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98 GSX600F Dropping cylinder, bogging

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  • 98 GSX600F Dropping cylinder, bogging

    Hi All,

    Sorry that my first post here has to be a wanting help one, but I'm afraid I have completely run out of ideas with this. I've had my bike almost 6 years now and I've worked on it an awful lot, but it's still stock, everything I have done has been maintenance related. I commute around 40 miles a day on it to give an idea of use. It has 44000 miles on the clock

    So about a year back I notice the bike is bogging intermittently, which is a huge pain because every time I would be ready to diagnose the thing it would run fine for a few days, but when I'd be out on the roads and open it up, it would bog awfully, felt like a 125 when it did.

    Now, in addition to this it appears to be dropping a cylinder or two on idle, which is a consistent problem now, its fine when started up (I always need choke to start it now - didn't use to), but as soon as it's warm, it starts this ticking noise, and dropping a cylinder I'm assuming (here's a video I'm hoping will help understand what's happening, this is after a 20 or so mile ride ).

    So here's what I've done so far -

    --rebuilt carbs, including all new gaskets and jets, but I didn't touch the pilot jets and thoroughly cleaned them - I've rebuilt carbs a fair few times before so I would be surprised if there's an awful error in my rebuilding, but who knows.

    --New air filter, checked all sealing between intake manifold, carbs and airbox

    --new spark plugs, and checked that all the spark plugs spark and look healthy when removed and earther against the frame (admittedly I only did this for a few seconds, perhaps I could be missing a problem that only happens only every 30 seconds or so. (Also no noticeable fouling of a particular plug during any of the times I have taken them out during the time I've had this problem)

    --New fuel filter, and checked for any kinking of the fuel line

    --balanced carbs, and checked all the vacuum lines are on properly and blanked off where needed

    -- removed head and checked all valve clearances (some seemed a little tight if anything - none were badly out of spec)

    Now I can't for the life of me work out if the bogging was too rich or too lean, I've tried using a tiny bit of choke when I noticed it was really bad, but I can't say I could tell there was a difference to be honest, and too much choke wants to flood the engine like I'd expect.

    Last thing to mention is the bike is restricted via blanking plates (Since I bought it) which restrict the amount of fuel air into the intake, however I am fairly certain that the bike didn't bog for a number of years with these, and definitely has been able to idle reasonably with them, I am loathe to blame them.

    Maybe it's an electrical problem - alternator/ spark generator related? Where would I start with diagnosing this?

    Anyway thank you so much for any help you might be able to give, I really do appreciate it.

  • #2
    Just a guess fuel starvation..
    Check your getting decent flow through the tap
    when it bogs crack the tank open to see if it's developing a vacuum in the tank.

    Rebuilding carbs would definitely do the pilots maybe this is why you need the choke

    good luck these probs can be a real pain

    I'd take the blanking plates out have not heard of these on this bike before
    Last edited by Lachie; 07-16-2021, 01:42 AM.
    “Anything that happens, happens. Anything that, in happening, causes something else to happen, causes something else to happen.
    Anything that, in happening, causes itself to happen again, happens again. It doesn’t necessarily do it in chronological order, though.”
    ― Douglas Adams


    • #3
      Thanks lachie, I'll try to get the tank open next time I feel it bogging. Is there any way to check that the fuel flow is reasonable and/or the petcock vacuum is sufficient?
      When I balanced the carbs they're all pulling a similar amount so I would guess swapping the vacuum line onto another carb won't show me if I have a problem.

      The blanking plates restrict the bike to 49bhp which is a requirement for my A2 license here in the UK, I'm afraid I took my test quite early and if you do your bike test before the age of 25 I think you have to do the whole thing again (and all the expense that goes with that) if you want to ride unrestricted. Even though I ride daily the test centre near me still want me to take several lessons before each test, and charge for hire during the test, so it works out to around 3-400 gpb which aside from finding the time or test availability (they're all booked up from the covid cancellations) is quite hard to justify at the moment. Sorry to ramble!


      • #4
        Ah I see about the blanking plates.

        No real flow spec's just see if it's flowing out reasonable and looks good solid flow.

        Should be a small plastic fuel filter just inside the petcock, you could have a look just unbolt the petcock and it's there.

        It doesn't take much vacuum through the vacuum hose so doesn't sound like it would be that.

        Even if you getting some (if not enough) fuel through the tap and you need the choke to start it, that bit sounds more like jetting.

        Check all hoses are not kinked or obstructed

        “Anything that happens, happens. Anything that, in happening, causes something else to happen, causes something else to happen.
        Anything that, in happening, causes itself to happen again, happens again. It doesn’t necessarily do it in chronological order, though.”
        ― Douglas Adams


        • #5
          Fixed!!! For posterity and for all of those searching forums for answers - check your ignition coils and spark plug caps! So I had written off the ignition because I could see sparks from all spark plugs when I grounded them on the frame, and I had checked the primary and secondary resistance of the coils before. The primary resistance was within the spec but I couldn't get a reading on the secondary so I thought I was doing it wrong. Recently I took the caps off and they were all around 15ko, way above the 5ko they should be. Then I checked the secondary resistance and they were in the mega ohm range! Around 150 mega ohms on one and 10 on the other! I replaced the spark plug caps and the coils, and checked all resistances were within spec and now on the 15 min test ride I just did it runs like a dream!

          What I think was happening, was it was dropping spark on occasional cylinders and then the unburnt fuel was causing it to be rich and bogging. But no spark plug was individually worse in colour than others so pretty difficult to diagnose!

          Hopefully this can help others because it's a real pain when it doesn't run right.