Airplane: Piper Warrior II (O-320 engine)
Airport: CYGK (north shore of Lake Ontario
Time: afternoon
Weather: CAVU; 15°c OAT; prevailing westerly winds, but airport in local light lake breeze from the south
About five miles back from landing today, I noticed the engine running rough whenever I reduced power — it was fine above 2,200 RPM, but started vibrating at any lower setting, enough that the plane (carbureted) shook noticeably. I accepted the vibration, landed, and taxied to the ramp, with the plane still shaking. I switched mags while taxiing and tried different mixture settings, with no difference. When I tried to shut down with the mixture lever, the engine kept running (even at full lean), and I had to stop it with the mags.
Your diagnosis? (I’ll post the answer in a day or two).
Land and Hold Short 
Carb ice?
That was our first guess chatting on the ramp, but it had two (apparent) strikes against it:
– carb ice wouldn’t (AFAIK) explain the engine still running at full lean
– carb ice is rare in the Warrior, because the air intake is right next to the muffler, which heats the air by default
Per my G+ comment; I’m guessing gunge or bad seals in the carb.
Carb ice is the go to answer (unless of course you’re fuel injected), until you mentioned the engine not stopping. If mixture idle cutoff doesn’t work then fuel is entering the system from somewhere. I’d have to check the POH to be sure but obvious sources are mixture control (cable broken or stuck at some position), the primer (stuck open) or the carburetor itself. Regardless, I’m guessing that at higher RPM that the excess fuel wasn’t that noticeable since the engine is consuming close to max as it is but at lower RPM one or more (but obviously not all) cylinders were being flooded, causing the rough running. If I had to hazard a guess, if say the carburetor since if it was the primer leaking you should have experienced roughness during taxi, which presumes that the primer broke during start up.
@douglasr is correct — the primer knob had simply popped very slightly open.
I usually verify it in my pre-landing checks, but since the roughness started *before* then (when I initially cut power for descent), I was distracted planning a high glidepath (so that I could make the runway in case of complete engine failure), and didn’t pull to verify that the primer was locked.
I’ve forgotten to lock the primer after starting (which shows up in the run-up) but I’ve never had it open in flight.
Me neither. I figure it must have been right on the edge of being locked — either I turned it too far, or not far enough.
David, do you have an engine monitor widget in the plane? Is a slow leaning part of your runup (the extra fuel could’ve shown up as a larger rpm rise / later drop than usual)?
I knew this because the same thing happened to me when I had a Cherokee 🙂
Always triple checked it was tight after that..