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#21
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Bad plug. Lead fouling isn't the only reason they go bad. If it was a wire,
agressive leaning wouldn't have done anything. Alex "Jay Honeck" wrote in message news:cI9od.66570$V41.11973@attbi_s52... Our JPI EDM-700 engine analyzer may have paid for itself today. Here's the scenario: Mary is PIC. During her run-up, the left mag dropped a bit more than the right. She leaned it back (the usual fouled-plug procedure) and it smoothed out to within specs. Still not as smooth at the right, but with less than a 75 rpm drop. Climb out was normal, with that O-540 pulling like a tractor at over 1000 feet per minute. Life was good, especially after surviving the Wisconsin-versus-Iowa Big Ten football game... Upon leveling out, and getting everything cleaned up, I was messing around with our new CO detector, which is mounted on the pilot's-side at elbow height. (Recommendation: Don't reach across your pilot like this unless you're married to her! :-) As I was tinkering with it, my eyes were drawn to the EDM-700, which was clearly showing something amiss with the #2 cylinder. EGT was over 1630 degrees (we try to keep them under 1500), and the CHT was way low -- like in the low 200s. We tried leaning, and enrichening, and different throttle settings, but that cylinder stayed way out of line with all the others. Strangely, nothing sounded amiss at all, and the engine seemed to be performing normally. I was about to suspect something wrong with the JPI, when I suggested that we try switching to one mag, then the other, in flight. On the right mag, all was well -- but on the left mag, the engine started shaking, and the #2 EGT shot off the scale! Switching back to "both" everything smoothed out. In fact, we still could not detect anything amiss "by ear" -- but the #2 cylinder was still "out of whack" with the others. Since we were closer to our destination than home, we elected to continue on and land. On the ramp Mary did a run-up, and the #2 cylinder was still clearly not firing on the left mag. We then ran it up on "both", and leaned it severely -- again, the usual procedure for a fouled plug. After this it was better, but not perfect -- so we de-cowled the engine, and checked everything thoroughly. Other than the #2 cylinder's bottom spark plug wire being oily (actually, all the bottom wires are), I couldn't find anything amiss. I wiggled all the wires, made sure they were not bent over too sharply, checked all connections at the mags -- and then went to dinner. Of course, this bothered me throughout dinner, and I really couldn't enjoy myself. On the return flight, with me acting as PIC, I did a VERY thorough run-up, with a long, brutally leaned 2000 RPM test. Everything tested and ran fine -- and remained fine all the way home, at all throttle, prop and mixture settings. What's going on here? We burn unleaded car gas, so it seems unlikely that we had a lead-fouled spark plug (we haven't bought avgas in several weeks) -- could it be a bad plug? Or a bad spark plug wire? Can a mag fail in such a way as to only hinder a single spark plug from firing? I'm going to call my A&P tomorrow to have him check it over, but anyone got any ideas? -- Jay Honeck Iowa City, IA Pathfinder N56993 www.AlexisParkInn.com "Your Aviation Destination" |
#22
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Jay Honeck wrote:
I didn't have time to work on the plane today, but I'm gonna yank that plug and have my mechanic test it in his brand new spark plug tester -- the kind that actually pressurizes (or does it lower the pressure?) it while testing. Increases...no matter what the ambient pressure in, the cylinder is many times that inside. Now if you're testing the ignition leads.... Of course you could bite the bullet and spend the $25 on a new plug. |
#23
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Jay,
Hope you find out what it is. Let me know if you do so I can learn from you. BTW, I was getting 1,400fpm on the new cylinders last week........I couldn't believe it. Trip |
#24
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On Tue, 23 Nov 2004 04:30:36 GMT, "Jay Honeck"
wrote: snip I didn't have time to work on the plane today, but I'm gonna yank that plug and have my mechanic test it in his brand new spark plug tester -- the kind that actually pressurizes (or does it lower the pressure?) it while testing. If that's not it, I'm betting on the wiring harness. Swap all the plugs top-to-bottom. If it is a bad spark plug (which may pass the bomb test just fine) the next time the problem rears it's head it will be on the other magneto. If your local guy has a high-tension lead tester, testing the leads is simple and fast. You could check just the one with the "issue", or when swapping the plugs top-to-bottom, you could check them all. As someone else indicated, oil on the lower plug wires is a bad thing, and not what I would consider to be typical (or acceptable). I'm curious, what type of mags are installed? How much time is on the engine/mags since you bought the airplane/had the engine major'd? TC |
#25
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"Jay Honeck" wrote in message news:cI9od.66570$V41.11973@attbi_s52... Our JPI EDM-700 engine analyzer may have paid for itself today. Try exchanging the upper and lower plugs on the #2 cylinder and see if the problem switches to the other mag. Don't clean or do anything to the plugs or harness, the first priority is to identify the problem. It has to be in the ignition system. Mike MU-2 Helio Courier |
#26
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A couple of days ago I posted about a problem with my #2 cylinder. Watching
my JPI EDM-700 in flight, the EGTs on #2 shot to over 1600 degrees, and the CHT sank to 200 degrees. After trouble-shooting, it became apparent that the condition became much worse when set on the left mag only. In other words, the #2 cylinder was not firing -- or, at least, not firing consistently -- when on the left mag. On "both", the only symptom was the bar graph on the EDM -- the engine sounded perfectly normal. Yesterday I removed BOTH spark plugs from the #2 cylinder (I figured "why not?" as long as I was at it) and had my A&P test them in his pressurized spark plug tester. Both were clean, and checked out fine. He re-gapped them, and I reinstalled them. They were off, according to my A&P, so I believe my next step will be to re-gap ALL the spark plugs. I also sprayed the oil-saturated wiring harness down with contact cleaner, which did a fine job of "de-greasing" them. All connections appeared fine, although I didn't take the mag connections off. I figure I'll take her around the patch when I get time, to see if all is now well. (It WAS well on our way home, if you recall, after leaning severely during a long run-up.) My mechanic said what I did MAY fix it, if it was just a chunk of crud on the spark plug causing it to mis-fire. But he also said it could be a bad lead wire, or a bad distributor inside the mag. Thanks to all who have responded -- is there anything else to check? I find it hard to believe that I could have had a fouled plug, given the fact that we usually burn unleaded mogas, but perhaps it's a plug that only fails when hot? -- Jay Honeck Iowa City, IA Pathfinder N56993 www.AlexisParkInn.com "Your Aviation Destination" |
#27
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Jay Honeck wrote:
He re-gapped them, and I reinstalled them. They were off, according to my A&P, so I believe my next step will be to re-gap ALL the spark plugs. disclaimer: I'm no mechanic, but I've cleaned and gapped a few spark plugs in my life, both automotive and aircraft. It's possible you just got a piece-o-junk in the gap and it was dislodged when the plug was removed, and it will be just fine forever onward. You could also have an invisible crack in the insulator that only shows up when hot or under combustion chamber pressure, in which case the problem will reappear. Some people intentionally make the gap wider than spec, particularly on a plug that has a tendency to foul. My understanding of the rationale is that the wider gap gives a more reliable spark, but causes higher voltages in the high tension wiring, which might cause other symptons, like crossfiring. Apparently you can routinely get away with a gap that's wider than spec. Dave |
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