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Rough Running M20C. Fouling?


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Okay Mooney friends, got some troubleshooting to do. On a long (2hr) cross country a couple days ago my '67 M20C, she began to develop an occasional stutter while flying through rain. Thinking it was carb ice, I applied it, but that didn't seem to be the case. I got to clear air, tried leaning, enrichening, and she still held a slight hesitation and occasional RPM flicker for the remainder of the flight. Over the airfield at 3,000, a mag check produced a MAJOR loss of power on the left mag, accompanied by backfiring. Ground run up produced the same result, barely staying running. A mechanic friend on the field looked things over and found that the plugs (with 500 hours) to be worn, with three of them showing signs of shorting "two completely dead, one on its way out" according to the mechanic. He replaced all 8 plugs with brand new Champion plugs, runup checked okay. 

I just flew the plane, all checked perfect on the run up. I did a long taxi out to look things over, and leaned thoroughly while taxiing out. (OATs around 90F) Takeoff, climb, and cruise to the next airport over were perfect. On takeoff for the return, all appeared normal. Climbing through 1,500ft, I experienced a significant loss of power lasting about 5-10seconds. Mixture was full rich, All temps and pressures looked good. Thinking it was fuel related, I made a turn towards the airfield and switched fuel tanks, checking all normal emergency items. The issue cleared completely, and I spiraled up above the field. An airborne mag check above the field produced an extremely rough running mag, similar to before. I pulled power and decsended down to the field. After rollout, I did a mag check and 1,700 and 2,000, both of which were clear. After taxiing in, I did a full power run to try to replicate the symptoms. With the left mag selected, she slowly began to run rougher, eventually developing the same backfiring, which disappeared after switching back to both, but kept a hesitation while on both or left. 

Thoughts? Mag, plug? Running too rich, fouling plugs repeatedly while operating at the higher temps? I've never run it in temps this warm before. I'm thinking that's the case. 

Edited by CJpilot316
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PP thoughts to follow...

1) What plugs did you have in before...? (Champion). Exact part number if you have it...

2) What plugs did you go with this time...? (Champion???). Exact part number...

3) Are you familiar with Champion and their ever increasing internal resistance...?

4) Are you familiar with how this internal resistance can affect the health of a magneto...?

5) When was your mags last OHs?

6) Don't toss out the evidence yet...

7) read up on the effects of too high of a resistance caused by bad plugs, on magnetos...

Wish I could point you to the thread.  But you can find it or somebody else can point you in the right direction...

PP thoughts only, stuff I learned by reading MS...

Wish I had sunnier news than that. :)

Best regards,

-a-

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Since it sounds like it is only doing it on the left magneto, that pretty much narrows it down for me. 

Internal failures of the magneto include sheared teeth on the distributor drive, heavily carbonized points or carbon in the distributor portion of the mag, bad leads that start to break down with heat, or a bad coil. Bad coils often also tend to get worse with heat. 

Probably time to send off that mag for a 500 hour inspection (about $700) and while it's out check the harness. 

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Just now, Andy95W said:

Since it sounds like it is only doing it on the left magneto, that pretty much narrows it down for me. 

Internal failures of the magneto include sheared teeth on the distributor drive, heavily carbonized points or carbon in the distributor portion of the mag, bad leads that start to break down with heat, or a bad coil. Bad coils often also tend to get worse with heat. 

Probably time to send off that mag for a 500 hour inspection (about $700) and while it's out check the harness. 

Or one of the new Champion plugs on the left mag is bad

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4 hours ago, neilpilot said:

Or one of the new Champion plugs on the left mag is bad

Yep! Clean, gap, test and swap plugs top to bottom, see if the problem moves to the right mag--if so, you have a bad plug. If not, then you have a left magneto problem. Been there, done that, glad to finally have it behind me.

Bonus for your F:  your left magneto is not hiding behind the enclosed battery, so you have less to take apart to get access. That stupid batters box has four bolts through the firewall into the pilot footwell, including one behind the brake master cylinder . . . so it will be easy[ier] for you.  ;)

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