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Shortly after takeoff and following a normal mag pre flight check at barometric field pressure (30 inches) the right engine started missing and running rough. Reducing power and troubleshooting led to the following: ---oil temp, pressures, fuel pressure, CHTs were all normal and manifold heat had no effect ---ran rough on BOTH with oscillations of plus minus approximately 100 rpm ---ran extremely rough on LEFT only with missing and rpm drop off of 400 rpm before changing back to Both or Right ---ran smooth on RIGHT only ---on the ground did a mag check at 1500 rpm, right engine ran extremely rough on Left only with what sounded like backfiring ---engine ran smooth on Right mag only ---on Both roughness not really discernable ---on shutdown at idle, 600 rpm, place right engine mag on Left only and engine died I am thinking that the LEFT mag, which only has 50 hours (in less then one year operation) since overhaul by Tulsa as part of a majored engine, is crossfiring??? Any other thoughts?? or what to check for? Many thanks!! |
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Was that a question? If it walks like a duck, quacks, etc etc etc...lol |
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Had the aircraft been in wet conditions or freshly washed? And just because a mag is new doesn't mean it can't go bad. Jimbob |
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The aircraft/engine were "dry" as the aircraft is hangared, had not been recently washed, and the weather was dry. Stuff can go bad at any time but I was curious if there was any type of previously experienced early failures with these mags....as I don't have the years of experience. |
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sounds to me like the mag hatched |
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Sounds like the mag to me -- you could change the distributor block to see if that is the culprit 1st, and proceed to the mag itself if the problem remains. I have a couple spares here, if you want to try this. While you're in there, look at the points gap (remove the back cover) -- should be close to .010, and the points should be clean and not pitted. I have some points too, if ya need 'em. If you change the dist block and reset the points, and it STILL won't run right, it could be a coil....by golly, I have some of those too! Might have a look at the plugs too, but a 400rpm drop would be about 1/2 of 'em not firing -- all at once? not likely... |
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I spoke with Sam Thompson at Tulsa yesterday. From the description I gave he is fairly certain that the phenolic drive(?) gear that is internal in the Bosch mag sheared. Sam indicated this is known failure of these phenolic gears. If you are not aware Tulsa has an STC on a brass gear ($ 450) to replace the phenolic one. Tulsa is sending out a replacement mag n/c under warranty though I am paying the upcharge for the brass gear. Great customer service!! Vendors like Tulsa certainly will get my business and support! |
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Consider that they designed that gear to fail first rather then the drive gear on the engine. Im sure they weighed the odds but I dont hear about that many failures of the phenolic gear. A lot cheaper and easier to replace then the engine side gear. Plus, isnt it possible for metal shavings to contaminate the oil supply from that area? Just being the devils advocate here. Im always suspicious of better mousetraps. Im curious about that diagnoses though as you stated that though it ran rough on the left it WAS running right? Tom |
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Me, Murphy, and Rube Goldberg have built the finest mouse trap available. Stupid mags anyway! Sam's the best in my book, he's (TAE) been my favorite for 25 years. Tom, I left some part numbers with yer answering lady today, look forward to hearing from you. Jimbob |
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JimBob, I finally got a chance to get upstairs and take a look. I dont have any of those units. Those would be unusual to find in a parts house anyway. No one would have ordered them for stock. Check the salvage yards. Tom |
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Received and installed the replacement mag from Tulsa Aircraft Engines. Normal mag check on run up at field barometric pressure. Investigating the "bad" mag did reveal the the teeth on the phenolic gear were no longer teeth....most were worn to stubs. |
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I had the gears strip in one mag on my J3 Cub. It was turning out of time and did some really strange things. I had just taken off and simply turned around and landed. |
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I took off 18 @ LIT with 3500 lbs UPS one morning . You have an upslope after passing the interstate. I cleared the road but wasn't going to clear the hill. Eventually I figured out I had a bad mag missfiring and killing the air fuel charge. After I turned the mag off and it ran good. As usual maintenence was slow so I flew it 3 days on one mag Aint 135 wonderful Still it beats the 3 days I hand propped a 18 morning and night to make the run. |
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As part of your annual inspection you should check the backlash on your phenolic mag gear. It's a very easy 10 minute job. If it's getting anywhere near 010 replace it or IT WILL FAIL. Under some conditions the resulting misfiring can damage your engine or cause a drastic loss of power. |
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I am in the middle of my annual and the engine cowls are off right now. How does one check the backlash on the phenolic mag gear? Is the procedure in the maintenance manual? Steve Oxman H18 BA-650 |
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Remove the back covers from the mags. Rotate the cam back and forth, if you've done it for 30 years you know what .010 feels like, if not use a dial indicator or put a mag timer on it. Yes it's in the manual. I've seen them with yellow tags out of overhaul with wore out gears. I won't run a mag with more than .006, the consequences of loosing an airplane over a 200 dollar gear don't make any sense. |
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