Differential Mag Drop

[Graueradler]

My left mag consistently drops very little and less than my right mag. I have checked the timing (dead on for both mags) and the P-leads (by turning the switch to OFF and by checking for the proper gounded condition in the test positions) to make sure that the right mag isn't sneakily helping out the left mag. Any thoughts on why? I'll collect some data and post it the next time I go flying. Engine is an O-320 Lyc. with Slick mags. Plugs are relatively new massive electrode Champions.

 

[Tom-D]

My left mag consistently drops very little and less than my right mag. I have checked the timing (dead on for both mags) and the P-leads (by turning the switch to OFF and by checking for the proper gounded condition in the test positions) to make sure that the right mag isn't sneakily helping out the left mag. Any thoughts on why? I'll collect some data and post it the next time I go flying. Engine is an O-320 Lyc. with Slick mags. Plugs are relatively new massive electrode Champions.

Most engines will have what you discribe, and it is considered normal.

 

[poadeleted20]

Most engines will have what you discribe, and it is considered normal.

A very small drop on one mag, and a large difference between the drops is not, to my knowledge, "considered normal." In many (most?) light plane engines, a differential of more than 75 RPM is considered a "no go" item as it indicates one mag isn't contributing much to the mix. Before making any judgements, I'd like to know just how much each side is dropping, and what the difference in drop is. I'd also like to know what happens to EGT on each side, preferably in each cylinder. I think a bit more information is needed to determine whether or not there's a problem worth worrying about.

 

[Henning]

My left mag consistently drops very little and less than my right mag. I have checked the timing (dead on for both mags) and the P-leads (by turning the switch to OFF and by checking for the proper gounded condition in the test positions) to make sure that the right mag isn't sneakily helping out the left mag. Any thoughts on why? I'll collect some data and post it the next time I go flying. Engine is an O-320 Lyc. with Slick mags. Plugs are relatively new massive electrode Champions.

First, check and see if one mag is firing top plugs on one side and bottoms on the other. If one is firing all tops and the other one bottoms, this can happen. Typically the harness design prevents this but some of the things I've seen... If that isn't it, Check the points gap and set them equal, reset the timing, and then check the dwell. If the dwells arent equal, you have a worn points cam on one of your mags. Then there is a possibility of one mag coil being weaker than the other. Once you've spent $1500 on the ignition system, now you can go to the induction system, especially since you have a carb. A leak in the system (best way I have found to find an induction leak is to run the engine just above idle so it's smooth, and then start spraying Starting Fluid on all the joints and fittings and fasteners, if there is a leak, you'll hear it when you spray on it). Have fun finding it, or if it is still within tolerances, forget about it until it isn't within tolerances, and by then the problem may have made itself more obvious. There are other tests to do so you can "read" the plugs. If you're really interested, shoot me a note and I'll explain them to you, they'll allow you to isolate a "different" cylinder if that is actually what it is.

 

[Let'sgoflying!]

I believe it is remotely possible that although the ignition switch passes the kill test by shutting the engine down in the "off" position, when in the left (or right) position it might not be grounding the other mag properly and you will get no drop. But you did see a small drop, didn't you Bob?

 

[waldo]

I had the same type thing happen to me when I bought my plane. One mag would drop 75-80 rpm and the other would drop 20 rpm. My mechanic found, what he called, "internal" timing error. He removed the mag and set something inside the mag, put it back together and timed it. Now both mags drop 75 rpm. I have no idea what a mags "internal" timing is .

 

[Henning]

I believe it is remotely possible that although the ignition switch passes the kill test by shutting the engine down in the "off" position, when in the left (or right) position it might not be grounding the other mag properly and you will get no drop. But you did see a small drop, didn't you Bob?

While it may be intellectually possible as to the switch, due to design and construction of the switchs most commonly used, it isn't likely unless the switch is obviously falling apart, and even maybe not then.

 

[Tom-D]

A very small drop on one mag, and a large difference between the drops is not, to my knowledge, "considered normal." In many (most?) light plane engines, a differential of more than 75 RPM is considered a "no go" item as it indicates one mag isn't contributing much to the mix. Before making any judgements, I'd like to know just how much each side is dropping, and what the difference in drop is. I'd also like to know what happens to EGT on each side, preferably in each cylinder. I think a bit more information is needed to determine whether or not there's a problem worth worrying about.

"""My left mag consistently drops very little and less than my right mag"""

Where did he say "LARGE DROP" 25/50 is very common.

0/75 I'd worry about,

0/0 I'd worry about

10/75 I'm flying.

50/100 I'm flying

100/150 legal in most engines. i'm flying.

 

[Let'sgoflying!]

While it may be intellectually possible as to the switch, due to design and construction of the switchs most commonly used, it isn't likely unless the switch is obviously falling apart, and even maybe not then.

agreed, that is why the use of the word 'remotely'.
and these switches have had their problems.

 

[Graueradler]

Thanks for the responses. Its been over three weeks since I flew the plane so don't remember the data exactly. Thanks to fatigue and many hours in a big silver tube, I've got a hell of a cold that I have to get over before going flying again (If I never get on another airliner, it will be too soon). I am getting a drop on the left mag but 50 maximum. Maybe more like 30 to 40. Differential is 50 or greater but not exceeding 75. The harness is top on one side and bottom on the other for each mag just like it is supposed to be. One mag did have a new coil, points and condenser put in about 75 hrs ago. Just checked the log book and it was the left mag so that may be it. I had the switch apart at the last annual and it was in very good condition. I was chasing what turned out to be a loose nut on the P-lead of the right mag and causing an intermittent connection. That nut is very definitely tight now.

 

[Henning]

Thanks for the responses. Its been over three weeks since I flew the plane so don't remember the data exactly. Thanks to fatigue and many hours in a big silver tube, I've got a hell of a cold that I have to get over before going flying again (If I never get on another airliner, it will be too soon). I am getting a drop on the left mag but 50 maximum. Maybe more like 30 to 40. Differential is 50 or greater but not exceeding 75. The harness is top on one side and bottom on the other for each mag just like it is supposed to be. One mag did have a new coil, points and condenser put in about 75 hrs ago. Just checked the log book and it was the left mag so that may be it. I had the switch apart at the last annual and it was in very good condition. I was chasing what turned out to be a loose nut on the P-lead of the right mag and causing an intermittent connection. That nut is very definitely tight now.

What you have there is nothing to worry about.

 

[poadeleted20]

I am getting a drop on the left mag but 50 maximum. Maybe more like 30 to 40. Differential is 50 or greater but not exceeding 75.

I agree with Henning -- well within normal limits.

 

[Tom-D]

I agree with Henning -- well within normal limits.

Isn't that what I said in my first reply?

 

[poadeleted20]

Isn't that what I said in my first reply?

You may have, but you didn't have the numbers when you did, and I wanted the numbers before taking a position. "A little" from one person ain't the same as "a little" from the next one.