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Vent bowl problem and cylinders not firing

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    Vent bowl problem and cylinders not firing

    I have a couple of problems with a 1980 GS550 that I could use a little advice on.
    This bike has 16000 miles on it and has not been run for at least a few years. The OEM ignition had lost the spark output on the #2 and #3 cylinders which probably was the reason it was parked.
    The bike has had the following maintenance done so far:

    New Dyna S Ignition
    Complete Carb. teardown and cleanup including the new O ring kit. As per the docs on this and the basscliffs site(They were not really in that bad a shape other that a little varnish)
    Valve clearances where checked and all were in spec.
    New plugs
    Rebuilt kit on the fuel petcock
    Sealed the airbox

    Problem 1: I put everything back together and fuel would not flow into the carburetors until I pulled off one end of the crossover line that ties the vent ports of the 1-2 carbs to the vent port on the 3-4 carbs. At this point I could start the bike and run it. If I put the crossover line back on I lose flow to the carbs again. This sounds like there is a float bowl vent port plugged somewhere but I don’t know where it would be. It will still only run if the crossover tube is pulled.

    Problem 2: The #2 and the #4 cylinders do not want to fire at Idle. This is after a carb sync and balance. Both of them will pick up at around 1500 to 2000 RPM or at idle if I pull the choke out .

    Stuff that has been checked to try and track down the problem:
    Checked for air leaks
    Switched plugs from the good to the bad cylinders
    Pulled the carbs down again and verified all ports and jets clear, settings ok and same size jets in all the carbs and that CV slides were free and not sticking
    Checked the float levels with a drilled a drain plug and with clear tubing attached
    Verified Battery charge and good voltage to the new Dyna S unit
    Verified solid engine and frame grounds

    The initial cold compression check on the engine was not very good with a high of around 90 on #1 and #3 and a high of around 60 to 65 on #2 and #4. This was a roughly a 5 second test with the throttle wide open all the plugs pulled. Subsequent tests after 5 minutes of run time showed about the same values. Surprisingly a cylinder leak down test was less than 3 PSI loss at 100 PSI test pressure on all the cylinders. (I’m still scratching my head on that one). Pouring a little oil into the cylinders brought the compression test up to +175 on all the cylinders so there is some wear of the rings/walls but the valves appear to be sealing well.

    The fact that I have ignition in #2 and #4 only while choked suggests that there is not enough fuel in the mixture going into the non firing cylinders. I suppose I could still have some carb issues that I have not found but now I’m leaning toward the compression is being too low for the spark to trigger the mixture at low RPM values.

    Any thoughts or advice on these issues would be appreciated

    #2
    I'm not following you regarding the vent lines being pulled? The vents should be open tubes, with the hosed draped on top of the airbox, plugged into nothing.

    Regarding not running well at low speed, where are the pilot screws set? I suggest 3-3.5 turns for the CV carb 550's.
    Ed

    To measure is to know.

    Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

    Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

    Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

    KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

    Comment


      #3
      That explains a lot! When I got the bike a rubber hose was plugged into the vent port between the right set of carbs and was looped across the top of the carbs and plugged into the the vent port between the the left set of carbs. (which now I see equals no fuel bowl vent). I just assumed that this was a bowl pressure equalization line and the bowl vent/s were somewhere else on the carb. I didn't realize there was a missing vent line. Are there some small filters on the ends of the vent hoses that sit on top of the air box?

      I have the idle jets set at 1.5 turns right now so I will give your setting a try. Thanks for the help.

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