I searched the net and have not had time to ask the locals yet, so I am asking some of you piston pilots out there what you think.

I took my nephew on his first short x-country for a burger today. He has the bug bad now (all my fault ;-). I have a habit before I park the plane (Cessna 172L) to do a mag check before I put her away. I was the last person to fly this plane and on the run-up this morning my LEFT mag ran rough. I did the normal clearing; 2,000 rpm, watched for the needle drop and put a tiny bit of mixture back in and waited for a slow count to 10. All was cleared and off we went.

We flew to our destination, performed a normal 45° entry to this uncontrolled field and a normal landing; the temp was a lot higher at this airport than it was at home with a density alt of 2,000 inbound and 3,000 leaving. Coming back home, I again performed a run-up and the same thing happened. Only the LEFT mag was rough and the right was fine. I cleared the rough mag again can continued on my way (avoiding parachute jumpers on the way)

I was given a straight-in landing (not my favorite) and I was too high on final. I performed a power-off 40° flap landing. The tower had a citation and a station air inbound for the same and I was placed in front of them. Tower gave me first to land but they told me to keep up my best speed (which I did) until I had to get my airspeed and descent stabilized. Over a small town closer to the runway I pulled the power back with carb heat ON and began my descent with runway in sight.

I had all 40° flaps in on short final and power idle. I will mention that both arriving at the first airport and coming home I always make sure to clear the engine now and again when I am at a low power setting by gently adding power and pulling it back again.

My landing was a very firm short field landing (as it should be with 40° of flaps) I again, did a quick run-up before putting the plane away.

Oh and both mags have been replaced only a few weeks ago. Here is my question...why is it always the LEFT and not the RIGHT mag that runs rough? I have never had the right mag do this to me. I still don't know why they should run rough at all, but at least I am able to clear it each time.

From what I can see, the LEFT mag seems to be connected to the lower spark plugs. Maybe some excess oil accumulating at the bottom and fouling them, while the top ones obviously stay clean?

That is what I got so far too, however this never USED to happen. Now it seems that it happens every single flight. Maybe the plugs are bad and should have been replaced when the mags did?

You know? It just occurred to me that this can be related to a 1/2 case oil leak that we are having...sounds logical to me. The oil leak appears on the nose after sitting for awhile. The mechanic looked at it and he won't do anything until the next major overhaul due in a year or so...hmm