I've got a 67 m20f, fairly new to me about 100 hours on her, 400 smoh.
Before attempting a flight the other day, I experienced some serious roughness during the run-up when I switched to the left mag. It ran fine on both & right, but when I switched to left I get serious roughness, and cylinder 4 CHT&EGT started plummeting. I figured it was a dirty plug so I leaned out, ran it up and waited. Took a few tries but eventually got it to run normal on L without roughness at about 1700 rpm.
For better or worse I was content with that and went on my merry way. In flight however I noticed my #4 egt was pretty high. It had been higher than the rest the past week or two, but I never thought too much of it. I noticed it once or twice creep into the yellow on my EDM900 (maybe 1550 i don't remember). And this was while trying to transition to LOP, where the only way I got it out of the yellow was to richen back to 12 gph or so. This caused me a little concern so when I landed again at home I did another runup on the ground, but tried the mag check at 2000 rpm instead of 1700, extreme roughness was back when switched to L, and same situation where #4 CHT and EGT would plummet. I was unable to remedy the roughness at this higher RPM.
Now would be a good time to mention my right mag is around 100hrs SOH and left mag is around 560hrs SOH. I've been meaning to get the left done the next time in in the shop, within the next few months anyway. I'm trying to figure out what might be going on here, if I need to go ahead with the mag immediately because it's probably the cause... and lastly if I'm at least moderately safe to make the 30 min top to the shop.
So do y'all think it could still just be a fouled plug on #4L, bad wire from the mag to that plug maybe? Or mag getting toast? Considering I can get it to run smoother at lower rpm it makes me think wire is not the issue. Of course I'd prefer it to be just a fouled plug. If I take it to the shop with the intent on doing the mag right now though I think it would be best to decide that before I head over there, so I can come with a mag in hand / on the way so I'm not parked in their shop quite as long.
Thanks for all the help. New owner, not super mechanically strong so I won't be taking apart the mag or anything like that myself, but I do want to have a good idea what could be going on before I get her in to the shop.