Rough R Magneto

[pablo]

Lately the R magneto on my 7AC Champ 65 Cont runs occasionally rough (150 -200 rpms drop), mainly when engine first started, and clears itself within 5minutes. We have checked spark plugs, harness, compression, etc. Sent magneto to the shop, where it was overhauled, finding no explanations, reinstalled, timed, and corrected for about 5hrs. Now doing it again.
Suggestions greatly appreciated.
Pablo

[joea]

Which plugs is this mag firing? Uppers or lowers?

Type of harness?

[pablo]

Joe:
Shielded Harness, firing the low spark plugs in the R side and upper spark plugs on the left side of engine (will confirm with my mechanic and let you know if this information is wrong)
Pabl

[Dennis]

I had a similar problem. Turned out to be one of the nuts on the mag wires at the back of the bendix mag was loose allowing moisture in. Once the mag operated for a little while, the moisture would evaporate but returned when the mag cooled off.

Dennis

[pablo]

Joe:
My previous information is wrong: the R magneto fires the upper spark plugs .
Pablo

[MikeB]

Pablo,
I know you've checked compression but how does it feel on a 'pull through' on the prop. Any indications of weakness? It does sound like a slightly stuck valve, commonly called 'morning sickness'. Also, try moving the plugs around to the other magneto and see if the problem moves. Like I used to tell my students "Troubleshooting is a process of elimination starting with the simplest things first".

MikeB

[pablo]

MikeB:
I know what you mean by how does it feel when the prop is pulled: I had a stucked valve sometime ago.
Progress Report: My mechanic changed the oil (25hrs), fool around with the spark plugs, double check wiring, etc. Did not find anything. R Magneto is behaving as brand new. Hope stays like that.
I appreciate everybody's opinion. My tool box is a Philips screwdiver...
Pablo

[flyingfool]

I used to have the exact same problem. It would run rough during run up. I would take off and come back for landing. Do a run up and it would be fine.

Here is what I did to cure it. At least it has not happened this year like it did last year

1) I started flying more.
2) I changed from 100% straight 100LL to a mixture of 75% autogas ad 25% 100LL
3) Started adding 2 oz. of MMO for every 5 gallons of my mixed fuel in #2 above.

I have not had the problem since. I do not know if it was minor lead fouling (I too checked the plugs and nothing was suspicious) or the simple fact I was flying more or the MMO and morning sicknes. But All I can say is that I no longer have this rough running right mag. So it was one or a combination of the three things above.

That was my experience, YMMV (Your Milage May Vary)

[MikeB]

Regarding the 100LL: as far as fouling plugs, it never seems to bother my A65. Granted, I tend to not make many cross country trips so it gets a lot of auto fuel. On the other hand, when I had my Cherokee 140, the Lycoming absolutely hated 100LL. I tried leaning when taxing and all sorts of tricks but nothing seemed to work. Especially when the trips were short hops and the engine didn't get up to temperature for any amount of time. On cross countries it didn't seem to bother too much. Once in a while I'd have to run it up and pull lean to clear the plugs. Pulling the plugs showed a lot of lead accumulated and plug life was fairly short overall.

The A65 now gets a mixture of 75% premium auto fuel and 25% 100LL and seems to work fine. Once in awhile it might get a shot of MMO.

MikeB