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Update on gas in airbox, Bad Petcock, how can I test it?

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  • Update on gas in airbox, Bad Petcock, how can I test it?

    An update from my gas in airbox problem (1978 XS1100, all original). Starting a new thread as this is specific to petcock issue.

    Quick background. Restoring bike after sitting for 13 years. After rebuilding carbs/petcock (vacuum) took for a ride or two and found gas in airbox. Suspecting petcock and carb issues, I drained the tank and took it off the bike. Took off the petcocks and tighten any loose screws (none were actual loose). I then put 2 gallons or so into the tank and tested the petcock by having it in the "ON" position and no gas coming out. Put it in "PRI" and gas flowed, Put in in "RES" and no gas flowing. All working as expected (I assume....). So put it back together went for a short ride. Let bike sit for a bit and then checked for leaks. More gas leaking in airbox. Took of the gas line to the left petcock and gas was flowing out of it even though it was in "ON" position. Grrr, I thought I verified the petcock was working correctly.

    So, apparently my petcock test was not very good. I am guessing when I emptied the tank and refilled it, the petcock sealed shut so no gas flowed. However, when I went for a ride, the vacuum open the petcock up (as it should) but once opened it never close.

    My question, does anyone have a better test for the petcock other than filling the tank and seeing if the petcock leaks in the on position? Can I simulate Carb opening up the petcock by sucking on the vacuum line?

    Thanks again.
    Steve

  • #2

    I think you realize you have a carb float needle valve problem. If gas gets into the airbox, it happened because the float valve didn't stop the flow of gas correctly.

    Getting back to petcock, yes, you can suck on the vacuum line to test the petcock.

    The petcock might be fine and your problem might be only the float. I had a float valve stick open while I was riding a couple of weeks ago, and the airbox was flooded when I stopped.

    However, you should test he petcock for sure because it will prevent a major problem if your floats don't stop the gas while the bike is parked.
    -Mike
    _________
    '79 XS1100SF 20k miles
    '80 XS1100SG 42k miles
    '81 XS1100H Venturer 35k miles
    '79 XS750SF 20k miles
    '85 Honda V65 Magna ~7k miles
    '84 Honda V65 Magna 48k miles (parts bike)
    '86 Yamaha VMAX 7k miles

    Previous: '68 Motoguzzi 600cc + '79 XS750SF 22k miles +'84 Honda V65

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    • #3
      Might want to take the petcocks off and apart and clean them along with rinse the tank.
      79 F full cruiser, stainless brake lines, spade fuses, Accel coils, modded air box w/larger velocity stacks, 750 FD.
      79 SF parts bike.

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      • #4
        You need to verify your float height settings. The should be set at (25.7mm +or - 1.0 mm). You measure from the base of the gasket surface to, meaning gasket removed, to the top of the float. You stated in your previous thread that you installed all new float needles and seats so they should be good. One other question that comes to mind. What is the condition of your tank and what has been done to it? The reason I am asking is to verify the condition as the whether you have or had rust in it and what you did to alleviate it. Also ... do you still have the internal filters off the petcocks and were they in good shape? It doesn't take but a small amount of loose crap to really mess with the carbs both performance wise and for the float needles to be affected. If you are sure that the tank is ok then I would definitely recheck my float heights.
        Last edited by cajun31; 04-27-2021, 10:31 PM.
        2 - 80 LGs bought one new
        81 LH
        02 FXSTB Nighttrain
        Jim

        Comment


        • #5
          Easy way to set float height on early style carbs with brass floats is to set them to (1"), with gasket removed as mentioned.
          2H7 (79)
          3H3

          Comment


          • #6
            Originally posted by cajun31 View Post
            You need to verify your float height settings. The should be set at (25.7 +or - 1.O mm). You measure from the base of the gasket surface to, meaning gasket removed, to the top of the float. You stated in your previous thread that you installed all new float needles and seats so they should be good. One other question that comes to mind. What is the condition of your tank and what has been done to it? The reason I am asking is to verify the condition as the whether you have or had rust in it and what you did to alleviate it. Also ... do you still have the internal filters off the petcocks and were they in good shape? It doesn't take but a small amount of loose crap to really mess with the carbs both performance wise and for the float needles to be affected. If you are sure that the tank is ok then I would definitely recheck my float heights.
            The tank was sealed by previous owner. Before we did anything we ran some gas through it and it look clean but I guess you never know. There are also inline filters (I don't know if that is standard or original owner added them.. I plan to go reset the carbs floats. I think I have it narrowed down to carb #4 that was leaking. I will check the float height, good to know it is height with gasket removed.

            Also, I did a vacuum test on the petcocks. The right one, when "ON" didn't flow gas until I sucked on the vacuum line. Gas flowed and continued to flow for about 30 seconds after I stopped sucking (is that correct, or should it stop right away?). Then went on the to left petcock. Once again gas didn't flow initially (which previously I took this to mean that the petcock was OK). Once I sucked on the vacuum gas started flowing, however when I stopped sucking, it just kept flowing.

            Both petcocks were rebuilt (I think from a kit from Mikes XS, but not sure). I plan to open up the left one again to see if I find anything.
            Oh, and I don't think I have the original internal filters of the petcocks but I will double check.

            Thanks for all the help.

            Can't wait to get this beast on the road.

            -Steve

            Comment


            • #7
              Here is some info with pictures on how to get the vacuum operated fuel taps working correctly. The tutorial shows a different type valve than yours but very similar.

              https://www.xjbikes.com/forums/threa...-w-pics.25058/
              2H7 (79)
              3H3

              Comment


              • #8
                Originally posted by bikerphil View Post
                Easy way to set float height on early style carbs with brass floats is to set them to (1"), with gasket removed as mentioned.
                1" is about 25mm. If the float is not closing wouldn't it be better to go towards the plus side of the +/- 1.0mm. Meaning he needs to set them a little higher then the required 25.7mm. Of course when he inspects he may find that the floats are set too low right now.
                2 - 80 LGs bought one new
                81 LH
                02 FXSTB Nighttrain
                Jim

                Comment


                • #9
                  That little difference would never cause the overflowing, something else is wrong in there.
                  Couple possible causes:

                  Brass float has pinhole and not floating
                  Gasket washer bad under seat
                  Worn out needle/seat
                  Float binding on pin or hanging on bowl gasket
                  Debris in fuel
                  Cracked/pitted carb body
                  Last edited by bikerphil; 04-28-2021, 07:13 PM.
                  2H7 (79)
                  3H3

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