3 plugs - perfect tan color, 1 plug - dry fluffy carbon

Discussion in 'Mechanics Garage' started by flummerylove, Feb 27, 2012.

  1. flummerylove

    flummerylove New Member

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    Greetings. After countless hours, I am SO close to getting my 90' dialed. 3 plugs are reading perfect tan and 1 is black dry carbon. Tinkerin w/ stuff saved my ass and sold me some cally carbs which I cannibalized mostly for the inlet needles. Didnt even need to mess with the tangs...all four were perfect - 9mm.

    After a successful carb balance, I have been playing with the mixture screws. I can kill 3 cyls when turned fully IN except 1. And guess which one...same cyl as dry carboned plug.

    So what might be the variables now? Still incorrect float level in one cyl? Not yet checked compression in that cyl.
    Doesnt it all point to incorrect float level? Other?

    System is charging proper. Good batt. new plugs
     
  2. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    Unfortunately, it does sound like fuel getting past the float needle :pout:

    It might be interesting to try swapping the float and needle with one of the other carbs and see if the problem follows to that cylinder.

    cuz pulling V4 carbs is soooo much fun
     
  3. ridervfr

    ridervfr Member

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    try measuring petrol that flows out when you drain each float bowl, compression check would be nice. Use a new spark plug for the bad cylinder, could be a bad one. Good luck!!! and keep us posted...
     
  4. flummerylove

    flummerylove New Member

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    Yeah good idea on the swap. Will do that def. And measure fuel when I drain em. Updates to follow
     
  5. ZEN biker

    ZEN biker New Member

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    I agree with tink he is a carb god! Have you checked the idle jet? it could be not all the way in and giving you too rich a mixture at idle and off idle. There is also (if your carbs have it) the acceleration pump, these can leak and let fuel in just on engine vacuum. If you dont find anything wrong, you could just do the carnal sin of running a hotter plug in that cylinder.
     
  6. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    Measuring the fuel in the float when draining is a great idea but doesn't practice well.

    It's tough to be able to measure accurately enough, and the first bowl you attempt to drain will drain the whole fuel rail so you get a huge amount but it doesn't mean that anything is wrong.

    Thanks Zen - I don't know that I'd say I'm that good but I do enjoy the puzzle that carbs present.
     
  7. flummerylove

    flummerylove New Member

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    Copy that tink. No accel pump on my 90. Zen you mean idle jets, not mixture screws? I have never been able to get the bike to idle without using some throttle stop screw (whtevrs the proper term). If we are talking mix screws, usually just left them at 1.5 turns out. My carbs were all kinds of flooded so mix screws didnt do anything then anyways. However now 3 are good, one is flooding. I can kill the 3 good cyls when mix screw is turned IN til it "lightly seats". I REALLY want to get the bike to idle w/o stop screw, but perhaps this is not her lot in life...
     
  8. squirrelman

    squirrelman Member

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    Some fool over-tightening the idle mixture screws can damage the carb, or the point can be ruined, causing an over-rich condition around idle, but the most likely fault is an improperly set fuel level in that carb or clogged air jets around the intake or dirty pilot jet air passages or holes.
     
  9. ZEN biker

    ZEN biker New Member

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    if your intent on the brigs & straton carb setup (bolt up and set idle mix once) then you may have to pull that one and fully disassemble it, use soft copper bell wire to clean all the ports and double check all needles and seats. I dont know your carbs, but generally all carbs meant for power applications will have the base minimum of a fuel metering jet and needle (in the float bowl) a idle mix control jet and needle(on the side that you adjust 1- 1.5 turns from lightly seated) off idle bypass jet (no adjustments but some can be changed) Idle air control port(if plugged will act like rich mix) and a power jet and needle (not usually adjustable but most can be changed to match your power setup. Usually found under an epoxy cap to prevent tinkering). jargon aside they concepts and practices are the same for just about any carb including lawnmowers, the difference is how they are controlled.

    I would check the fuel metering jet and its seat, some of the needles have a rubber cap to make them more fool proof but I have found that older ones tend to leak because of the type of rubber they are made of just wasnt meant for fuel applications or the heat your carbs see during riding. when in doubt clean everything with a mild cleaner and never use break clean untill the end.

    another thought: is the carbon on the plug chaulky? or more of a powder that coats it. if its chalky like dry water color paints you might just need a new valve seal cover. This may need the head removed as you have to take the spring retainers out to change this little rubber cover inside the valve spring. if you have access to a bore-scope have a look down the plug hole and look for carbon on the piston face. if its flakey then is oil that being burnt if its powdery then its fuel. note this only works for gas engines, desiels burn desiel oil so will have flakey carbon on everything.
     
  10. Dukiedook

    Dukiedook New Member

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    Hook up clear hoses to the drain nipples and turn them up- instant piezometer to check the actual static head in each carb bowl, that will tell you if that rich bowl is high or not.
    This technique is used on CB's with good success all the time when people have rich condition after cleaning their carbs for the millionth time.
    Make double sure you carbs are truly clean though, the idle circuit does not take much to clog it or the float needle. Carb cleaner and compressed air through all orifices.
     
  11. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    FTW Dukie! Nice suggestion.

    You could run it that and know right where the level is. Keeping a fire extinguisher nearby of course.

    I've learned my one new thing today. The question is what did I forget I order to make room for that new tidbit?
     
  12. ZEN biker

    ZEN biker New Member

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    never mind what you forgot, its what you learned that you should be worried about!
     
  13. Dukiedook

    Dukiedook New Member

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    Just make sure on the homemade clear tube piezometer to build a harness or something that keeps the tubing upright or else gas will spill.
     
  14. squirrelman

    squirrelman Member

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    Most likely cause--as mentioned above--is a leaky float needle on that carb.
     
  15. flummerylove

    flummerylove New Member

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    Yanked those bastards today. The float in the problem cyl was a teeny smidge high so adjusted. And per tinks suggestion swapped needles/floats with adjacent bowl. Reassembled, did a proppa plug chop and checked problem child again. STILL black dry fluffy. Bike has new valve seals. Today, after chasing this for umm..32-ish months I feel like I have lost the battle. Bad thoughts of well, you know. :frown: PLUG.jpg
     
  16. flummerylove

    flummerylove New Member

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    SO.. another good set of carbs with new needles/seats :faint2:
    Might this point to a cylinder that is worn out of spec or compression issue? Closer than ever and greatest resistance...
     
  17. squirrelman

    squirrelman Member

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    Could be bad rings, but not so likely on one cylinder only.



    Just to check, i'd switch ignition coils from a tan plug to the black one.

    CDI can fail to fire on one cylinder only, to my surprize, but that plug will be clean, not black usually.
     
  18. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    compression test can't hurt
     
  19. flummerylove

    flummerylove New Member

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    Right. Bike has new rings.
     
  20. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    [​IMG]
     
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