Running on rear cylinder only

Discussion in 'Engine, Fuel and Exhaust' started by rfranz1952, Dec 27, 2008.

  1. rfranz1952

    rfranz1952 Member

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    I pulled into a bud's place yeaterday--stayed for 30 minutes or so. When I tried to start the bike, it would run only on the rear cylinder. The front is getting fuel, but it's not burning--except occasionally when it ignites out through the carb.

    The bike was running great before I stopped. Pulled strong, no unusual noises, no missing, hesitation, or backfiring. The most recent plug check showed the plugs were "normal". No sign of running either rich or lean.

    What we know now:
    We are getting fuel into the cylinder--enough so that after turning it over a while, fuel leaks out the exhaust ports.
    We are getting good spark--confirmed instrumentally and visually.
    We have visually checked and replaced the intake gaskets. No indication of an intake leak.
    We are getting 150 lbs plus compression in the affected cylinder.
    We have visually examined the parts of the intake valve that can be seen through the intake port. Still shiny and clean--no signs of burning.

    Oh yes, the top end is new--it has about 150 miles on it.
     
  2. glider

    glider Veteran Member

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    How and where did you check for spark?
     
  3. rfranz1952

    rfranz1952 Member

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    We put a "spark detector" between the plug and the wire. We also pulled the plug, laid it on the head and turned the engine to visually observe the spark. Bright, blue-white flashes at both electrodes.
     
  4. gsxrboy96

    gsxrboy96 Member

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    So does the bike not pop or spit or anything,? you say its igniting out the carb? sounds either like a bad plug and or coil.
     
  5. glider

    glider Veteran Member

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    Try switching the plugs between cylinders and see if the problem moves to the other cylinder.
     
  6. Ken S.

    Ken S. Active Member

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    You can see spark
    You know you are getting fuel.

    Spark and fuel mean it should fire. Obviously if those two factors are present outside and inside the cylinder then combustion has to happen.

    Unless, you are getting too much fuel, flooding out the spark.

    Can the fuel be shut off on that Bike?

    If so, pull rear plug wire and wet the front plug with fuel from a can and reinstall and try to start and see if it fires. If it does fire for a moment, then I could see it being too much fuel injected into the cylinder retarding spark.
     
    Last edited: Dec 28, 2008
  7. rfranz1952

    rfranz1952 Member

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    No history of popping or spitting. However, when I try to start it now,we do get some fuel standoff outside the carb, and that occasionally ignites.

    We've switched plugs, as well as changed to new. No effect on the outcome.

    There is no question that when we pull the plug from the front cylinder, it is wet with fuel, and that affects the ability to spark. The rear cylinder fires. So, the next questions are:

    1. Why is there too much fuel? The bike has not been running rich.
    This bike has a Mikuni HSR 42 carburetor. When I upgraded the top end to a 1250, re-did the heads, and changed the cams, I rejetted to carb to match the new configuration. If the pilot jet did not get tightened enough and has come loose, or fell out into the bowl, that could conceivably create a rich enough condition to match the symptoms?? If so, then the second question is:

    2. Why only the front cylinder?
    Hmmm. Maybe (I'm speculating) the rear spark is a shade stronger than the front, and just enough to keep the fuel firing. We do get nice puffs of black smoke out the pipe when we can get it to catch for a few moments.

    To be honest, I think its a long shot, but it is certainly worth dropping the bowl on the carburetor to check before we pull the heads or cam cover to check the other possibilities. Thanks for the suggestion.:hii That is something we had not thought about.
     
  8. gsxrboy96

    gsxrboy96 Member

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    Before i pulled the cam cover, i Would simply pull the push rod covers and make sure both valves are working ( espically the exhaust) because the intake is certainly working if the plug is wet, If the exhaust moves, or you can feel the cylinder working at the end of the exhaust pipe, then you have a ignition problem, because you have fuel, compression, and its not the timing if the rear cylinder is running correctly, so i would say you have a bad coil, the only reason i say this, is I always thought if i had a nice blue spark, it would fire the bike off, MY sons pw50, had fuel, comp, spark, and it would not start for the life of me, and i ended up replacing the cdi and coil, and the bike fired second kick, so dont bank on your ignition being ok if you see the spark, im sure there is a way to test the coil with resistance, but i couldnt tell you how, ill lookin my repair manual and try to scan that for you.
     
  9. rfranz1952

    rfranz1952 Member

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    Yes, it is an NRHS build. 1250 kit w 10.5-1 pistons, Stage 2 lite headwork, SE536 cams. Like I said, its been running great, and nothing unusual (noises or behavior). We did not change lifters or pushrods, those are still stock w 25K miles on them. I think it is a definate possibility that one of the lifters gave up the ghost with the new springs and higher lift on the cams. However, I don't want to tear it down to that level until I exhaust other, less intrusive possibilities.

    Dan gone for the holiday weekend--I will call him on Monday--but wanted to understand as much as possible when I talked to him.

    Are you on the other (Bert's) forum as Hobbituk? Avatar's the same, and he says he is from close by where Tolkien lived. However, he has a new nightster, I believe. Good taste, that fellow!:D
     
  10. rfranz1952

    rfranz1952 Member

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    Last night I installed a new Crank Position Sensor as those are a known problem with 2004 and 2005 Sporties. If anything, it made things worse. With my Daytona TC-88 ignition, I could get no spark at all, but when I installed the stock ignition module, I got sporadic and weak spark—but not enough to actually start the thing. Notice that the observed behavior is now different—I’m not getting good spark any more—with new plugs yet. But the engine is fully cold now too.

    Tonight I played with it some more. I changed the plug wires from the pretty blue ones (Thundervolts from J&P) to the old ugly stock wires.

    It started!! Now it ran ragged as hell, and I think it is getting actual ignition only on about every other cycle, but both cylinders are firing. Still quite a bit of fuel standoff outside the carb. I think the “back cylinder only” impression from the first day was the result of the front cylinder flooding out to the point where it did not ignite at all.

    About all I’ve got left is the coil, even though I have been told they “rarely” fail. We shall see.