Fixing fouled spark plugs
My partner drew the long straw--the left seat. After pulling the airplane out of the hangar and preflighting, we climbed aboard and in a few minutes were taxiing to the active runway. While working through the Before Takeoff checklist, a familiar problem reared its head. When my partner cycled the magnetos, or mags, one yielded a drop in engine speed of about 100 rpm, the other a 300-rpm drop.
The owner's handbook for our airplane says to check the magnetos at an engine speed of 2,200 rpm. The procedure is one we're all taught from the beginning: Switch one mag to Off, then back to Both, then the second mag to Off, then back to Both. If one switch controls both mags in the airplane you fly, you'll probably turn the switch--which may be the key--from Both to Left, back to Both, then to Right and back to Both.
Switching off one mag to check the integrity of the second should result in a slight drop in engine and propeller rpm (in a direct-drive engine; on a geared engine propeller speed is lower than engine speed). Our handbook says the drop in rpm that results when switching off one mag should not exceed 125 rpm, and the difference in drop between the two mags should not exceed 50 rpm.
The drop in rpm on the mag check occurs because only one plug, rather than two, is igniting the fuel/air mixture in each cylinder. When both mags are working properly and firing both spark plugs at the same instant, the fuel/air mixture burns more quickly and efficiently, which yields a higher rpm for the same throttle setting. The Before Takeoff mag check tells us whether or not each mag is performing properly. It also tells us if the spark plugs associated with that mag are firing properly.
It stands to reason that a significant drop in engine/propeller rpm noted when checking the mags before takeoff means one of two things: Either the operating mag is defective, or one or more of the spark plugs associated with the operating mag are fouled. A fouled plug means that deposits from the lead additive in avgas have collected on the spark plug electrodes, or the plug tip is coated in sooty, unburned carbon or oil. Carbon deposits are indicative of an overly rich fuel/air mixture. An oil-fouled plug is evidence that oil is seeping by worn piston rings or the intake or exhaust valve guides.
Either condition can cause the spark that jumps between the two electrodes on the spark plug to be weak or even nonexistent. A heavy deposit can bridge the gap between the spark plug's electrodes, preventing any spark from occurring. A weak or nonexistent spark will not properly burn the fuel/air mixture that has been sucked into the cylinder on the piston's intake stroke and pressurized on the compression stroke.
No spark means no combustion; a weak spark means poor combustion at best. Either results in a loss of power signaled by a drop in engine rpm and a noticeable roughness in the engine. That's not the way to start a flight, and that is why we do a mag check before takeoff. Really, it should be called a magneto/spark plug check.
Back to the runup area and our Before Takeoff check. The 300-rpm drop we saw on the mag check was more than twice the allowable maximum. Same for the difference between each mag. Not good, and it wasn't the first time we'd seen this.
The quick way to find out whether a mag drop problem is due to a defective mag or fouled plugs is to try to clean the plugs on the spot. It's a simple procedure. After noting a large rpm drop with one mag, place the mag switch back to the Both position, advance the throttle to increase rpm, then slowly lean the mixture until the engine begins to run rough. Maintain that throttle/mixture configuration for a few seconds to allow the lead deposits or carbon coating to burn off, then advance the mixture and re-check the mags at the proper rpm.
The procedure helps to clean the plugs and restore smoothness to the engine. If the roughness remains, it's time to scrub the flight and taxi to the maintenance shop to have the magnetos and plugs checked. On this day, we were able to clean the plugs and make our flight.
Plug fouling is nothing new where I live and fly, in southwest Florida. The hot and humid ambient air is less dense than in cooler climates, and less-dense air tends to make for a richer fuel/air mixture. Even though our airport is just a few feet above sea level, the density altitude often can be 1,500 feet or higher.
I've always made it a practice to do something while taxiing on the ground that we pilots routinely do in the air after leveling off at cruise altitude--lean the mixture. The vast difference in air density on the ground and at altitude means we have to lean the mixture at cruise to achieve efficient, economical fuel consumption. But, while leaning at altitude is a must, leaning on the ground is more of a personal choice.
Our airplane has Textron Lycoming engines, and Lycoming does not address leaning while taxiing--even though the owner's handbook warns that "idling at low rpm...may result in fouled spark plugs." Instead of leaning, Lycoming recommends a ground idle of between 1,000 and 1,200 rpm to avoid lead fouling, and to steer clear of idling with the throttle closed.
The engine manufacturer also recommends that, after landing and before shutting the engine down, the pilot idle the engine at 1,200 rpm for one minute, then 1,800 rpm for 15 to 20 seconds, and finally at 1,200 rpm to shut down by pulling the mixture control to the Idle Cutoff position.
The problem I have with those recommendations is that taxiing at 1,000 to 1,200 rpm means riding the brakes to keep taxi speed in check. Also, it is downright discourteous to pull onto the ramp or into a parking space and then run the engine at 1,800 rpm, even for just a few seconds.
So, why not lean while taxiing? The throttle setting is so low when taxiing or sitting in line for takeoff that there is no chance of damaging the engine with high cylinder head or exhaust-gas temperatures or detonation by operating on the lean side of peak. Nine years of taxiing around in hot, humid Florida air has demonstrated to me that leaning while taxiing works to keep spark plugs relatively clean.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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