Credit to Peter MacPherson:
I have a 1980 Sundowner with an O-360-A4K engine. Lately, no matter how aggressively I lean, I get a 200-300 rpm drop on the left mag when I do a mag check during run-up. It’s always on the left mag, and never on the right mag. Is this a bad plug that keeps getting fouled? I clear it every time, by running the rpm up to 2200, and leaning it out until rough for 15-20> seconds. Any ideas would be appreciated.
Credit to Dan Kirby:
Peter,
The best thing I’ve found is to switch plugs top to bottom. Right mag fires top on one side and bottom on the other, and vice-versa. If your trouble goes to your other mag, it’s a plug.
Credit to Bob Steward:
The side draft HA-6 carburetor on the -A4K/J/M series has bedeviled many pilots, and has a few problems that are not understood by most every instructor or pilot I’ve ever met. This carburetor runs MUCH too rich at low power settings, and can easily foul a plug between start up and the run up pad. In fact it probably was between pulling off the runway and taxiing back to the ramp at the end of the LAST flight, and the time from re-start to the next run up, that was the
rest of the time period for the lead fouling.
The only way I’ve found to get good service out of this engine/carburetor combination is to start the engine normally, set the idle speed to 1000 RPM, and then lean to peak RPM (usually 1150-1200 RPM), and then retard the throttle to the RPM needed for warm up and taxi out. Going back to rich mixture for the run up is assured, because the engine will die for lack of fuel if the throttle is advanced beyond about 1400 RPM. Upon landing, after clearing the active, stop and set the RPM to 1000, and then re-lean to peak RPM and adjust throttle as needed to taxi back to the ramp.
Using this process, one will eliminate the cause of most every lead fouled plug with an O-360-A4K/J/M engine, without an illegal modification of the carburetor (which also fixes the problem, but we won’t go there).
As to why it’s ALWAYS the Left (in your case), you need to understand how the mags are wired to the spark plugs. Each cylinder has 2 spark plugs, a top and a bottom. One is fired by each mag, and to even out mag drop, each mag fires the top plugs on one side of the engine and the bottom ones on the other. So your Left mag is firing 2 bottom plugs and 2 top plugs. Gravity causes the bottom plugs to be more likely to foul, since the deposits flow down into the space between the insulator and the outside shell of the plug, and on the top plugs the deposits tend to drip OUT of
the plug into the combustion chamber.
So you likely have a plug or cylinder problem that is “helping” the fouling occur with a little oil or some other contributing factor that makes that particular plug more likely to foul. You could try swapping the bottom plugs from one bank to the other and see if the problem migrates to the
right mag, which would indicate a weak plug (cheapest possible problem) or if it stays with the same mag, it might indicate a bad plug wire, or carbon tracking in the mag, or possibly a problem with that one cylinder.