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Slight mist of fuel out of carb?

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  • Slight mist of fuel out of carb?

    Working on a scooter and I noticed this problem.... anyone want to hazard a guess?

    Oh and it's got a bit more Crank pressure then I'd like... Compression is good or not good depending on the number of strokes and if it catches just right... cause it has spiked at 150 and been as low as 70...

    Just never seen variable compression... hmmm I have had bad compression gages that simply won't work on a cylinder at Bonneville .... dang thing would work on all but say 2 holes after a 275hp Nitrous run for 60+ seconds... but that's just bad gages... LOL

    Keith

  • #2
    Re: Slight mist of fuel out of carb?

    had my triumph bonnie break a ring and do that ? hard to say good luck

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    • #3
      Re: Slight mist of fuel out of carb?

      What kind of motor is it Kieth? 2 stroke, 4 stroke?

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      • #4
        Re: Slight mist of fuel out of carb?

        On a single carb per cylinder motorcycle engine its common to see a cloud of fuel in the air horn. Most bikes (stock) when jetted for the stock aircleaner and airbox wont run right with them removed. Ususally you have to increase jet size or raise the needle on the carb slide to compensate for the missing box.

        The "mist" I believe is caused by a short intake tract combinded with intake reversion pushing the fuel/air mix backwards. This happens in a normal intake as well its just contained inside the manifold and we dont see it. If its a two stroke you might have a weak reed valve contributing to it as well.

        As far as the variable compression, are you doing the compression test warm or cold? If the bike is a smaller one or two cylinder motor it may have a compression release that may be affecting the compression check. Basically it bleeds off the compression untill the bottom of the stroke of the kickstarter. (Im assuming your kickstarting the motor, if its electric start then disregard this)

        Whats the year, make, model, specs, problems your having with the bike?


        Chris 8)

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        • #5
          Re: Slight mist of fuel out of carb?

          it's a single cylinder 500cc Buell Blast... basically it's a Tonya bike... or going to be when shithead gets it running...

          K

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          • #6
            Re: Slight mist of fuel out of carb?

            Not a moto expert but my wheel horse and my sons go-cart both do that , Thats why they have tops on the air breather isn't it !? . LOL

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            • #7
              Re: Slight mist of fuel out of carb?

              I think there is a bit of reversion as a matter of course, this turned out to be in the bore... the cylinder was trashed... VERY tight spec's and this one was Really loose and scored from overheating... the head's been ported, and it's got aftermarket cam's... Nice stuff but most likely stuff that's not going to be happy just being a street bike...

              we'll get some replacement junk on the way and make it a happy thing...

              K

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              • #8
                Re: Slight mist of fuel out of carb?

                Keith,

                What you are describing is known as "fuel standoff." This is the reason velocity stacks were created. The velocity stack will contain this "standoff" so airflow across the top of the carburetor does not blow it away (as in the case of a tunnel ram sticking up through a hood) and causing a lean condition. This is also a good reason to run an air cleaner on your carburetor. How many times have you heard of guys having to jet up their carbs (enrichen them) when they take the air cleaner off? It isn't because the engine is getting more air, but rather, the fuel standoff is being blown away from the top of the carb, thus leaning out the mixture.

                I learned this years ago (back in the early 80's) when I first started hotrodding my VW Bug. I hope this helps.


                Ron
                It's really no different than trying to glue them back on after she has her way.

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                • #9
                  Re: Slight mist of fuel out of carb?

                  I've also seen people run diffusers under thier hood scoops for the same reasons. Most of the one's I've seen have been on Buick's with Stage 2 scoops and really they look like and supposidly are are just large diameter collanders.
                  Central TEXAS Sleeper
                  USAF Physicist

                  ROA# 9790

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