To Lean or Not to Lean While You Taxi
Aviation is full of tribal knowledge from pilots, both past and present. One trick that often comes up, usually in the early part of your flying career is, “you should lean the engine while on the ground to prevent spark plugs from lead fouling.” In theory, this seems like a completely logical method. Less fuel equals less lead deposits in my spark plugs. So why do Lycoming Engines’ Operators Manuals recommend different?
Is there a better way? Not surprisingly, Mother Nature throws more unexpected factors into the situation, and while you might feel better trying, you can’t fool Mother Nature.
No you can't and when operating as per Lycoming I always had far more Pb deposits, who are they trying to kid here?
WHY SHOULDN'T I LEAN DURING TAXI?
To start, Lycoming Engines’ Operators Manuals clearly instruct the pilot to keep the mixture at full rich during ground operations and engine run-up. By leaning on the ground, you may not entirely eliminate the problem of spark plug fouling. While you’re introducing less lead into the system,
you are also reducing the cooling effects of the fuel.
COMPLETE BS, at any power the combustion curves applyYour airplane’s cooling package is not designed for prolonged periods of zero airspeed operations. Of course, we have the propeller wash, but it isn’t the same as having airspeed blowing over your air cooled Lycoming Engine. Leaving the mixture rich helps cool your engine since you don’t have that airspeed yet.
Supporting a myth by telling the truth…clever trick 
Taxi and take-off can be a busy time for a pilot, especially if you are at a busy airport. Imagine you are at a complicated international airport. You’ve leaned your mixture to try to prevent the engine from fouling up. The tower is yelling at everyone,
(Really??)making things stressful. There are jets all around you with engines big enough to suck up your entire airplane.
(Sounds like a Greenpeace or PETA commercial- Are you scared yet?) You are given a challenging departure procedure and you are trying to brief yourself. Before you know it they give you a clearance to take-off-- not in 2 seconds, but right now. You start your take-off roll and realize the engine seems weaker than normal. Maybe it’s even got a little vibration to it. But you are committed now, and you’re running out of runway. Because of the heavy workload, you may have forgotten to push the mixture back to full rich, and it is negatively affecting the engine’s power for takeoff and climb. This happens more than you might think.
(Only if they are poorly trained, Line up and go in a LYCOMING is simple, boost pump on mixture rich and GO!!!!Add to this that Lycoming’s Service Instruction 1094 recommends take-off be conducted at the full rich setting when at density altitudes below 5,000 feet; above 5,000 feet density altitude may require some leaning to achieve acceptable power and smooth operation.
Complete myths designed to scare you. The APS method does everything it should and nothing it shouldn't. Do it right and the engine will stumble and almost quit…..you will push the red knob in GUARANTEED
WELL THEN, WHAT SHOULD I DO?
Make sure your mechanic is doing all of the proper spark plug maintenance. Lycoming Engines recommends spark plugs by part number in our Service Instruction 1042. It may be a good idea to check your plugs to make sure previous owners or mechanics installed the correct part numbers. Also, the spark plugs should be cleaned, gapped, and rotated at least every 100 hours of operation. If you are operating Champion spark plugs, information on those procedures can be found on their website here: Aviation Service Manual.
Ensure that your carburetor or fuel injection servo have the idle settings correctly adjusted. A proper idle set up will help reduce the possibility of lead collecting in the cylinders and spark plugs.
Your mechanic should run the aircraft until the engine oil temperature is up to the normal operating temperature before making any of the following adjustments:
Idle Speed - Increase the throttle to roughly 1,700 RPM and rapidly reduce it back to idle. Generally the airframe manuals recommend the idle speed be set between 600 RPM and 700 RPM. If it is outside the specification, it should be adjusted using the idle stop screw.
Idle Mixture - While at idle, slowly pull the mixture lever back to idle cut-off. You should notice a slight rise in engine RPM before it shuts off. Lycoming Engines recommends a rise of 10-25 RPM. If there is no rise, the engine is too lean and needs to be richened slightly. If it rises more, the engine is getting too much fuel and needs to be leaned.ALL GOOD ADVICE
4. Study the figure to the left. Lead fouling is greatly affected by the temperature of the gasses inside the cylinder. How can we take advantage of this? Shortly after you start your engine and have positive oil pressure, increase your throttle setting to obtain an engine speed between 1,000 RPM and 1,200 RPM. This will allow enough heat to be generated to properly scavenge lead from the cylinders. Anything less and the temperatures inside the cylinder will cool enough to form lead and carbon deposits, even if the engine is leaned.
1000 RPM is enough, think noise and stone damage
5. Just prior to engine shut down, increase the throttle setting to about 1,800 RPM. Operate here for 15 to 20 seconds. Reduce the throttle back to 1,000 RPM to 1,200 RPM and immediately shut down the engine using the mixture control. This will ensure the engine gets properly scavenged of lead just before shut down.
Bad idea, heat the engine up to achieve what? Make a lot of noise, rotating props are best not where people are around….and I never found doing or not doing this made any difference, nor do thousands of others.
FINAL THOUGHTS
Don’t be afraid to question procedures that aren’t found in the Pilot’s Operating Handbook (POH). Poor practices which then become bad habits may have been passed down from pilot to pilot for generations; leaning during ground operations is one example. Always refer to the aircraft’s POH for the approved practices or operation instructions. If someone recommends something different than the POH, ask them to provide approved data to allow you to operate the aircraft using their method.
Don't be afraid to question your POH, much of what is written in them is not scientifically correct, is misleading or contradictory to the engineering data in the same manual……so Be a critical thinker. Just because it is a POH does not mean it is correct, approved BS is BS all the same.