Tips to reduce spark plug fouling

Q: In my pilot club meeting last month, a pilot complained of spark plug fouling before takeoff in our Grumman Cheetah with 150-hp Lycoming. I suggested that after starting the engine, and before taxiing, she lean the mixture, keep it lean while taxiing and put on full rich for run-up. However, if there is a delay — say she’s fourth in a line of four airplanes for takeoff — she should lean until cleared for takeoff.

Another member cautioned that leaning while taxiing can burn up a cylinder. I say that while taxiing the engine rpm is too low for a lean mixture to burn a cylinder. My brother says not leaning may, in addition to fouling plugs, cause a valve to burn or other damage.

What say you?


A: I can tell you this subject is quite common at flying clubs worldwide.

If spark plug fouling is a consistent problem with a specific aircraft/engine combination, there may be some simple things you can do to reduce or eliminate the problem. If the problem is experienced by other aircraft on the field using various types of engines, then there are other ways of dealing with this annoying situation.

If we take your particular situation with the Grumman Cheetah using the Lycoming O-320-E2G engine, I’d first take a look at the condition of the spark plugs. There may be a chance the problem you’re dealing with may be eliminated simply by changing to a different type of spark plug, such as a hotter plug that may be better suited to your type of operation. Your maintenance facility can provide advice on the various options of spark plugs approved for your particular engine model.

Among the great plugs that have been on the market for several years are the extended nose massive electrode spark plugs, such as the Champion REM37BY or its equivalent. These spark plugs were originally designed for the O-235-L series Lycomings. When the high compression 235-L series engine came on the market the problem with lead fouling was quite common and this style of plug proved to be a real asset in reducing the lead fouling problem. Approval for many additional engine models to use this spark plug soon followed. Good service reports from the field keep these plugs a popular choice.

There are several other things that could contribute to this annoying problem, but from the brief information you provided, it sounds to me like some simple operational changes may be in order. Even though it’s not uncommon to hear pilots say they manually lean during taxi to reduce lead fouling, it gives some of us concern that there is always the possibility that they’ll forget to return the mixture lever to full rich for takeoff. Taking off with the mixture leaned can cause serious engine damage and possible engine failure.

An alternative method to manually leaning, and to lessen spark plug fouling, is to increase the engine operating rpm during taxi to 1,200 rpm or so. This causes the core nose temperature of the spark plug to reach a temperature range where it can scavenge the lead as it’s produced before it gets to a point where it causes erratic engine operation. I’d recommend you try this method to see if it will help.

If the problem is common in other aircraft on the field and there is an alternate brand of fuel available, this may offer some relief, assuming different additives are used by different fuel suppliers. Unfortunately, most GA airports only have access to one brand of fuel.

The fuel additive TCP is approved for use in all Lycoming normally aspirated engines per Lycoming Service Letter L190 and has been used to alleviate lead fouling. NOTE: You must make certain this product is approved by the airframe manufacturer before use. It is not detrimental to any of the engine components, but the engine manufacturers cannot speak for it’s compatibility with airframe components.

Just as a matter of course, you might want to make an engine logbook entry when you make any changes to your current operation so you’ll know if any of the changes make an impact.

Hopefully this information will help improve your overall operation at little or no additional cost to you.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


  1. Bill Friece says

    Incident occurred in exactly the same way, and location, on two separate days.

    Plane has been running fine, and was running fine, for at least ½ hour on both days.

    No problems on run ups or full throttle maneuvers, plenty of fuel.

    Intercepted the ILS, (11) miles from the runway at 3000’, flew it in for 7-8 minutes.

    Executed a missed approach, engine ran rough, and aircraft was unable to hold altitude.

    Switched Fuel Tanks.

    Engine continued to run rough, then after a few seconds, ran fine.

    Total duration of rough engine was 10-15 seconds.

    Landed, did a full run up (all good), then took it around the pattern without further incident.

    At the IF, 11 miles out (3000’) configured as follows:

    Day 1 Day 2

    Power: 1900 1900

    Fuel Tank: Right Tank Left Tank

    Attitude: ½ Bar Down ½ Bar Down

    IAS: 90-100 90-100

    Mixture: Full Rich Leaned Slightly

    Carb Heat: Off On

    Fuel Pump: On On

    At Decision Altitude (365’) reconfigured as follows

    Power: Full Full

    Attitude: ½ Bar Up ½ Bar Up

    Mixture: Full Rich Full Rich

    Carb Heat: Off Off

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