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Thread: '81 R100 running on 1 cylinder

  1. #1
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    '81 R100 running on 1 cylinder

    My airhead is only running on 1 cylinder. I pulled the carb & cleaned it. It's getting plenty of fuel. Next I pulled the spark plug & held it against the engine block while turning it over to check for spark. Plenty of spark. Next I switched the coil wires. The coil is okay. Any suggestions how to troubleshoot this problem ?

  2. #2
    Douglas Williams
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    Is the plug in the non-running cylinder wet or dry when you inspect it after running? If wet, then I would suspect the ignition starting with the plug wires. If dry, then maybe the fuel isn't getting there. How about valve adjustment. Are they closing all the way or possibly not, allowing poor compression. A few years ago, on my RT, it was the spark plug wire. Good luck.
    Sent from a Galaxy, far, far away

  3. #3
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    Check your valve adjustment on the nonrunninsg cylinder. An adjuster might have become so loose you effectively have no compression.
    You really should not be grounding spark plugs that way with the electronic ignitions, very possibly damage Hall sensor or trigger unit under tank.

  4. #4
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    Quote Originally Posted by jforgo View Post
    You really should not be grounding spark plugs that way with the electronic ignitions, very possibly damage Hall sensor or trigger unit under tank.
    As long as the plug is grounded when turning the engine over, it shouldn't be a problem. It's only when the spark has no way to find ground.
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  5. #5
    On the Road
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    Sounds like a valve clearance has closed up and holding a valve open. A similar thing happened to me recently. I've been breaking in freshly rebuilt heads and suddenly one day the bike was VERY hard to start. Checked valves and found one exhaust clearance had all but closed up. I retorqued the heads and my valve clearance returned to exactly where it should be. Bike started up perfectly afterwards.

  6. #6
    James.A
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    Facts

    Fuel
    Air
    Compression
    Timing
    Spark

    These are the basic functional elements of the internal combustion engine.

    Like ATL83 says, adjust your valve clearances to correct specs, (intake and exhaust-both sides),.. and then check the compression. You would be looking for a significant compression loss on the non-running side.

    You didn't state the mileage on the bike, but if the "pulse air" emission system is still in place, that will make the exhaust valves run hotter and need close monitoring on clearances and compression.

    You claim to have fuel and spark, so what would be left to check? Since your problem is side-specific, that would eliminate timing. (unless you have an aftermarket Dyna ignition, or some similar set up, that allows for independent timing of the cylinders, and independent failure).

    So, if your F and S and T are good, that leaves air and compression.

    (credit where due)....I first saw this acronym (FACTS) posted by Joe Cuda, and it has served me well.

  7. #7
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    Plugs can"blow out" under compression. This happend to my son's bike a little while ago. Good spark when grounded outside the engine but in the engine, no fire. They short when the spark encounters the resistance of the more dense compressed air in the cylinder.

    If you still have the miss after checking the FACTS, as stated above, try swaping plugs side to side. If the miss follows the plug - Bingo!

  8. #8
    Registered User der verge's Avatar
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    Quote Originally Posted by 40401 View Post
    Plugs can"blow out" under compression. This happend to my son's bike a little while ago. Good spark when grounded outside the engine but in the engine, no fire. They short when the spark encounters the resistance of the more dense compressed air in the cylinder.

    If you still have the miss after checking the FACTS, as stated above, try swaping plugs side to side. If the miss follows the plug - Bingo!
    This is true. As "atmospheric" pressure (the atmosphere the spark plug is in) rises, so does the resistance of the air in that atmosphere. You do not see this all that often on a naturally aspirated engine, but it does happen, and is one of the first things you would check for if you have a force fed engine.
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  9. #9
    James.A
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    Quote Originally Posted by 40401 View Post
    Plugs can"blow out" under compression. This happend to my son's bike a little while ago. Good spark when grounded outside the engine but in the engine, no fire. They short when the spark encounters the resistance of the more dense compressed air in the cylinder.

    If you still have the miss after checking the FACTS, as stated above, try swaping plugs side to side. If the miss follows the plug - Bingo!
    Great Idea! In fact, I once found a failed spark plug on a friends airhead by pulling a known-to-be-good spark plug out of my own bike and putting it in his on the suspect cylinder.

  10. #10
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    Did a compression test & the dead cylinder shows only 62 PSI or so.. took the head off & sent it out for a valve job...I know, I know- you should do both cylinders... thanks to all of you airheads for your input. This forum alone is worth the price of the membership and more..

  11. #11
    Registered User lmo1131's Avatar
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    I know, I know- you should do both cylinders...
    so... .. . what kind of compression did the "good" cylinder have when you did the test with the carburetors disconnected?
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

  12. #12
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    ONLY 86 PSI- sounds a little low ?!

  13. #13
    Registered User lmo1131's Avatar
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    ONLY 86 PSI- sounds a little low ?!
    My '73 R75/5 sucks 135 on both sides.

    Yankem' both, and pull the jugs while you're at it.

    Not much point, in my opinion, of taking the time to do the heads, without doing the rings too. You may find out that all is needed is a hone job and new rings ... may.

    But you'll eventually have to pull the heads again to do a ring job. Might as well be now. Course... if you like wrenching on the bike, you could always wait for the heads to get back, break out a set of new head gaskets, bolt it all up, and then re-check the compression. And then do it all again if it isn't right... .. . head gaskets are cheap
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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