Ask Paul: What’s going on with my spark plugs?

Q: I have a 1962 Piper Cherokee 160 with the O-320-D2A engine. The engine had a complete overhaul in 1992 and only 500 SMOH. In addition, 20 hours ago the engine had an annual in which an oil change was done, new plugs, and compressions in the 78/80 range.

What I have noticed is when I taxi the airplane without flying it (I always lean on taxi), the next time I do a run-up the left mag performs poorly and I have to lean and “burn off” whatever is getting on the plug(s), then the airplane runs fine. This has happened to me twice recently. The oil is clean…not black after 20 hours and I have only added about 1.5 quarts in 20 hours.

Any clues to what the problem could be?

JOHN KITTS, via email

 A: John, I hope we can solve your problem with a relatively easy approach. Since you didn’t mention the specific spark plug you recently had installed some 20 hours ago, I’ll offer a few comments regarding spark plugs. A quick check of Lycoming Service Instruction 1042Z or its latest revision is a good place to begin to learn what specific spark plugs are approved for your O-320-D2A engine and learn more specifics as to what the manufacturers information means.

First, you should choose a spark plug that would work best with the type of flying you do. As an example, if you do mostly local flying, you may be better off using a hotter spark plug, such as a Champion REM40E or an equivalent approved spark plug.

Speaking of spark plug heat ranges, this can be confusing too, so referring to the manufacturers specifications is recommended. As an example, on the Champion and Unison (now the Tempest product line) spark plugs, the higher the number, the higher the heat range or hotter plug, while the SL Auburn spark plug uses a lower number for its higher heat range plugs, making it more important to check the manufacturers specifications. So, if you are not running a hotter plug now, I’d recommend that you consider that option to begin with.

If you suspect spark plug fouling from lead deposits, which I would find unusual because you mentioned that you lean during taxi, then you may want to consider the Champion REM37BY, which offers better service when lead fouling occurs.

The most important thing I can tell you besides this spark plug information is just be certain you return the mixture to full rich before commencing your takeoff roll!

The second thing you may want to try is complying with Lycoming Service Instruction 1294, dated Jan. 25, 1974 — so you can see it’s been around for some time and, in most cases, is very successful when situations like you’re experiencing have come about. I’d suggest you have your maintenance technicians refer to this Lycoming publication, which details alternate ignition cable routing, and move forward from there. It’s really very simple and the only cost is for the labor to do the work with no cost for any parts on most engine installations.

Regarding the rest of the information you provided on your engine, I’d say all is well, including the oil consumption you mentioned.


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


  1. Daniel Carvalho says

    Hi Paul,

    Could you please help me finding the problem with my O-235-L2C engine, since it’s overheating on cylinder #1. It as a few hours since Overhaul (+/- 20hours).

    It was installed new cylinders from Lycoming (not overhauled units) and the spark plugs are the Tempest UREM37BY. The #1 spark plug is full of lead fouling.

    I’m operating the aircraft in training in Romania.

    could you please help me?

    Thank you in advance.

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