Fouling Plugs

Question The rear jug misfire and it keeps fouling plugs on my 1991 Sportster. I took it to the local Harley shop and they took me for 900 dollars, no fix. I’ve changed plugs, wire, coil ignition. Do I have to rebuild the engine?

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Answer First thing you should check is your charging system. A bad charging system can make a bike misfire, or just run really bad. If the ignition system is all complete and known good; you can isolate it from the rest of your wiring harness to see if you have a problem within your wiring harness. Remove the +12V wire that powers your coil. This wire is usually white, or white with a black tracer. Make a good jumper wire and run it from the positive battery terminal to the coil post, where you removed the white +12V wire.

Make this jumper so that you can quickly/easily unplug it when you need to kill the ignition (you will not be able to turn off m/c until the wire is disconnected). If this solves the problem, you need to search for a dead short within your wiring harness. If your charging system tests good, your ignition system is new and tests good and you still have this problem after isolating the ignition system; unfortunately, you may be looking at a mechanical problem.

Related Videos:
Charging System

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8 Responses to “Fouling Plugs”
  1. jim

    my 2013 FLSTC has the brake light sticks on I adjusted the front brake lever many times no help I took it apart switch works proper when out of housing when the brake lights on the red light comes on that’s four the security system but I don’t have a security system on fob no nothing what a mystery would love some input thanks

    • Customer Service Techs
      Customer Service Techs

      Hi, Jim. If you have an aftermarket brake lever, you may have an alignment/contact issue that you may not be able to correct. That is a common issue. Otherwise, you may need to replace your front brake switch.

  2. Kurt

    I have 2000 Road Glide – upgraded newer stylehigh volume oil pump from Harley Davidson- I have experienced a problem where it seems there is no oil at startup – this happened about three times – can you tell me what I may be hearing and the fix for this – thanks Kurt

  3. NOEL acevedo
    NOEL acevedo

    Hi, my 1999 road king fires up and runs with the choke on ,if it is running with choke on and I take the choke off it shuts off. Will not fire up unless the choke is pulled on any suggestions.

  4. Ronald

    my ’95 bad boy started fouling rear plug then eventually it started fouling both plugs, front and back…after reading things online I decide to put S&S SUPER E on bike thing carb was the problem…I was wrong…after getting new carb installed, and new plugs, I rode bike home from shop [about a mile]…next morning rode bike to store [about a mile] and filled up with gas…in less than 2 miles,both plugs fouled again, that soon…what do I check to fix this problem of plugs fouling [black sooty plugs]?…