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TLDR - I am renting a PA-28 181 (engine Lycoming O-360-A4M), and have noticed that the right mag is rough at low power settings. During before takeoff power checks, both magnetos operate as expected, but when making dead cut checks both after start and prior to shut down, the right mag consistently runs rougher than the left - is this normal behaviour?

CONTEXT - I've been told, by the club that I am renting this aircraft from, that the engine has always been like this, and that it is characteristic behaviour, but I've been becoming a bit more dubious as the problem seems to be worsening, and I will now often get backfires and VERY rough running (600-900 rpm) on the right mag during dead cut checks. The left mag runs perfectly smoothly during dead cut checks at about 1100 rpm (I conduct dead cut checks with the engine at 1200 rpm for reference). The engine also seems very reluctant to settle at 1200 rpm before power checks are conducted, and consistently jumps between running fast at 1500 rpm and then running unevenly at about 900 rpm. After power checks are made, the problem seems to rectify itself, and the engine will happily run at 1200 rpm.

FIXES - I can't find anything in the POH with this set of symptoms, and dead cut checks do not specify maximum rpm drops / tolerances (although for reference, I can tell you that it is uncomfortable sitting in the plane when the right mag is isolated during dead cut checks). Carb heat issues can be largely ruled out, since I always check this on the ground. Fouling seems unlikely given that the plane is always leaned for taxi, usually leaned at cruise, never flown especially high, and never flown or taxied at low power settings for extended periods (it flies out of a small airport, and off block to take off time is never more than 5 minutes).

I've mostly flown the PA-28 161 rather than 181, and have not flown enough different airfames of this type to know if this kind of anomaly is harmful or not, although another pilot not from this club has expressed concerns. I would much appreciate any advice!

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    $\begingroup$ I'm not a certified A&P, but I do know that Lycoming engines have an impulse coupling on the left magneto only. This is why you perform startups on the left. Magnetos naturally have performance issues at low RPM. Perhaps the quick power cut check is short enough for the impulse coupling to keep the left firing smoothly, while the right mag struggles under the low RPM. $\endgroup$ – Aaron Holmes Aug 2 '20 at 17:05
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    $\begingroup$ When is the last time the mag was pulled and checked? We just had a mag on our aircraft experience similar issues, and the mag was really bad. Our ignition switch was also shot. $\endgroup$ – Ron Beyer Aug 2 '20 at 18:03
  • $\begingroup$ Unfortunately I don't manage the maintenance, so I don't know when the mags were last checked, but something wrong with the right mag was my first thought. The aircraft just had its annual, but not being the engineer, I don't know exactly which parts are checked during that. I do know that the engine is old but low hours however. Could you please clarify what you mean by "similar issues"? $\endgroup$ – Anonymous Aug 2 '20 at 18:38
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    $\begingroup$ We had a rough mag on one side, originally we thought it was just fouled plugs on the ground or using carb heat at low RPM (in the pattern). Eventually it started misfiring quite a bit and was followed by a flight that had a loss of power and inability to climb (luckily they made it back to the field). The aircraft had an annual 2 months prior, they pulled the mag and found it had really bad contacts. As far as I know the dead-mag checks worked fine (RPM drop was OK), but it would run rougher on one mag than the other. $\endgroup$ – Ron Beyer Aug 2 '20 at 22:46
  • $\begingroup$ Just to be clear, the mag was not pulled at annual, the rough engine existed before the annual and got worse afterward. They didn't check the mag at annual and we were a little upset about that, but I don't think it's a required annual item. Some mags are life limited parts $\endgroup$ – Ron Beyer Aug 3 '20 at 0:49

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