Switch off engine no stoppy..p-leads?

simtech

simtech

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Simtech
Went flying yesterday and went to an airport which was a 10 minute flight to pick someone up. Upon leaving the plane was rather hard to start so my passenger said at next shut down turn off the mags instead of the mixture to help keep fuel in the lines. Okay ill try that.

We get back to drop him off and I turn the ignition switch to off and the engine just about stopped rotation and it start to diesel. So it would almost quit and then make another rotation and almost quit and make another rotation until I finally pulled the mixture to cutoff. So after about 10 minutes I go to fly it home and hard to start again. As im getting close to home the rpm drops like 50rpm and I had to throttle back up for that 50 rpm and everything was fine no roughness or anything.

So im starting to think I might have a P-lead problem??? Or would that cause the engine to keep running like normal? I didn't do a mag check when I got back but all mag checks during the day were within specs. Or am I being paranoid??
 
I don't understand why you think you have a P-lead problem. You run too rich and have a big dose of carbon on your exhaust valves that caused your engine to 'Diesel' when you turned off the key while still feeding it fuel. Opening the throttle should have shut it down as well. What is the engine? Are you FI or do you have a carburetor?
 
simtech said:
Went flying yesterday and went to an airport which was a 10 minute flight to pick someone up. Upon leaving the plane was rather hard to start so my passenger said at next shut down turn off the mags instead of the mixture to help keep fuel in the lines. Okay ill try that.

We get back to drop him off and I turn the ignition switch to off and the engine just about stopped rotation and it start to diesel. So it would almost quit and then make another rotation and almost quit and make another rotation until I finally pulled the mixture to cutoff. So after about 10 minutes I go to fly it home and hard to start again. As im getting close to home the rpm drops like 50rpm and I had to throttle back up for that 50 rpm and everything was fine no roughness or anything.

So im starting to think I might have a P-lead problem??? Or would that cause the engine to keep running like normal? I didn't do a mag check when I got back but all mag checks during the day were within specs. Or am I being paranoid??
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Does it shutdown normal with the mixture? Then why the mag switch?
 
FWIW, I did just what you did last week to answer the same question. I turned off the mags at the switch and the engine (O-320) stopped immediately. No dieseling or anything. Mixture was full rich, engine hot. FWIW
 
BigBadLou said:
To check proper function of the mag switch and P-leads.
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If you are doing a mag check and get a 50 rpm drop in the "L" and the "R" positions, doesn't that indicate proper operation?

Even if I switch to "off" I only do it very briefly as it will be obvious the engine will stop. Then shutdown with mixture.
 
Henning said:
I don't understand why you think you have a P-lead problem. You run too rich and have a big dose of carbon on your exhaust valves that caused your engine to 'Diesel' when you turned off the key while still feeding it fuel. Opening the throttle should have shut it down as well. What is the engine? Are you FI or do you have a carburetor?
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Just wondering why when I killed the switch it stayed running kind of. On the ground I run really really lean. Maybe I forgot to lean on this one when I taxied in. And this is a new to me plane so im not sure how the previous owner leaned on the ground, though I watched him do it on the test flight so I assume he leaned all the time. it the carb version the 0-360. So consensus says no p-lead problem huh..
 
BigBadLou said:
How does the engine behave with the mag switch in the L or R positions?
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like normal on the mag check. r right drops 100 RPM and the left drops 75-100 RPM.

while in flight the engine randomly dropped 50rpm with no input from me. I throttled back up to get it. But I would have thought regardless switch to off and engines stops completely. mine did not I had to go full lean to get it to stop dieseling.
 
simtech said:
like normal on the mag check. r right drops 100 RPM and the left drops 75-100 RPM.

while in flight the engine randomly dropped 50rpm with no input from me. I throttled back up to get it. But I would have thought regardless switch to off and engines stops completely. mine did not I had to go full lean to get it to stop dieseling.
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You have a constant speed prop? How are you seeing random drops in RPM? Can you hear or feel a change in RPM or is it a false indication issue?
 
bnt83 said:
You have a constant speed prop? How are you seeing random drops in RPM? Can you hear or feel a change in RPM or is it a false indication issue?
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fixed pitch. I heard it drop ever so slightly. this is on a Cherokee 180. Unless Im being overly sensitive.. could be! haha
 
simtech said:
Just wondering why when I killed the switch it stayed running kind of. On the ground I run really really lean. Maybe I forgot to lean on this one when I taxied in. And this is a new to me plane so im not sure how the previous owner leaned on the ground, though I watched him do it on the test flight so I assume he leaned all the time. it the carb version the 0-360. So consensus says no p-lead problem huh..
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If it's 'Dieseling', that means the mags are cold in P-leads functioning, otherwise it would run better.
 
simtech said:
fixed pitch. I heard it drop ever so slightly. this is on a Cherokee 180. Unless Im being overly sensitive.. could be! haha
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Mechanical tachometers are very..... precise.

How many people grease the tach cable?
 
Henning said:
If it's 'Dieseling', that means the mags are cold in P-leads functioning, otherwise it would run better.
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Okay I hope Im not sounding dumb here but what do you mean by cold in P-lead functioning?
 
There's a tach cable???? hahaha just kidding and yeah I don't know when it was lubed last so not really sure how accurate the tach even is. I know I heard the drop and saw it in the tack. Just going off of what I heard and how the mag is going to wonder if its all related or Im being paranoid. haha
 
simtech said:
Okay I hope Im not sounding dumb here but what do you mean by cold in P-lead functioning?
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Should have read cold and P-Lead functioning.

The mag is in a permanently charged state. With no intervention, they will always provide a source of ignition to a plug. The P-Lead provides that intervention by providing a path to ground for the points circuit before the points so you never collapse the field on the coil and generate the current for the spark. It does this typically through a switch and to the airframe. If the P-Lead is not functioning, it cannot provide this ground circuit, this is typically a bad switch, broken wire, or missing screw. That is one mode of failure. The other mode is the wire chafes on something and finds ground when when you have the switch open for "hot mags", this will shut down an otherwise functioning mag.
 
simtech said:
There's a tach cable???? hahaha just kidding and yeah I don't know when it was lubed last so not really sure how accurate the tach even is. I know I heard the drop and saw it in the tack. Just going off of what I heard and how the mag is going to wonder if its all related or Im being paranoid. haha
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There might be, there might not be. I still missed the application. Some tachs are electric generator type. You can check the calibration of your tach with some fluorescent lights at night.
 
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Henning said:
Should have read cold and P-Lead functioning.

The mag is in a permanently charged state. With no intervention, they will always provide a source of ignition to a plug. The P-Lead provides that intervention by providing a path to ground for the points circuit before the points so you never collapse the field on the coil and generate the current for the spark. It does this typically through a switch and to the airframe. If the P-Lead is not functioning, it cannot provide this ground circuit, this is typically a bad switch, broken wire, or missing screw. That is one mode of failure. The other mode is the wire chafes on something and finds ground when when you have the switch open for "hot mags", this will shut down an otherwise functioning mag.
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Okay that makes sense now. Ive read up on the mags and I too figured if the p-leads were failing it would continue to run like normal and not what it did. So what causes the dieseling? and what could cause the 50rpm drop or is that not related and in my head? A buddy of mine is a A&P and he says I have to many what ifs, it ground check is good go fly. that easy to him. haha Which I have been too. so....
 
simtech said:
Okay that makes sense now. Ive read up on the mags and I too figured if the p-leads were failing it would continue to run like normal and not what it did. So what causes the dieseling? and what could cause the 50rpm drop or is that not related and in my head? A buddy of mine is a A&P and he says I have to many what ifs, it ground check is good go fly. that easy to him. haha Which I have been too. so....
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Typically hot deposits of carbon on the exhaust valve and a correct lean running mixture that doesn't cool it quickly. It provides a retarded point of ignition that sets off the fuel further into compression than the spark would have. Another reason is the wrong heat range spark plugs, if you have a choice of two heat ranges for your engine, choose the lower number if this is a persistent problem.
 
simtech said:
did you mean an incorrect lean mixture?
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No, correct mixture within the 'nose' of the mixture ratio, where it should be to run correctly and efficiently leaned to run near the Lower Explosive Limit, or LEL, of the fuel. Had you opened the throttle quickly it would have died, same for if you went full rich as it would cool the combustion chamber too low to sustain combustion.
 
well good to know I leaned it right then! haha well I wont worry about the issue unless it gets worse and good to know. I have a short 1.5 hour flight this weekend to a fly in and I was contemplating not going because of it and getting with my mech.
 
simtech said:
We get back to drop him off and I turn the ignition switch to off and the engine just about stopped rotation and it start to diesel.
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So did the 120, the 170, the 172, and the 182 until an old-timer at Oshkosh a few years ago taught me the trick of switching off the mags and simultaneously giving the engine full throttle SLOWLY.

Haven't had the problem since. And, the hot engine starts OH SO MUCH easier with this method. Try it.

Jim
 
With a carburetor I can't imagine it making a big difference starting. It still leaves a fuel charge in the cylinder though so is an unacceptable technique no matter what. Use a proper priming technique if you are having trouble starting.
 
By the way, the mag drop may tell you the p-leads are working but it doesn't tell you the MAGS are OFF in the OFF position. I had an ignition switch that you could do all the mag checks you want (even turning the key to OFF and letting the engine die) and when you pulled the key from the ignition at least one of the mags went hot again. Those old bendix mag key switches wear out...they were cheezy enough to begin with.
 
Now you know why we shut down regular aircraft engines (e.g., not Rotax engines) using mixture rather than mags. BTW, who gave you the idea to do otherwise?
 
The use of the mags to shutoff the engine won't make it any easier to restart, either. I'm not sure where this old wives tale gets started. It's quite likely going to make the hot restart WORSE by leaving fuel in the lines to boil into vapor and lock up.
 
flyingron said:
By the way, the mag drop may tell you the p-leads are working but it doesn't tell you the MAGS are OFF in the OFF position. I had an ignition switch that you could do all the mag checks you want (even turning the key to OFF and letting the engine die) and when you pulled the key from the ignition at least one of the mags went hot again. Those old bendix mag key switches wear out...they were cheezy enough to begin with.
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I've had one physically apart, back together, and it even worked afterwards!

I lost one spring out of the stinker for about 3 hours. It wasn't the same size of an ACS switch I had laying in my tool box either. :mad2: Quite the test of patients that day was...



 
Henning said:
With a carburetor I can't imagine it making a big difference starting. It still leaves a fuel charge in the cylinder though so is an unacceptable technique no matter what. Use a proper priming technique if you are having trouble starting.
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I bet he also has tired magnetos. Not getting a decent spark out of the impulse mag. I wonder when those mags were last checked?

Dan
 
bnt83 said:
Even if I switch to "off" I only do it very briefly as it will be obvious the engine will stop. Then shutdown with mixture.
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I've been told that switching to 'OFF', then immediately back to L/R/ON is not recommended due to backfiring.
 
F01LA said:
I've been told that switching to 'OFF', then immediately back to L/R/ON is not recommended due to backfiring.
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Not a problem at a very low power setting.

I pull the throttle ALL THE WAY out then mags to off quick then back on. You test two things at once: a) the engine won't die with the throttle all the way out b) the engine will die if you turn the mags off.
 
F01LA said:
I've been told that switching to 'OFF', then immediately back to L/R/ON is not recommended due to backfiring.
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Well you don't want to do it at other than idle. Rod Machado refers to this as the CLICK-CLICK-CLICK-CLICK-BANG mag check. At idle, there won't be much unburned fuel to be reignited in the exhaust.
 
weirdjim said:
So did the 120, the 170, the 172, and the 182 until an old-timer at Oshkosh a few years ago taught me the trick of switching off the mags and simultaneously giving the engine full throttle SLOWLY.

Haven't had the problem since. And, the hot engine starts OH SO MUCH easier with this method. Try it.

Jim
Click to expand...

This may have been appropriate for the 120 as I believe some didn't have a mixture control. The rest should be shut down with the mixture.

Brian
 
F01LA said:
I've been told that switching to 'OFF', then immediately back to L/R/ON is not recommended due to backfiring.
Click to expand...


if you turn the mags off, leave them off, wasn't the idea to shut the engine off? Well then let it die.
 
brcase said:
This may have been appropriate for the 120 as I believe some didn't have a mixture control. The rest should be shut down with the mixture.

Brian
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Those old airplanes often had the Stromberg carb, whose mixture control was a back-suction type that will not cut off fuel flow at idle. I have one in my Jodel. At shutdown I shut the fuel valve off and let the carb run dry. I don't want the hazard a live mag might present if there are still fuel vapors in those cylinders.

Dan
 
Dan Thomas said:
Those old airplanes often had the Stromberg carb, whose mixture control was a back-suction type that will not cut off fuel flow at idle. I have one in my Jodel. At shutdown I shut the fuel valve off and let the carb run dry. I don't want the hazard a live mag might present if there are still fuel vapors in those cylinders.

Dan
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Every body did it that way, because those carbs leaked like a colander.
 
Another very quick mag evaluation check is to see how well they work at low rpm such as at idle. If the points are bad, there won't be a clean point break and as a result the spark will be weak.

Certainly not a quantitative evaluation, but it does give an indication of a degrading mag.
 
Well..I talked with a new mechanic today and I think Im done worrying about it. Too many different things Im hearing. This mechanic said the only time he had heard it diesel after shutting the mags down was when the switch is bad. So he recommended not flying it and having the switch checked out because if bad it could cause both mags to ground out and shut down. UGH...so.....could be true.. But..

Had I not shut it down like I did I would have never known and it shuts down normally when pulling the mixture which leads me to believe everything is fine. Im just learning what is normal to me in my new to me plane.
 
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