Proper leaning can set engine power parameters for best range, maximum endurance, best economy, best speed, or anything in between. Best economy runs the engine at peak power. This is the top of the EGT scale. The best power is slightly richer by about 100 degrees EGT temperature. This uses more fuel with a slight improvement in airspeed. Modern engines can be operated with manifold pressures of square or even over square (manifold pressure higher than RPM) without harm. Cruise power settings lower than 75% can be flown at peak EGT.
The standard leaning procedure is to lean by pulling the mixture until the engine runs rough, and then turn it in until it smooths out. If using an EGT, pull the mixture until the EGT gauge read as high as it will go. You may need to reposition the EGT needle if it goes all the way to the top. Once it has stabilized at a high point that the engine has gone rough, enrich the mixture to get at least a fifty-degree drop. Proper leaning will coincide
Witha throttle back to full idle, fuel flow is set by the idle adjust screw. Full idle is usually about 600 RPM in light aircraft engines. At idle you cannot taxi nor keep the engine running if the weather is cold.
Typically, 800-1200 RPM are the ground operating speeds. At these power settings leaning has noticeable effect. You should lean regardless of the fuel used. At these powers leaning can be done until the engine coughs; then enrich a bit. An engine that dies when you add power is a good indicator of correct low power leaning. You cannot damage the engine by leaning It is possible to foul the plugs if you do not lean enough, regardless of the fuel used.
By operating an engine at low temperatures with a rich mixture you are sure to reap a harvest of carbon deposits. With leaded fuel you will harvest lead deposits. Proper leaning will solve the carbon-fouling problem but will not prevent lead fouling. Only by keeping the engine temperatures high enough to keep all the lead vaporized can you prevent lead fouling. The leaner the mixture the hotter the engine and the less fuel to supply lead needing vaporization.
One of the most common operating problems is on starting. The throttle is in so far as to cause the engine to operate at relatively high power before the oil has had a chance to circulate. A surging engine start will cause excessive wear throughout the engine. Abusive operation may involve taxiing at high power and holding the brakes on to keep the speed down. This can use 50 hours of steel off the camshaft and scuff the pistons against cylinders. Keep the starting rpm low and let the oil work before moving. If it is cold the oil will be so thick that some oil passages will be plugged, you will get squirts instead of sprays and there won't be much splash for the splash-lubricated parts. High starting oil pressure is indicative of plugged passages.
If you have ever taken an air filter and cowling off and observed the action and location of the throttle (accelerator) pump vs the prime (look where the primer activated fuel goes in) you will see the wisdom of NOT pumping the throttle to prime. Also it is important to prime and then IMMEDIATELY turn the starter, as even he primer fuel will drain down to the carb and intake box. The intake box is where you DO NOT want the fuel to be. I am talking about typical Lycoming carb on the bottom trainers and the like.
Abusive engine operation creates thermal shock to the engine and its parts. Ideally every engine would be preheated. 110V preheat systems now exist. Pre-oilers are in existence. Every cold engine start has metal to metal parts scraping each other without oil film separation. Abrupt throttle operation, cowl flaps, and rpm adjustments cause variations in heat and cooling sufficient to damage even the most rugged of engines. Do what it takes to keep the engine warm within its operating temperatures and avoid extremes of heat and cooling.
Keeping an engine log can prevent some failures. Keep a record of fuel flow, oil pressure, temperatures, and electrical readings. Record oil consumption and changes. Have the oil lab checked for metal to determine what and how much wear is occurring. Some is normal but too much of one kind is significant. Poor lubrication causes excessive wear.
You can take better care of your engine if you avoid those throttle changes that cause sudden heat changes. Abrupt throttle movements cause cylinder head, exhaust header and turbo cracks. A sudden power application or sudden shut down (As when stopping) can cause bearings to coke up, overheat and seize. Counterweights can be de-tuned by sudden throttle movement.
Under certain conditions an engine may be unable to obtain fuel to the carburetor due to vapor lock. If the fuel lines have curves that allow the formation of hot air pockets the fuel may be unable to force its way through the blockage. A hot engine compartment may make it impossible to start the engine and under certain conditions the blockage may exceed the ability of a pump or gravity to move the fuel. A dent or a crimp in a fuel line can decrease fuel flow ability. A dent in a fuel injector line is particularly dangerous where takeoff performance is required.
A pilot who has predetermined power and configuration to be used in a journey is reducing his piloting workload. He knows ahead of time the Vy/Vy speeds to be used in climb. He has picked an altitude for winds and fuel efficiency, he pre-plans his descent to make the best use of altitude for airspeed, and arrives at the pattern for an efficient arrival. Additionally, he knows the trim, mixture and power changes before the needs occur.
The bible of engine operation is the POH but each aircraft model year varies so only a POH specific to the aircraft and year should be used. When an aircraft is a mixed breed such as having a later engine different than that of the aircraft year, you must develop your own POH.
Lycoming has some very specific suggestions for high performance aircraft. Don't lean below 5000' when using climb power. Don't reduce manifold pressure over five inches at one instance. Better to do one-inch every one-minute. Maintain 15 inches during descent with rpm set to lowest cruise to prevent piston flutter. Fixed propeller operations should be limited to 400-rpm reductions at a time. Descents faster than high cruise and more than 1000 fpm are not recommended.
Leaning is performed to get the maximum power with the best air to fuel mix. Excess fuel will cool the engine, but too much can dilute the oil needed to lubricate the cylinder wall. This causes excessive engine wear. An excessively rich mixture tends to foul plugs in smaller engines. A lean mixture will gives better power, but excessive leaning will cause higher temperatures in the combustion chamber.. Excessive leaning over a period of time can cause serious engine problems and lack of power. Aircraft with an exhaust gas temp gauge, (EGT) gives positive indicators in the leaning needed to attain best operation. Without the EGT lean by incrementally pulling the mixture until the engine noticeably loses power and then put the mixture in until you get the max rpm back. This setting is correct for a specific altitude for that time. Any other altitude or time requires new settings.
Partial power takeoffs are worse for engine than the use of full power. Full throttle opens up a fuel jet that allows extra (cooling and not used) fuel to enter the cylinders. This can be demonstrated if you have a cylinder head temperature gauge.
