QUESTION: I have had instructors insist that I lean the engine while taxiing my O-320 Lycoming.
My current Citabria has a JPI 700 engine analyzer with fuel flow.
In the testing that I have done, the EGT will change less than 20° and fuel flow less than 0.1 gallon when adjusting mixture at less than 1,000 rpm. I have leaned until the engine chokes before there is much of a change. I suspect that the mixture control does not work in the idle circuit and the leaning procedure is either for other types of engines or an old wives’ tale. What say you?
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ANSWER: Ah, the old instructor’s lean while taxiing subject again! Morris, this is a subject that comes up quite often during any gathering of those who fly general aviation piston powered aircraft, so let me offer my thoughts on this.
First of all, I hope your instructor explained why he encourages this practice and the main reason it’s done. For those of you who may not be familiar with this procedure, it’s generally done to relieve and/or prevent spark plug lead fouling.
Assuming your instructor wants you to use this procedure to prevent lead fouling, there are a few other things you might want to check, also. Make certain your IO-320 engine is using the proper spark plug, as called for in the latest revision of Lycoming Service Instruction 1042. Just installing a hotter spark plug could be the answer to any spark plug fouling issues you may have. Be very careful to review the Lycoming Service Instruction, paying particular attention to how the various spark plug manufacturers identify the heat ranges of their plugs, i.e., higher numbers don’t necessarily mean a higher heat range or hotter plug.
Also, if spark plug fouling is an issue, you may find that using one of the extended nose spark plugs, such as the 37BY series, will alleviate the situation.
From the operational side of the picture, there are a couple of things you can do as well. First, try to avoid prolonged periods of idling and power-off descents.
I believe engine manufacturers have been reluctant over the years to recommend leaning during taxi to avoid spark plug fouling because of the fear the pilot would fail to return the mixture to full rich prior to takeoff. We all know what that could lead to and, trust me, it isn’t good.
Let’s cut to the chase. One of the best ways to avoid spark plug fouling is to bring the engine RPM up to around 1,200. This causes the nose core temperature of the spark plug to increase to a point where it will scavenge lead build-up by design. Some manual leaning at this point will assist in bringing the temperature up and should prevent any lead fouling from occurring. Don’t forget to refer to the airframe manufacturer’s “”Check List”" prior to takeoff.
Morris, don’t be concerned with what the fuel flow or the EGT does while operating per the aforementioned, but I’d suggest you observe your CHT and not allow it to approach the maximum allowable, which I doubt it would do during the short time you’d be using this procedure.
For those of you who are operating engines utilizing carburetors, you may use this procedure if you are having lead fouling problems. I would suggest that you confirm that your carb heat system is fully functional and not allowing any hot air to enter the induction system when the carb heat is in the off position. If there is any leakage of hot air, this could cause a rich mixture condition, resulting in excessive lead build up on the spark plugs. Even though you may be able to improve the engine operation as far as lead fouling goes, using the procedure discussed, it would be wiser to take corrective action now to repair the carb heat system and improve the over-all operation of your engine. Fuel savings would be an added benefit.
Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.