Mag Drop

[Releew]

I have a dual magneto setup on my 1992 M20J. It is a Bendix 3000 series. I recently sent the Mags off to Quality Aircraft for a 500 hour inspection and because I recently experienced a 175RPM drop on the right Mag during a run up. There was a series of parts changed on the Mags including new gears, points, capacitor and impulse coupling. Mag was installed during the annual inspection and engine was timed. Aircraft fires up and runs smooth on both mags but I am now getting a 200RPM drop on both Mags. The drop is real as you can feel the loss of power and roughness. All plugs were verified good under pressure and the wires are new. I did an in flight Mag check at 2.5’K and did not feel ANY loss of power or see a RPM drop on either Mag. As always… I would appreciate any advice! Thanks....Rick

[romair]

Sounds like the timing may be a bit off. Had the same thing happen to me, took it back, they retimed the internal timing, but upon installation the mag sat a bit differently than before onto the case. So I am not sure if it was an internal or external timing issue, but that's what fixed my problem. That's after I went through all the spark plugs and the wire harness to make sure that's not where the problem was.

[MB65E]

Internal timing is tricky on some mags. The little points can be hard to torque down with out slipping. I'd test the harness and the ohm reading on all the plugs, before you pull the mag off again.

What is the ground rpm drop after a hard flight?

-Matt

[aaronk25]

I hope they didn't screw up and time it at 20 degrees instead of 25degrees. If so cht would be down and cruise speed at a given fuel flow would be down by 5kts

[jetdriven]

Our dual mag engine was originally timed at 20 degrees, and the mag drop was around 125 RPM. As Aaron reports, the speed was off by 5 knots as well.  If you open it up, do not reuse the breaker point cam screw if you loosen it (like when you set the E-gap). It is coated with a special threadlock and is one time use only. A couple mechanics didnt know that, and adjusted the breaker point cam then retightened the screw. In both cases the screw backed out, the mag quit firing on both sides, and the aircraft crashed on the next flight.

 

Anyways, check the E-gap, timing, and the points. All 3 of those are needed for a good strong spark.

[Bob - S50]

I hope they didn't screw up and time it at 20 degrees instead of 25degrees. If so cht would be down and cruise speed at a given fuel flow would be down by 5kts

Good info.  If I'm not mistaken, under those conditions not only will the CHT be down, but the EGT will be up.  Anyone else heard that?

 

Bob

[jetdriven]

Yes, Bob, that is correct. All else equal, cranking the mag from 20 degrees to 25 will go from a CHT of 330 and EGT peak of 1550 to something like a CHT 370 / EGT peak 1500.  Every plane and every engine monitor is different, but it sure is noticeable.

[Bob - S50]

When you did your mag check, did you do it full rich or leaned?

 

From what I've read, if you lean to peak or even LOP before the mag check (I lean until just past max RPM) you could get a much larger RPM drop than normal.  To be honest, I don't even look at the tach on a mag check.  I look for an EGT rise on all four cylinders and listen for a RPM drop.  When I go back to both I want to see/hear both of those reverse.

 

Bob

[jetdriven]

Lycoming put out a service letter that said you can do the mag check when leaned to peak RPM, which correlates to something near best power mixture, ~80 ROP.  The prop is on the low pitch stop on the ground anyway so its a fixed-pitch propeller in this case.  http://www.lycoming.com/Portals/0/techpublications/serviceinstructions/SI%201132B%20%2806-18-2010%29/Magneto%20Drop-Off.pdf

[Releew]

Thanks for all the great replies!

I have not changed the start up or lean procedure to the run up area. I do lean for taxi. The first drop was noted between two short hops. I have never seen a drop of this magnitude leading me to believe all else was normal and it was an individual Mag problem. The 175 RPM drop was on the Right Mag only. With the 500 check needed I spoke to the Tech at Quality and explained what I saw. Mags were returned, installed by the AP and timed. With no change in procedure to start, lean while taxing and run up I now get a 200RPM drop on both Mags. Running on Both Mags I do not see any loss of power or any increase in CHT or EGT’s

[aaronk25]

The more you lean on run up the greater the mag drop will be.....

[Releew]

Understood.... But if the plugs were wet, as in full rich from start up the the run up area(about a 5 minute taxi)would that also produce a Mag drop? I keep going back to the fact that my procedure hasn't changed. The plane was flown for 60 hour plus before the initial 175 RPM drop on the right Mag. That's what I do not understand. Thank you for your input?

[Bob - S50]

Do you have a 4 cylinder engine monitor?

 

Did the engine run smoothly or rough when you got the big drop?

 

Bob

[Releew]

The Engine runs rough cycling through the right or left mag. With switch in the Both position I do not notice any loss of power at all. In flight the EGT's and CHT's are in normal range. I do have a four cylinder engine monitor.

[Bob - S50]

The Engine runs rough cycling through the right or left mag. With switch in the Both position I do not notice any loss of power at all. In flight the EGT's and CHT's are in normal range. I do have a four cylinder engine monitor.

When you do a mag check watch the EGT's.

 

Do they all rise when you switch to a single mag?

 

Do any of them fall or disappear when you switch to a single mag?

 

Do the EGT's all drop back down when you switch back to Both?

 

Bob

[Releew]

Thanks everyone for their experienced replies!! Problem is now solved. This is going to be hard to believe but it was a mixture issue. My protocol was to lean for taxi to about mid range and until I saw the egt's increasing. During the annual the AP cleaned the injectors in which a lot of what looked like carbon was flushed out. The only thing we could come up with was... with clean injectors I was simply not leaning enough for taxi. Pulling the mixture back past the point I was performing the previous run ups stopped the excessive 200 RPM drop down to ~50 per side. I just made a one day trip from New Orleans to Nashville and the plane performed flawlessly! Once again....Thanks!!!

[orionflt]

Thanks everyone for their experienced replies!! Problem is now solved. This is going to be hard to believe but it was a mixture issue. My protocol was to lean for taxi to about mid range and until I saw the egt's increasing. During the annual the AP cleaned the injectors in which a lot of what looked like carbon was flushed out. The only thing we could come up with was... with clean injectors I was simply not leaning enough for taxi. Pulling the mixture back past the point I was performing the previous run ups stopped the excessive 200 RPM drop down to ~50 per side. I just made a one day trip from New Orleans to Nashville and the plane performed flawlessly! Once again....Thanks!!!

Are you running your mag check with your mixture leaned?

 

Brian

[Releew]

Yes, I am running leaned for the mag test. I start up normally, lean for taxi, run up in the same leaned position. After about a 5 minute taxi, run up, then full rich for take off.

[Hank]

I taxi lean, go Full Rich for runup, then back to lean if I have to wait. What does your POH say about the runup? Mine says Full Rich.

[Marauder]

I taxi lean, go Full Rich for runup, then back to lean if I have to wait. What does your POH say about the runup? Mine says Full Rich.

Mine too...

[jetdriven]

Since then Lycoming published a service letter about excessive mag drop. Check mag drop at peak RPM for runup. Set power to your specified RPM, then lean to peak RPM, reset power to specified RPM, then check mags. Actually if the RPM is off the POH number by 100-200 its no big deal. Its not a critical value.

[Releew]

Thanks for the info.