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3 out of 4 ain't bad. One cylinder not firing.

Discussion in 'XJ Technical Chat' started by cdnjj, Jun 20, 2020.

  1. cdnjj

    cdnjj Member

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    Bringing my '82 650 Seca back to life after an extended hibernation and got her running today on 3 out of 4 cylinders.

    #4 is not firing (cold pipes).

    I've checked the coils and plug caps and they are in spec. Brand new battery. There is spark at the cylinder and I can see un-burned gas sputtering out the tailpipe at 3k rpm. Swapped plugs and caps and no affect.

    I know the carbs are a suspect here though I did a quick clean in them and had drained bowls prior to storage.

    Two things I am concerned about is the diaphragms cracking and the engine side carb boots leaking. If a diaphragm is torn or boots leaking what symptoms would this produce?

    Before I go back into the carbs any words of wisdom on other things I should check out?

    Thanks!
     
  2. Glen F.

    Glen F. Active Member

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    Just had the same problem. Check your float levels, dry and wet. Wet level is most important.
     
  3. tabaka45

    tabaka45 Well-Known Member

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    Don't know about the diaphragm, but if the boot is cracked so that it is sucking air into the engine it will be too lean to fire. I had a similar problem on my bike and it turned out to be a badly torn gasket between the boot and the engine. You can try blowing some propane --or some people use starter fluid--around the boot and see if that causes it to fire. Is the plug on #4 dry or wet? I hope that is condensation and not gas blowing out of your tailpipe.
     
  4. SQLGuy

    SQLGuy Well-Known Member

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    If you have the air box side boots off, you can lift a slide and see how it drops down. If the diaphragm is torn, it will drop much more quickly than the others. With it running, you can look and see how the slides react to throttle opening. If #4 reacts pretty much the same as the others, then the problem is probably related to a clogged passage or float level.

    Have you checked compression?
     
  5. cdnjj

    cdnjj Member

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    Thank you for the suggestions, I will try these before going deeper though the the propane method sounds a bit dangerous!

    The plug is coming out wet so some fuel is definitely getting to the cylinder.

    Anyone know what diameter hose is needed for the fuel level test?
     
  6. XJ550H

    XJ550H Well-Known Member Premium Member

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    6mm id will work..

    unlit propane is safer than carb cleaner it does not stay on motor or boots

    also check that nipple covers are sealed
     
  7. Eric Hughes

    Eric Hughes Member

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    I have a very similar issue and will be following along.
     
  8. cdnjj

    cdnjj Member

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    I went to do the fuel level test (with the clear tube) and no fuel came out. Engine was running and tank on prime. Tested on the other side and it worked fine...curious.

    Gave up being lazy and pulled off the carbs tonight and I'll open up the bowl to see what's what and report back.

    Oddly the engine side boot did have quite a bit of fuel on it so it was getting through the carb somehow.
     
  9. Glen F.

    Glen F. Active Member

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    No need to have the engine running.
     
  10. cdnjj

    cdnjj Member

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    Just to close this out after cleaning out the bowl and checking fuel levels I noticed 2 things:

    1. A significant crack on the carb boot. I wrapped it in self sealing silicone tape.
    2. The fuel mixture screw was adjusted way out (I think it was 7 full turns out and the others were 5 or 6). I reset them all to 3 turns.

    It fired up fine on all 4 cylinders!

    Thanks for the suggestions.
     
    Huntchuks and Rusty81 like this.

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