spark plugs

Discussion in 'Engine, Fuel and Exhaust' started by harleydad1000, Nov 30, 2010.

  1. harleydad1000

    harleydad1000 Member

    I have a stock 1980 sporty with fresh top end, Bendix carb, and Dyna S ignition with 5 ohm coil. Been running HD 4R5 as the recommended plug based off the stock ignition with a 3 ohm coil. The equal is a Autolite 4123 which I have been running with some success in getting the plug to burn right. The recommended plug for a 79 sporty is the HD 4R but that ignition was points with a 5 ohm coil. The 4R is a little hotter plug but about 1/8" shorter than the 4R5. I assume the 4R will be flush with the head or slighty recessed. Does anyone see any issues running the 4R?
  2. Jack Klarich

    Jack Klarich Expert Member

    May not get enough flame front to burn all of the fuel, I would go with the longest reach plug you can get JMO
  3. wilks3

    wilks3 Junior Member

    The more exposed "tip" you can run the better. But, if high comp or high rpms the tip might get too hot= preignition, which is bad. If you have good luck running the Autolites keep running them.
  4. Hoople

    Hoople Account Removed

    Trying to read plugs with today's fuel (10% ethanol & no methyl tert-butyl ether) is almost impossible. You can change the mixture from 13.9:1 down to 12:1 and see no change at all as far as color goes.

    I also have never seen a reason to change plugs according to the resistance of the primary of a coil. I can't see how a plug would even care what the value of the primary coil resistance was. The plug itself is more concerned with the combustion chamber then it is with the available voltage or rise time of the spark.:)