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Slick Mag dropping more that usual

mikehoover

Well Known Member
I have a Slick impulse mag on the left and a Pmag on the right. During my run ups, I am used to having a 60 rpm drop when the left Slick mag is grounded, and a 90 rpm drop when the right Pmag is grounded. This is an O-320-D1A from Vans and has performed well for 115 hours (minus the right side Slick mag failure at only 11 hours.)

Recently, I noted that my left Slick mag started dropping on run up to about 120 when I was used to it dropping about 60. Also, there is an abrupt jerk when I switch off the mag in run up and had intermittent skipping. We removed the mag to find a light coating of oil on everything. It was cleaned, a contact kit installed and remounted on the engine. When running up, I get the same 120+ drop but the intermittent skip is gone.

All through this, the Pmag is performing as expected.

One other thing is that when I pull the throttle all the way out, the rpm is dropping to around 550 rpm where I am used to 650.

Can this be a coil or capacitor problem in the Slick?
 
Could be those or a bad plug. Start by checking the plugs, including resistance.
 
Worn breaker cam? Did you look at it? The spark gets weaker with wear as the break point starts to occur after E gap.
 
What am I missing?

I must be reading something wrong. It sounds like you are saying the drop is greater than it was when the mag is switched off.
i.e. When you switch the mag off you now get 120 drop where you previously got 60.
If that's the case, the Pmag is running at lower RPM than previously. That doesn't sound like a mag problem. That sounds like Pmag problem.
 
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I must be reading something wrong. It sounds like you are saying the drop is greater than it was when the mag is switched off.
i.e. When you switch the mag off you now get 120 drop where you previously got 60.
If that's the case, the Pmag is running at lower RPM than previously. That doesn't sound like a mag problem. That sounds like Pmag problem.

Mel,
Now you are making me doubt myself :) With a switched key, when you put the key to L, you are essentially grounding the Right mag and running on the left and Vice versa, correct?
 
Mel,
Now you are making me doubt myself :) With a switched key, when you put the key to L, you are essentially grounding the Right mag and running on the left and Vice versa, correct?

Assuming it is wired properly, correct.
 
I must be reading something wrong. It sounds like you are saying the drop is greater than it was when the mag is switched off.
i.e. When you switch the mag off you now get 120 drop where you previously got 60.
If that's the case, the Pmag is running at lower RPM than previously. That doesn't sound like a mag problem. That sounds like Pmag problem.

Yes I did say it backwards. Sorry for the confusion. When the Pmag is grounded, I traditionally get a 60 rpm drop on the Slick, but now am getting 120. The Pmag is working as expected.

We "think" we have isolated the problem to the Slick by disconnecting it from all wiring (i.e., unfettered live mag) and running up with same results. I say we "think" because I don't want completely assume it is a mag problem and blindly overlook some other possibility. Checked the cleaned, tested spark plugs, used another set, and ohm tested the harness wires. They all seem to be within spec.
 
same

that is pretty much exactly the issue i was having. I ran the savvy analysis mag stress test...emailed in data and they said there was a very slight variance in spark on the the mag. (no issue with lightspeed side) and still within test tolerances. It was different than i was used to...and i was not confident...so I sent it to aircraft magneto service for a full 500 inspection and its perfect. (i had 580 hrs so it was due anyway) A few hundred well spent. 100% happy to know its perfect. They did a great job.
 
that is pretty much exactly the issue i was having. I ran the savvy analysis mag stress test...emailed in data and they said there was a very slight variance in spark on the the mag. (no issue with lightspeed side) and still within test tolerances. It was different than i was used to...and i was not confident...so I sent it to aircraft magneto service for a full 500 inspection and its perfect. (i had 580 hrs so it was due anyway) A few hundred well spent. 100% happy to know its perfect. They did a great job.

Ironically, my brand new slick was dropping about the same RPM (120) on a brand new engine and since I did not have much other reference and it was within the tolerance of Lycoming spec, I did not concern myself with it much. At about 480 hours, I sent my slick for the 500 hour inspection and since reinstalling it back, I get only 60 RPM drops and about 70-90 with my PMAG which is on the right. So not exactly sure what they have done that it is preforming better than when brand new !!!!
 
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