Q&A / Tech

Mailbag: How to Cure Spark Plug Fouling During Idle

Q: I own a 1971 Dodge Demon with a 400. The 10.5:1 compression engine has a forged crank and “LY” rods, and has a balanced bottom end. The heads are 906 castings port matched to a Weiand Team G intake and 2.5-inch headers. The heads have also had port work done. The cam is a Mopar Performance grind with .509-inch and 292-degree duration at .050-inch lift. The problem I have is when I let the car idle for 10-15 minutes, the spark plugs get fouled (the motor always runs on the rich side). I thought the problem was with the Holley 0-3310 750 cfm carburetor so I replaced it with another carburetor of the same style. The problem is still there. I changed jets from #72s to #68s, put in a 2.5 power valve, but that didn’t help. When I turn the idle mixture screws in, the engine doesn’t shut off. Could the problem be with my ignition? I have an electronic distributor with mechanical advance, the “chrome” ECU, an ACCEL Super Coil, and ACCEL 300+ wires. The car also has a Torqueflite transmission with a 3,000 rpm stall TCI converter and a Cheetah manual valve body. The Demon probably weighs 3,000-3,200 pounds. Is the 0-3310 right for this car?

A: The classic 0-3310 is not a bad choice for your Demon; however, a 750 cfm double pumper would give you maximum performance. But even the double pumper will not help your plug fouling problem. Here are some things to do to solve that:

  • Keep idle speed around 880-900 rpm in drive.
  • Check your distributor advance curve. You should have 10-12 degree initial timing with 36-38 degrees total by 2,500 rpm.
  • Keep engine operating temperature around 180 degrees.
  • Switch to an MSD 6AL series ignition and a Blaster 2 coil.

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  1. I have something to add. Early Chrysler vehicals resistored the ignition through the switch and if you are powering you ignition with the original wire to the coil then that’s a problem. and if you then have an internal resisted coil on with that then that’s two problems.you need forteen volts at coil positive when running. With most electronic ignition.

  2. My 440 is the same basic combo, it likes 20 degrees initial timing. Other than that, the advice given was spot on.

  3. Did I miss the vacuum advance which helps avoid this issue, or….. WHY DID THE FACTORY PUT THIS ON HERE ????

  4. You stated mixture screw adjustment had no effect. Might check the stop screw for the secondary butterflies. Accessable on the underside of the base plate. often the flies are open too far which will make the mixture screws non-functional.

  5. Victor J Glaviano says:

    I had the same car years ago in the early 90s. It was a Scat Pack car, came with triple deuces. You think you have problems, I could never keep the deuces tuned. Took them off put a single plane intake and a Holley 650 DP and it ran much leaner and my plugs were more evenly burned grayish brown. Also I tuned my carb by running 70, vice 65 jets, 4.5 power valve. Hotter plugs, sorry can’t remember which!
    Only difference is I had a 340 in it. My advice is to do what I did above and what really helped me was going back to my favorite… FOMOCO, haha sorry couldn’t help it! It was a good car.

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