Starts and runs great, but then....

Discussion in '3rd & 4th Generation 1990-1997' started by rangemaster, Jun 11, 2019.

  1. rangemaster

    rangemaster New Member

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    ...after it warms up for about 3 minutes it starts running on only 3 cylinders. (It's dropping No. 1 - left rear as you sit on the bike). Pulse generator checks out, and when you pull the spark plug boot you can hear it sparking so the coil must be good. Put a new plug in-same result. Any thoughts much appreciated and thanks in advance.
     
  2. squirrelman#1

    squirrelman#1 New Member

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    when that happened to my gen2 bike it was a bad cdi box.
     
    Last edited: Jun 12, 2019 at 8:36 AM
  3. RllwJoe

    RllwJoe Member

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    what model and year? If it has a wax unit, that may be suspect.
     
  4. rangemaster

    rangemaster New Member

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    It's a 1997 750-4th Gen w/ carbs. What's driving me nuts is that if you wait 15 minutes, It starts with just a touch of the button and will run PERFECT for 3 to 4 minutes. Then one cylinder drops off. Fuel pump is good, new plugs, coils test good. I HATE replacing parts but I'm at a loss to diagnose the problem.
     
  5. Brian Rodgers

    Brian Rodgers New Member

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    It sounds like the issue has a heat-related component. Usually that points towards "electrics' for me.
    Is it always the same cylinder that drops? You could maybe tell which cylinder by feeling the exhaust headers to find which one is coolest.
    A thermometer (infrared or other) might be better than fingers for that task !
    If it is always the same cylinder that drops out, then focus on that (probably coil, maybe wires, maybe spark plug).
    Rotate the components that can be swapped to another cylinder; see if the 'cold' header pipe follows.
    Brian
     
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  6. RllwJoe

    RllwJoe Member

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    Brian may be on to the problem. The coil could stop working when it is hot. Ask a local mechanic if it can be checked out of the bike. If so, remove the coil and bring it to them to check.
     
  7. SweViffer

    SweViffer New Member

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    I've had somewhat similar problems with two of my bikes.

    On my VF1kF it was one of the CDI-boxes that had corroded, and when warm one cylinder went silent (at first, after a while both cylinders on that CDI box stopped working). The main problem was that some PO had mounted the CDI boxes upside down, so that water ran down inside and didn't have any way out.

    On my ST1100 (V4 at least) the fuel pump stopped working when warm (known error on those), and shut off fuel delivery intermittently. I had it running on first three cylinders then two then none. Waiting for the pump to cool down solved the problem, at least for a while. VFR's have the tank up above the carburetors unlike the ST, so I don't know how that might influence behaviour.
     
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