RPM drop during Mag check

by Stuart Johnson in 912 / 914 Technical Questions 13 years ago

Can anyone tell me what an acceptable RPM drop is during a Mag check on a 912-ULS? My engine sat for 4 months this summer while I was out of the country and the drop is about 350-400 RPM at 4000 RPM and is the same for both sides. Previously, the drop had been in the 150 to 200 range. Also, do magnetos go bad gradually? I thought that they either worked or didn't.

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Re: RPM drop during Mag check

by Roger Lee » 13 years ago

Roger Lee

128 5.28k 4.98k

Hi Stuart,

Usually the mag drop around 3400 rpm is 50-70 and the drop for 4000 is around 80-100. The 350-400 drop tells me you most likely have a poor plug. Yes it could be a a plug wire or something else, but if it was okay when you put it away and it has sat for a long time then that is why I am guessing a plug issue until proven otherwise. It's the easiest and cheapest to check and work on. If it where me I would toss the plugs, gap them between .023-.027 (.027), apply my thermal conducting paste and try again. My guess is you will be okay.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: RPM drop during Mag check

by Stuart Johnson » 13 years ago

Stuart Johnson

2 4 0

I think the plugs are OK. I just pulled them and checked the gap (.028 is where I have been keeping them). They don't have many hours on them, but they have been in for a year. I also have been using the thermal paste, but I didn't reapply this time.... it looked like they were coated OK. I'll try new pugs since that's easy. Is there anything else I should check?

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Re: RPM drop during Mag check

by Roger Lee » 13 years ago

Roger Lee

128 5.28k 4.98k

Hi Stuart,

Hours on the plugs are the problem. I would bet one is contaminated or just bad. It could also be a bad connection on a spark plug cap where the plug wire screws into the plug cap. With only a 300 rpm drop you have just one plug acting up. Remember let's try the easy thing first. I would wait on playing with anything else until you do the plugs. You may make it worse or you may fix it and never know which one worked and what the real problem was. The plugs run on AC current. Here is an inexpensive little tester that helps figure out which plug or wire is bad. It cost about $12 and all you have to do is hold it up by the wire and see if it flashes while the engine is running.

ACTester.JPG (You do not have access to download this file.)

ACtester1.JPG (You do not have access to download this file.)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Stuart Johnson

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Re: RPM drop during Mag check

by bill martin » 13 years ago

bill martin

10 757 0

Roger,

Two quick questions.
Does this AC tester work on a 582?
I have two of the newer (black) oil filters for my 912. Now that there is an even newer filter, do I junk the ones I have, or use them?

Bill.

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Re: RPM drop during Mag check

by Roger Lee » 13 years ago

Roger Lee

128 5.28k 4.98k

Hi Bill,

If you have some of the old filters then use them up. They are fine.
The AC tester should work on most plug wires. You only need AC voltage for it to work. I have not actually tried it on a 582, built it should work. If something was 12V only then you can get the same tester in 12V I believe.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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RPM drop during Mag check

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