Piper Cherokee magneto drop

jmarine225

jmarine225

Pre-takeoff checklist
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Jmarine225
I have a 1969 piper Cherokee with bendix magnetos. I’ve recently noticed during a run up the left magneto drops around 100 rpm. Approx 50 more than the right. It runs smooth, no issues and I’ve tried burning it off with little to no effect. Curious if this is a problem or not. Sometimes on the left, the tach needle will jump a bit higher or drop a bit lower. It’s a 2700 rpm so I’ve now been paying attention to that. At full throttle during takeoff I max at approx 2600 rpm. Again, I’m a newbie er and this is all new to me.
 
Sounds totally normal to me.
 
Could be something as simple as a slightly lead fouled spark plug. As @Ryanb said, if it's within limits fly it.
 
Ryanb said:
It’s within limits. Fly it!
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With that being said, what are some other signs or symptoms something may be wrong with it? Just concerning because a month ago, it didn’t drop that much.
 
Your drops will vary depending on lead fouling. This is not an issue. Fly it, lean it when appropriate and it'll stay within limits. The goal is not to have the smallest drop possible, simply to ensure it's within limit (L drop, R drop, spread).
 
Also, the timing my be slightly different between mags and is easy to test. What can be concern in more roughness on one mag vs. the other.
 
It could be timing differences (as has been said). If it's not rough (and you said it wasn't) I wouldn't think it's lead fouling. The fact that it could be timing and it's changed in the last month would make me want to get it checked. If the timing adjustment is moving, that could be bad. (I am NOT an A&P, just a shade tree mechanic of cars.)
 
Do you have an engine monitor that reads egt per cylinder?

I'm guessing you have different advance on your mags.
 
Some of the comments here are a bit misleading.

I see nothing at all that would indicate a mag timing issue. A mag drop that exceeds the allowable limit, but otherwise runs smooth, with no roughness would be a sign of late engine timing. That is not what the OP described. Most “bad mag checks” are spark plug related. The spark plug is fouled and shorted to ground or is open, but the magneto is not able to fire it, even though it’s working normally. A typical bad plug will cause an immediate drop of 250 or more RPM at the mag check, which is not what is described here either.

There is nothing unusual about a 100rpm drop or a 50rpm differential. That’s quite common, at least in my experience. If you consult your POH, you will find the acceptable tolerance limits for the magnetos during the run-up. In the Archer I fly, it’s 175 max drop and a 50rpm maximum differential and it should be no different for the Cherokee 180. Fly it!
 
Ryanb said:
Some of the comments here are a bit misleading.

I see nothing at all that would indicate a mag timing issue. A mag drop that exceeds the allowable limit, but otherwise runs smooth, with no roughness would be a sign of late engine timing. That is not what the OP described. Most “bad mag checks” are spark plug related. The spark plug is fouled and shorted to ground or is open, but the magneto is not able to fire it, even though it’s working normally. A typical bad plug will cause an immediate drop of 250 or more RPM at the mag check, which is not what is described here either.

There is nothing unusual about a 100rpm drop or a 50rpm differential. That’s quite common, at least in my experience. If you consult your POH, you will find the acceptable tolerance limits for the magnetos during the run-up. In the Archer I fly, it’s 175 max drop and a 50rpm maximum differential and it should be no different for the Cherokee 180. Fly it!
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But how do you account for the recent change in RPM drop?
 
Ryanb said:
Some of the comments here are a bit misleading.

I see nothing at all that would indicate a mag timing issue. A mag drop that exceeds the allowable limit, but otherwise runs smooth, with no roughness would be a sign of late engine timing. That is not what the OP described.
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You may not think it’s timing related, but I disagree (and I work on this stuff every day). A difference in timing between magnetos will absolutely cause a difference in rpm drop during a mag check.

As far as spark plugs being bad/fouled goes, some of you guys need to fly better maintained airplanes if you have that much trouble.
 
Anything is possible, but given the statements that were written, I see nothing that would scream timing issue. Without having been present to see the changes noted during the mag check over the short period that he’s owned the airplane, it’s impossible to say what’s going on. It could be OCD, paranoia, mag going bad, who knows. All I’m saying is that by what was written, there is NOTHING that is out of limits.

Unless it’s been a long time since the mag timing was checked, it’s unlikely that it’s changed since he’s began flying this airplane. I understand that timing can easily drift two or so degrees in 3-4-500hours, but it doesn’t happen overnight. If the contact points erode, the timing will advance. If the cam point wears, the timing will decrease, but at this point the evidence we have for any of that is far too weak to make any conclusion. Best for OP to keep flying the airplane and see if the mag check drops become more extreme or any other changes are noted.
 
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Since the OP identifies as a "newbie", I'll repeat this leaning advice.

Lean to the extreme on the ground, during engine warmup at 1000-1200 rpm, before takeoff, and after landing. Lean until advancing the throttle causes the engine to stumble. This will ensure you cannot take off in that condition. As long as your engine power output is low (< 65% ?) you can't hurt the engine with radical leaning.

Why radically lean? Our Cherokees burn leaded fuel using the additive TEL. The fuel also contains a lead scavenger--ethylene dibromide. To work EDB needs a temp of about 1600F. Leaning raises the combustion temperature and reduces the amount of lead being burned.

For more information, and suggestions on how to warm up your engine and why see:
https://www.shell.com/business-cust...entre/technical-talk/techart-18-30071600.html
 
Your mags are probably ok, but they are not timed exactly the same. They might be a few degrees apart. If mag drop is within limits...go fly...leaning like Domenick suggested is also a good idea to prevent plug fouling.
 
Ryanb said:
speculating, you don’t know this for sure.
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One of the posters probably had it correct. I am a newbie and paranoid something might be wrong so I’m evaluating everything to be sure. To add to the original post here, my mag drop max’es around 100 rpm. Since that drop is 100, the other drop is quite less, around 50 at most. The tach gauge bounces around a little bit to get a precise number. Someone else mentioned an egt. I only have the standard egt gauge which functions correctly during leaning etc.
 
jason gula said:
One of the posters probably had it correct. I am a newbie and paranoid something might be wrong so I’m evaluating everything to be sure. To add to the original post here, my mag drop max’es around 100 rpm. Since that drop is 100, the other drop is quite less, around 50 at most. The tach gauge bounces around a little bit to get a precise number. Someone else mentioned an egt. I only have the standard egt gauge which functions correctly during leaning etc.
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No harm in that, I’ve done plenty of that as well. I will repeat it again - I see nothing in your postings that would be cause for alarm. Keep flying the airplane and monitor things. If the mag checks become more extreme or start to vary at a greater rate, than consult your A&P. Otherwise, fly it!
 
Mtns2Skies said:
OAT.
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One drops and not the other? Symptoms based on post 1 and 3 by OP:
1. RPM Drop on left mag is more than right mag (by itself, no problem)
2. Engine runs smooth on each individual mag (not a symptom, but (to me) says not a fouled plug)
3. RPM drop on left mag (only) is more than it was a month ago (this is what makes me wonder if the timing on that mag is changing/drifting over time)

I don't think OAT would account for one mag and not the other. But I'm open to being educated on how these are adjusted and work...
 
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