Q&A / Tech

Mailbag: Examining Causes for Sporadic Spark Plug Fouling


Q: I have a mild 383-cubic-inch small block with 9.5:1 compression, Edelbrock Performer RPM intake, and 750 cfm Holley double pumper carburetor. It also has a mild hydraulic roller cam and roller rockers. My question is in regards to gas fouling on some of the spark plugs. Specifically, I’ve noticed the fouling only occurs on a couple of plugs on the passenger side.

I’ve already rejetted the primaries in an attempt to remedy the problem. What else can I do, and why is it only on the one side?

A: Our suggestions with the fouling problem start with the type of choke operation you have. On the 750 mechanical secondary Holley, the choke is either hand-operated with a cable or controlled by a 12-volt source with a heated coil (electric choke). If the choke is not hooked up, you may be delivering too much fuel on startup. There may be too much fuel going to the cylinders closest to the carburetor, which happen to be on the passenger side.

When you rejetted the primary side of the carburetor, what happened? Was there any change? If there was, you may want to try to lean the primary jetting more on the passenger side than the driver’s side. Don’t go too far, as you might compromise the fuel/air ratio enough to damage the parts.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all

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  1. A common cold start practice in a hotrod with a carburetor without/disabled choke is to “pump” the gas pedal to make the accelerator pump(s) provide the needed additional fuel for a cold start-up. Problem is, this is often done prior activating the starter. The fuel travels to the rear cylinder(s) fouling the spark plugs as this article points out. Solution is to not “pump” the gas prior to starter activation. Turn the engine over first, then pump the gas; the fuel will get distributed evenly to all cylinders. Makes a huge difference.

  2. Kevin Talley says:

    Should be running a 650 hollwy instead of a 750.

  3. Agreed carb too big, 650 vacuum secondary

  4. This happens often. Use ngk copper plugs in the proper heat range. Other plugs foul easily. This works every time.

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