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VM Carb Confusion - '78 GS750

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    VM Carb Confusion - '78 GS750

    I'm in the rebuild process with my carbs and am officially, thoroughly confused. I suspect the engine on my '78 GS750 might not be the original, and the jetting and float height on my carbs doesn't match the resources I've found around this site.


    My main jets are 105 (100 is stock), pilot jets are 17.5 (15 is stock), and needle valve seats are 2.3 (not sure on stock number). The weirdest thing is what (on paper) would seem to be a very low fuel level - the float height is set at about 27.5 mm, where 22 - 24 is stock and the highest setting for the VM26, on the GS750B, seems to be 26mm. There's always the possibility the needle valves are just longer, aftermarket units and the fuel level will be fine. The bike ran when I got it, but not super well, and the airbox was a total mess and the intake boot o-rings weren't in great shape either. Maybe the PO was just adding fuel with the bigger jets to balance the extra air...? SO on to my questions:


    Should I start from square one and get the "proper" jets and set everything based on a 1978 GS750E? Or because I don't know what all has been done, would you recommend keeping things how they are and just use the clear tube method to set my fuel level to about 3mm as that seems to be the happy place for pre-80s 750s? This is my first encounter with carburetors, so I don't want to get everything to the point where I'm ready to tune just to have to tear it back down and replace a bunch of stuff.

    #2
    Now you know someone's been messing around in there yes. FWIW, mine ran fine with 102.5 main/15 pilot, e-clip in position 3. Float height 24mm.
    -Mal

    "The only reason for time is so that everything doesn't happen at once." - B. Banzai
    ___________

    78 GS750E

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      #3
      Yes to clear tubing to check fuel level. Float height is just a proxy for fuel level in the bowl. My experience was setting the float height in spec and having fuel level too high. Using clear tubing helped me find the correct float height for my floats.
      Jordan

      1977 Suzuki GS750 (My first bike)
      2000 Kawasaki ZRX1100
      1973 BMW R75/5

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