Not that I necessarily needed one, but I got a good lesson on the importance of doing the run up prior to takeoff this week...
It had been almost a month since I had last gotten to fly, so I was eager to get back up.
Work had been grueling over the last month, but I finally got a break, and after taking a couple of days to catch up on my sleep (literally) I was ready to take advantage of a beautiful Florida day and go flying.
My preflight was normal, the plane started normally, and everything seemed fine during taxi. However, when I went to perform the mag check, the drop on the right mag was horrible. My first reaction was " I've seen this before, it's just some plug fouling." I revved the engine a little higher, and leaned it out to clear the plugs. I brought the RPMs back down to 2000 and performed the mag check again. No change, just as bad. I tried once more to clear the plugs - no luck. I tried one more time, as I really wanted to fly. The engine ran so well on both, and so well on just the left, but the right was really bad. After a moment of temptation, good judgement ruled the day, and much as I hated to do so, assuming that the problem was simply that I was doing something wrong, I taxied the plane back to the ramp. Fortunately, the local A&P was at the airport, so I asked him to take a look. He was very agreeable, even though he was in the middle of something else, and walked over to check things out. As I had done, he performed the ground check and got the same result. He ran the engine RPM up, leaned it out, let it run, ran the check again, and got the same result.
He then taught me a cool little mechanic's trick. He ran the engine for a few minutes at around 1000 RPM only on the "bad" mag. He then shut the engine down, and removed the cowling. He explained that by running the engine only on the bad mag, the cylinder with the bad plug would run cooler, and by feeling each of the heads, he could tell which cylinder had the bad plug. It turned out that all of the cylinders were hot, so that trick didn't work, but then he showed me another trick. He took a spray bottle with water in it, and squirted each of the exhaust pipes leading from the cylinder. Sure enough, one of the pipes "sizzled" much less than the others when squirted. He figured that cylinder must have the fouled plug, and removed the top plug. When he looked at the plug however, it was clean. He reinstalled the top plug, and, when he went to remove the lead from the bottom plug, he noticed that it had come loose. Turns out that all of the leads on that side of the engine were loose. After getting them all tightened properly and reinstalling the cowl, he started the engine back up and it checked out fine.
The plane ran well for my hour of flying, and I had a nice time knocking off some rust (mine, not the plane's) performing a few touch-and-go's, and doing some Dutch rolls and steep turns as well as a little sight seeing.
So, lessons learned...
The engine really will run fine on only one magneto.
Follow the checklist religiously.
Do the right thing and take the plane back to the ramp if everything doesn't check out as it should - because it's much better to find out that there's a problem while you're still sitting on the ground safely at the airport...
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Posted by David at March 04, 2006 11:06 PM