Spark plug gap

Discussion in 'Electrical' started by 90FXRS, Oct 7, 2010.

  1. 90FXRS

    90FXRS Junior Member Contributor

    A question was thrown at me and I can't answer it so I thought I'd see what everyone here has to say. Why is there range, say .038-.043, not an exact measurement as with automobiles?
  2. Jack Klarich

    Jack Klarich Guest

    Odd, but if you have a good ignition system the larger gap will give you a bigger flame front, but it takes more energy to jump that gap:D
  3. editbrain

    editbrain Banned

    Wrong answer. Also you want to torque you plugs to spec. Because that is a big deal also.
  4. Retrop

    Retrop Active Member

    Set them at .40 and then go riding to clear out your head.
  5. editbrain

    editbrain Banned

    If it is a new bike then, yes, set to .040 and go riding. Not .40. They should come from Harley set to .040, but some are a little less/little more. I set mine to .040, and never had an issues.

    Spark plug gap does affect combustion, power used to create the spark, length of spark, and other things. It is important, and a big deal. Like I said before as well. Spark plug torque is very important. Remember that, and go by the service manual.

    .040 gap
    the plug that harley recommends
    the wire that harley recommends
    the torque you service manual call for

    Must follow steps.

    Harley has sensors that check the resistance that control sensor reading that affect the way your bike runs. Changing to plus and wires that are not recommend will cause these checks to fail, and your bike will not run correctly.

    NEWHD74FAN Experienced Member Retired Moderators

    Range is provided for acceptable accuracy, with tolerance to take into account...the erosion that occurs the moment you put them in and use them for 5,000 miles, remove, check cleanliness, file flat, regap with wire guage (wear contacts are grooved). Sound old school, why yes, so "old timers" use to set the gap a little "closer" but still in the accepted range.

    Nowadays, you have choices, and standard HD plugs are $8-$10 for pair so most do not even bother...just check gap in center of range and change out at 5000 rather than take time to "preserve" the old plug.
  7. kemo

    kemo R.I.P

    I have not heard of any one cleaning the spark plug and filing the electrode in years. I mean in 35 years. If any one wants one I have two Champion Spark Plug service station style cleaners and spark testers for sale. Both of them worked last time I used them. (about 35 years ago). Be a good piece for your nostalgic garage.
  8. Breeze3at

    Breeze3at Well-Known Member

    The little bag of sand with a air hose fitting and a rubber gasket on top to stick the plug into? I remember those.
  9. HDDon

    HDDon Experienced Member Contributor Retired Moderators

    I also file my electrods and I don't trust the setting from the factory as all kinds of nasty things can happen during shipping. I also check to make sure that they are not cracked (the white part). I also index my plugs (old racers habit). I also think that the correct torque is very important.
  10. editbrain

    editbrain Banned

    Sounds like you have been doing this wrong the entire time
    Here is a little info for you. This also applies to any ignition system using a coil and spark plug. Please do not give advise that could harm others whips.

    From: Spark plug gap

    The spark plug gap, along with the combustion chamber pressure and the ignition timing has a direct bearing on the amount of voltage you require from the ignition system. The bigger the spark plug gap, the more voltage you require to have the spark arc across the gap. The same applies when the combustion chamber pressure is increased. The spark plug gap also has a bearing on engine performance. The bigger the spark plug gap, the more air/fuel mixture will come into contact with the spark and the easier it will be to ignite the air/fuel mixture. However, it's not simply a matter of increasing the spark plug gap and the output voltage from the coil. Firstly, there is a limit to the amount of voltage the ignition system can handle and, secondly, there is an optimal spark plug gap that will best performance for your engine and your driving style.

    Ironically, the car manufacturer's recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010" for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010" for better performance. Ultimately, you'd need to run your car on a dynameter to find the best spark plug gap, and the right ignition timing for your engine.

    Remember that when you increase the spark plug gap you need more voltage from the ignition coil to create a spark across the spark plug gap. We'll discuss ignition voltage at a later stage. When a greater voltage is required to create a spark, cold starting and firing fouled spark plugs become more difficult. Therefore you should ensure that your secondary, high-tension ignition wiring is at least 8 mm in diameter, and that it is always clean, dry and in peak condition..