Cherokee 140 mag drop

jmarine225

jmarine225

Pre-takeoff checklist
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Jmarine225
So I have a 1969 Cherokee 140, with a 160 engine. It’s flown regularly, weekly basis with an engine that has roughly 450 hours smoh. Today during the run up the right mag was awful with a drop of around 300 rpm and the engine just running awful. We tried leaning it out and burning it off, but it ran even worse and the egt would skyrocket to the top of the gauge. Any suggestions on the issue here? Ran on both fine. Ran on the left fine. Hoping someone will tell me change the plugs and be on my way. Of course that’s my hopeful thinking.
 
Change the plugs and be on your way. :D
Run it on the bad mag for a bit. Quickly open the cowl and CAREFULLY check to see which cylinder is cooler. The guilty plug will be there. Follow the right mag lead to see which plug it is. Frequently it is the lower.
 
pmanton said:
Change the plugs and be on your way. :D
Run it on the bad mag for a bit. Quickly open the cowl and CAREFULLY check to see which cylinder is cooler. The guilty plug will be there. Follow the right mag lead to see which plug it is. Frequently it is the lower.
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This is the answer I’m hoping for and the answer the fbo gives me when they look at it.
 
Run it on the bad mag for one minute at 1000 RPM, then open the cowl, spit on your fingertip.
Carefully touch each exhaust stack, the good cylinder's stacks will sizzle, the bad cylinder's stack will not.

Or use a stripe of chalk on the stack, hot stacks will burn the chalk, the cold cylinder will remain white.
 
Or a harbor freight 20 dollar non contact laser thermometer.
 
biplanebob said:
Run it on the bad mag for one minute at 1000 RPM, then open the cowl, spit on your fingertip.
Carefully touch each exhaust stack, the good cylinder's stacks will sizzle, the bad cylinder's stack will not.

Or use a stripe of chalk on the stack, hot stacks will burn the chalk, the cold cylinder will remain white.
Click to expand...
 
As a new owner I checked the engine log book. Looks like a harness and two plugs were replaced in 2015. Besides this the entire engine was overhauled in 2014. Curious how long plugs should last and how do I know it’s not a bad magnet to? Just for knowledge so I do t get conned I to buying a whole new magneto, plugs , harness, etc.
 
Ryanb said:
Pro Tip - Let your A&P look at it
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Already at the fbo. Just curious so I know what may be wrong if it happens again and so I don’t get raped into fixing unnecessary stuff.
 
If you did the run up with full rich and saw the drop, it’s most likely the plug, to actually check the mags , lean to ROP and then do the mag test. Or so I have been told. Another good way to do a mag test is actually in the air, at the leanest settings. Mike B has written a lot about it.




I am not going to get into Mike B is no good discussion
 
WannFly said:
If you did the run up with full rich and saw the drop, it’s most likely the plug, to actually check the mags , lean to ROP and then do the mag test. Or so I have been told. Another good way to do a mag test is actually in the air, at the leanest settings. Mike B has written a lot about it.




I am not going to get into Mike B is no good discussion
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Exactly what happened. Full rich, up to 2000 as per poh, right magneto ran like junk stuttering and stuff, 300+ rpm drop which did t happen last week when I flew, although it seemed that mag was always a bit lower.
 
jason gula said:
Exactly what happened. Full rich, up to 2000 as per poh, right magneto ran like junk stuttering and stuff, 300+ rpm drop which did t happen last week when I flew, although it seemed that mag was always a bit lower.
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I am guessing there is not engine monitor, without that have you A&P decide which one failed/ too much lead fouling. Happened to me once, took 30 seconds to find out watching the engine monitor.

For future taxi ops, lean and lean some more. Lean to an extent that any further throttle will make the engine die, to prevent lead fouling. And get tempest plugs.
 
jason gula said:
So I have a 1969 Cherokee 140, with a 160 engine. It’s flown regularly, weekly basis with an engine that has roughly 450 hours smoh. Today during the run up the right mag was awful with a drop of around 300 rpm and the engine just running awful. We tried leaning it out and burning it off, but it ran even worse and the egt would skyrocket to the top of the gauge. Any suggestions on the issue here? Ran on both fine. Ran on the left fine. Hoping someone will tell me change the plugs and be on my way. Of course that’s my hopeful thinking.
Click to expand...

Had the same thing on my Cherokee a few years ago. A new (rebuilt) mag solved it. The bearing inside the mag had come from together.
 
So I spoke with the mechanic who pulled the plugs and inspected them. He said 4 plugs need to be replaced. He said the magneto is shot also and after log in the log books it looks like it was last overhauled in 2010, which he said is a really long time. Hasn’t been inspected or anything since then. He spun the prop by hand and showed me the mag isnt properly functioning, so new mag it is with some new plugs. Just checking if any of this makes sense since I’m new to this.
 
i also wonder what kind of pencil whipped annual it got... aren't they supposed to check the timing of the mag at annual time?
 
Yeah I actually went through the log books, as it was inspected in December at the time of the purchase. Mags were due for inspection at that time and not completed. Last overhauled in 2010.
 
WannFly said:
for plugs, buy tempest ones rather than champion
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That there. Even though Champion has fixed their resistor problem, there are plenty of old-stock plugs on shelves yet.
 
jason gula said:
Yeah I actually went through the log books, as it was inspected in December at the time of the purchase. Mags were due for inspection at that time and not completed. Last overhauled in 2010.
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That right there. Too many folks, pilots and mechanics both, don't take mag inspections seriously. "It still works," they say. Or "Never had a problem yet with those." I have taken off mags like that, still firing, and found that they were a few hours from failing.

Alternators, too. People fly them until they fail, and that, according to Murhpy, will be at night or in IMC or on a busy day in congested airspace.

As both pilot and mechanic I could never understand that sort of blind optimism. I have had some serious inflight equipment failures. Tends to make one a lot more sensible about these things. We all seem to have to learn the hard way......
 
Well am easy way to learn is to drop a quick $1,000 to get everything fixed, replaced and back running.
 
I'm taking a look at a mag for Dad's airplane, was last inspected 15 years ago IIRC, only has 477 hours since then, around 800 since factory rebuilt. So far all I really see is the ball bearings are kinda dry, still waiting for TCM grease to show up for that. The impulse coupling spring has enough corrosion on it that I wouldn't trust it to last another 500 hours, and they really aren't expensive so a new one it is. Oil seal may have a tiny leak, a new $9 seal seems like cheap insurance. A new $100 capacitor, $65 points and some other misc stuff and I'm figured about ~$275 in parts (excluding lubricants purchased which will be use to service others) an it should G2G.

Between family and friends I think we have about 10 TCM magnetos. I've never had a TCM fail but they do. I have had a coil die in Slick mag on me -in-flight before.
 
Plus I was told the other mag is going to be right behind this one and will fail since they are old.
 
Dan Thomas said:
As both pilot and mechanic I could never understand that sort of blind optimism.
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Boils down to $$$$ optimism. You can add parasitic drains that eat batteries every 2-3 years plus a number of other preventative measures to your list of mag insp and alternators checks. And while I'm the first to agree not every single optional item needs to be performed on an aircraft, when you do try to recommend a specific task that does reap a benefit vs cost, you usually get hammered for trying to make work up or "pay off my mega yacht." Fortunately for me, I was able to switch over and work with owners who thought along the same lines as me or who wanted to learn how to. Discussions like this always remind me of the saying, "pay me now, or pay me later." ;)
 
jason gula said:
Plus I was told the other mag is going to be right behind this one and will fail since they are old.
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Maybe OH the left mag, keep it as a spare, and replace it with a surefly?
 
Curious on this still. So there are bad plugs, which were located. So how do I know it wasn’t just bad plugs? How is it found the magneto is actually bad also, not just the plugs?
 
biplanebob said:
Run it on the bad mag for one minute at 1000 RPM, then open the cowl, spit on your fingertip.
Carefully touch each exhaust stack, the good cylinder's stacks will sizzle, the bad cylinder's stack will not.

Or use a stripe of chalk on the stack, hot stacks will burn the chalk, the cold cylinder will remain white.
Click to expand...
Or the plastic handle of a screwdriver, the cold one will not melt it.
 
Actually there are proven troubleshooting procedures to follow when trying to find a problem.

Surprising how well they work.
 
Gonna add another factor into this situation. As stated I ran it pretty hot for a bit on the bad mag trying to burn off any fouling. A mechanic told me to lean it way down until the engine backfires and sputters. Did that for a few minutes and obviously no good. Bad mag. Well today I happened to notice that all of a sudden my exhaust is cracked and corroded at the end of it. Definitely was not like that before this mag event. Any idea what the cause could be? Running the engine hot cause it?
 
jason gula said:
Gonna add another factor into this situation. As stated I ran it pretty hot for a bit on the bad mag trying to burn off any fouling. A mechanic told me to lean it way down until the engine backfires and sputters. Did that for a few minutes and obviously no good. Bad mag. Well today I happened to notice that all of a sudden my exhaust is cracked and corroded at the end of it. Definitely was not like that before this mag event. Any idea what the cause could be? Running the engine hot cause it?
Click to expand...
It’s unlikely doing that alone would cause a crack in the exhaust. It was probably on its last leg already and those times of rapid heat expansion and contraction was all it needed to finally crack.
 
jason gula said:
. Well today I happened to notice that all of a sudden my exhaust is cracked and corroded at the end of it. Definitely was not like that before this mag event.
Click to expand...
Corrosion didn't happen in the few minutes of running it like that.
 
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