I installed 2 rebuilt Bendix Mags. The mag drop initially was about 80rpm and a difference of 10-15 rpm between them. I have about 10 to 15 hours on them and the mag drop has continued to increase to about 120 on both ot them and about 25 between them. I checked the timing and it seems to be as close as I can get it. All cylinders around 70. Why is the mag drop increasing? Does an increase drop indicate to far advanced or retarded? What else could cause this?

- Make sure you are leaned out correctly during the runup and mag check. Slightly to lean or too rich can affect the drop
- Plugs have seen enough use to be slightly fouled (make sure to aggressively lean out on ground)

120 even drop is fine. If it gets worse then there is a laundry list of possibilities

I am looking for that laundry list where both mags have increasingly higher mag drops from about 80 to about 120 in 10 to 15 hours since rebuild. It seems like too short of a time for this to occur. The engine runs smoothly.

What is the condition of the rest of your ignition system? To high of system "pressure" can degrade the mags more quickly. Harness can develop resistance over time, there is a harness checking tool most shops have. Sparkplugs need to have 1000ohms of resistance and absolutely no more than 5000. I chuck em all the time for too much resistance.

There's a ton of good troubleshooting info here: https://www.aircraftmagnetoservice.net/magneto-troubleshooting-guide
Also fun to call up these guys and talk to Cliff, he's a genius.

Take a peek at the points. We had some aftermarket points do that in a Bendix mag. They wore really bad and that changes the "e-gap", resulting in a bigger drop. I also saw a set that had the rub block soft and it wore quickly, with the same result, but that was probably 30 years ago on a 600 series Slick.
John

Are the outside ambient temps higher now than before making a rich mixture even richer? That will cause higher smooth drops in RPM during the check.

Thanks. That is all good advice. These were rebuilt Bendix Mags bought from Aircraft Spruce. The increased drop is on both mags. They have almost the same drop. I plan to fly it some more and see if it goes beyond 120rpm. I will then open it up and check the timing. When I just checked it the timing may have retarded from 22 to 21 degrees. I would not think that would make much difference. It could also be my eyesight. It runs well on either mag. I think I may just be a little paranoid. When flying in the backcountry I want everything to be as close to perfect as possible.

120rpm drop is fine in an O470, I seem to recall 150rpm being the limit.

Hello, Everyone,
@flyer - I know it's been a few years since you reported these problems. I wanted to see if they got worse or the mag drops stayed in that range.

In the last few weeks, my 0-470-R started having really high drops on both mags. 100 to 125 had been common but now they are sometimes as high as 190 with popping when reducing power. but the mag checks are smooth. Found one cylinder with really low compression (under 30) that will probably have to come out - but I've had cylinders with low compression before that did not adversely affect mag drop...

Timing was spot-on on both mags - for mine it's 22 BTC

Thanks in advance!

Not spot on topic but be sure and lap the valves first before pulling a cylinder. I just had two at 30/80, lapped them and got 50 and 60 cold before even running it. Will check again at oil change...

wfd wrote:Not spot on topic but be sure and lap the valves first before pulling a cylinder. I just had two at 30/80, lapped them and got 50 and 60 cold before even running it. Will check again at oil change...

I tend to agree with this. But on the other hand I just pulled a cylinder that was coming in at 55/80 on a compression check. I just didn't feel good about it and pulled it off and it had 2 out of the 3 top rings broken. No signs of the broken rings besides the low compression. Normally I wouldn't pull a jug that was still reading 55, but I'm rethinking that strategy after this experience.

Sorry, assumed it was valve leaking

Sorry, assumed it was valve leaking

Were you able to identify that it was rings by where the compression test air was coming out? (Crankcase breather / oil dipstick)

I have lapped a valve in place with great success, but it was obvious the compression test air was mostly coming out the exhaust. Same engine a few years later, different cylinder, the air was clearly coming past the rings and audible in the oil filler / crankcase breather. That cylinder came off for overhaul.

PilotPeat wrote:Were you able to identify that it was rings by where the compression test air was coming out? (Crankcase breather / oil dipstick)

I have lapped a valve in place with great success, but it was obvious the compression test air was mostly coming out the exhaust. Same engine a few years later, different cylinder, the air was clearly coming past the rings and audible in the oil filler / crankcase breather. That cylinder came off for overhaul.

Assuming this was for me Peat. Ya I could tell it was coming form the crankcase. Still threw me though, as it really wasnt super low on the compression considering it had 2 broken rings. Just thought I'd share my experience.