started by Big Red, Nov 9, 2011.

  1. Big Red

    Big Red Active Member

    I hope someone has seen this before and can help. I already know the basics of an overflow problem and have tried all of them I know of. Float level, (As low as I dare go.) Needle valve, ( Let it set for a while, no leak problem there) Smaller jet, Just to see? ( Went all the way to a #64, Still gas SPEWING out of the overflow hole when giving it throttle only.)
    It starts and idles fine, but when I give it throttle fuel sperts out the overflow so badly it gets sucked back into the intake and, of course, floods the engine. I've also tried another brand new carb and get the same problem.
    Could this be a bad rings, engine blowby thing. What would or could cause fuel to kind of blow out of the overflow hole. And oh yeah, It's a stock HT all the way. It's a customer engine and I'm lookin kinda stupid here ,so PLEASE, Anyone got anything?
    Big Red.
    Last edited: Nov 9, 2011

  2. cloud_2901

    cloud_2901 Member

    Float has a hole in it and fills up with fuel? carb can never empty, just a thought.
  3. bideronit13

    bideronit13 Member

    try smaller diameter of fuel lines or get a valve were you can restrict the flow
    Last edited: Nov 9, 2011
  4. a/c man

    a/c man Member

    Stock exhaust clogged with oil ,restricting flow?
  5. motorpsycho

    motorpsycho Active Member

    well, lets see...
    he already said that he tried a new carb, which woudl eliminate a hole in the float.
    it does this with the original crb and a brand new carb, but that's not saying that the new float in the new carb doesn't have a hole in it.
    Try this (i know you may have already done this).
    I have noticed that on occasion the float can get cocked sideways in the float bowl.
    this will never let the float close the needle & seat, which will cause fuel to comeout of the overflow.
    Check to make sure that the float is in the bowl straight (or flat) and not cocked up on one side.
    also, if this bike has recently been turned upside down (to replace a tire or something) this can cause the float to get cocked sideways.
    It could be bad rings (blowby) because that would put a negative pressure into the intake, which would blow back into the carb through the main jet causing too much pressure in the float bowl, which COULD actually pressurize the float bowl and cause fuel to be blown out.
    But if you think about it, it would take quite a bit of pressure for that to happen, and more than likely IF this is what's going on, that extra pressure would probably just be blown out the rear of the carb.
    I've personally never seen this happen before, but it is a possiblilty i guess.
  6. Big Red

    Big Red Active Member


    Cloud, Didn't think to check the float on a new carb, I will.
    bideronit13, A smaller fuel line? Even if the line was a foot in diameter the float valve would shut off the flow.
    A/C man, Popped out the baffle, same thing.
    And before someone suggest's it, I also lowered the carb slide needle to the longest notch.
    We have now covered everything I've already checked or done.
    So Moto, I've had the carb's apart and back together a number of times, So I dont think the float is stuck. And I think yer right about the blowby, I think I would have noticed it blowing out the back of the carb. SO WTF MAN, I said I've never seen this before and it looks like no one else has either. Thanks everyone, and please keep the suggestions coming, I'll figure it out sooner or later.
    Big Red.
  7. blue 48

    blue 48 Member

    primer button touching float? let us know what it was wont you
  8. motorpsycho

    motorpsycho Active Member

    doubtful because 2 different carbs (an old one and a brand new one) give the exact same results.

    try running the engine and putting your hand over the throat of the carb and see what happens.
    it should kill the engine, but by blocking the intake air may give you a sign.
    I'm thinking that if you block the intake air, IF it is blowby, it will create more pressure when you block the carb throat.
    block the carb throat and see if more fuel comes out of the overflow.
    this would give more backpressure (if this is what's going on) and i'm thinking that it will pressurize the float bowl even more.
    I don't know, i'm just guessing, tryinig to make sense of what's going on here.
  9. Big Red

    Big Red Active Member


    Yeah blue, I tried lowering the float level as far as I dare so the primer rod was barely able to even touch the float when pushed all the way in.
    Thanks Moto, I'll try that tomorrow when he brings the bike back. It runs and he needs it for transportation, so I'm working on it on the fly.
    Big Red.
  10. HeadSmess

    HeadSmess Well-Known Member

    ok, its a stock nt carb right. nothing fancy. both of em?

    the overflow is just a passage from the top of the bowl to the air filter area.

    youre saying when given throttle fuel sprays from this passage. as far as i can work out.

    WTF? :jester:

    im also lost.


    vibration? i havent heard any talk of that.... and possibly...inlet manifold? or more suspiciously, the gasket? nicely matched up or has dangle berries?

    cus if the problems continuing with a swap of the carb... it has to be the engine somehow....

    how i dont know. weird. you got gremlins with fuel pumps playing silly buggers
  11. Big Red

    Big Red Active Member

    Yep, SPRAYS out. Don't think it's vibration. If loose intake then it would just suck air and run bad. I did go into the thing further and found a chewed up magneto key and burnt coil. It was getting spark so I didn't check that before. It might have been a little out of time from the bad key. I've got the engine out right now to fix a small crack in the frame. get it welded tomorrow and slap the engine in to see what happens.
    Big Red