Magnetos

[M20F]

I have never had to ground lean my F except in places like KTOR where you are 4000 MSL.  

I would echo Ross that properly setting idle mixture is more than likely the real solution.  Leaning can address it if set wrong but it is just working around the issue.  The exception being if you operate out of a very high airport. 
 
I have run Champion, Tempest, massive, fine wire, etc.  No issues with fouling plugs. 

[Andy95W]

The OP owns an M20C.  His carbureted engine is a very different animal than the injected IO-360, even when the idle is set correctly.

For cooling purposes, the M20C carburetor is set to make the mixture very rich.  Setting the idle is done at a very low rpm, but when the throttle is advanced to a normal taxi setting (about 1000-1200 rpm), additional idle jets open and allow more fuel to the throat of the carburetor.

That is why you have to lean so aggressively on the ground, even when the carburetor idle is set correctly.

[DJE22]

On 6/29/2022 at 10:18 PM, SessionDrummer said:

I bought a 1977 M20C last year, and I'm a first-time owner.

Just did the annual. Prior to that, the mags had been running rough during run-up and were due for a 500-hour. We sent them out, and no overhaul was necessary. But, they got a good once-over. My A&P was comfortable with the result, and let the plane out of maintenance. But, by my second flight, the run-up was rough again. Sent her back to maintenance and replaced the ignition leads. Everything checked out with the mechanic, and they ran smooth on my first couple flights. But, sure enough, they're running rough again.

The mags are Bendix, and the engine is the 0-360-A1D. I'm starting to wonder if it's something else.

Any ideas here?

It may be as simple as leaning it out (significantly)  immediately after start-up and while operating on the ground….

[SessionDrummer]

For those kind enough to have offered counsel on this thread....

It would appear that part of my problem was a dying cylinder (which I was surprised didn't come to light in the recent annual). Replaced it, and also adjusted the mixture control under the hood. With those items checked off, and also leaning aggressively during ground maneuvers.... the problem seems to have corrected itself.

[carusoam]

Great follow-up drummer!

Leaning vs. not leaning on the ground…. M20C specific…

If you don’t lean aggressively….

Small lead pellets collect in the lower spark plugs…

This only occurs when the engine is cool…. And fuel is extra rich…

 

The extra benefit of leaning aggressively…. If for some reason, the mixture gets forgotten…. It becomes obvious when you push the throttle in….

At low throttle… 1krpm you might see FF from 1.5 to 3gph… very lean to full rich…  when taxiing rpm should be near 700rpm….
 

The engine at idle won’t know it is lean…

The number of throttle pumps for priming is important to know… the accelerator pump, when it wears, may use a lot more pumps than when it is in good condition…

Keep your eyes open for carb wear issues…. If it hasn’t seen an OH in decades…..

 

Lets invite @Hank to stop by… he has plenty of M20C experience be can share…

Best regards,

-a-

[Igor_U]

23 hours ago, SessionDrummer said:

It would appear that part of my problem was a dying cylinder (which I was surprised didn't come to light in the recent annual). Replaced it, and also adjusted the mixture control under the hood. With those items checked off, and also leaning aggressively during ground maneuvers.... the problem seems to have corrected itself.

By a dying cylinder, do you mean shot piston rings would leak oil and foul the plug or some valve issue? I'd be interested to hear more...

[SessionDrummer]

I'm not very adroit with the mechanical stuff, so I probably won't answer to your satisfaction. But, here goes....

In the process of trying to diagnose, we had already replaced the engine leads and sent the mags out for a serious once-over (a full overhaul wasn't deemed necessary). Since I was leaning very aggressively during ground operations, and STILL getting some weirdness during mag checks on run-up, my A&P decided to double-check the cylinder compressions (as mentioned, it just sent thru annual in May). Sure enough, cylinder #3 was sketchy. And, then I found a logbook entry from about 500 hours ago where that cylinder was acting up a little bit. But, as far as I can tell, nothing significant was ever done about it. Since I was (very luckily) able to find a brand new cylinder, I decided to replace it.

That, along with a minor adjustment with the mixture control, seems to have covered a multitude of sins.