Engine rough idle
O-320-E2D, low compression pistons (150 hp), MA-4SPA carb. 350 hours since field overhaul.
Ran very nicely for about 7 years, then rather suddenly developed a very rough idle.
Diagnosis has shown a very rich mixture at idle, up to about 1300 RPM. At 1300 RPM if I lean aggressively, the RPM rises to about 1550. The engine stumbles irregularly, and produces puffs of black smoke out the exhaust below 1300 RPM. Have removed primer system, and blocked off cylinder ports with solid plugs.
I initially focused on the carburetor. I thought that the idle circuit in the carb might have gotten plugged. So I borrowed a carb to try, and it ran even a bit worse than mine. I have varied the idle mixture screw from fully in, to way out, and no difference in the symptoms!????
Here?s the strange thing: I have now tried 3 different carburetors, and the same symptoms show up with all 3 carburetors!
The engine runs progressively smoother at higher power settings, and seems to develop full power. Static RPM is 2150, which is no change from the last few years of flying. If I lean at full power, I can get a 40 to 50 RPM rise, as I apparently lean to best power.
What, other than a carburetor, could cause excessively rich mixture at idle/low RPM?
Mahlon?
O-320-E2D, low compression pistons (150 hp), MA-4SPA carb. 350 hours since field overhaul.
Ran very nicely for about 7 years, then rather suddenly developed a very rough idle.
Diagnosis has shown a very rich mixture at idle, up to about 1300 RPM. At 1300 RPM if I lean aggressively, the RPM rises to about 1550. The engine stumbles irregularly, and produces puffs of black smoke out the exhaust below 1300 RPM. Have removed primer system, and blocked off cylinder ports with solid plugs.
I initially focused on the carburetor. I thought that the idle circuit in the carb might have gotten plugged. So I borrowed a carb to try, and it ran even a bit worse than mine. I have varied the idle mixture screw from fully in, to way out, and no difference in the symptoms!????
Here?s the strange thing: I have now tried 3 different carburetors, and the same symptoms show up with all 3 carburetors!
The engine runs progressively smoother at higher power settings, and seems to develop full power. Static RPM is 2150, which is no change from the last few years of flying. If I lean at full power, I can get a 40 to 50 RPM rise, as I apparently lean to best power.
What, other than a carburetor, could cause excessively rich mixture at idle/low RPM?
Mahlon?