no RPM drop on mag check

John Stevens wrote:

Seems to me (a rank novice, almost) that the RPM drop is the
indication of actually switching off one of the mags to allow you to
test the other in isolation.

It usually is, but if the timing on both mags is retarded a little (about 10 degrees
does it on my O-320), the engine will run at the same speed on either or both mags.
This could be the problem, or, as others have said, the switch could be failing to
ground the mags in the "L" and "R" positions.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

I concur with several others - the timing's off.

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I remember reading another post in which the owner said that his ignition
switch was mis-wired. L & R were shorted together, so all three settings
resulted in both mags being energized.

Don't know if this is possible, but it's something to consider.

Adam

Whoever said this is not an issue is WRONG. Although I might fly the
plane with this problem (depends), I would get it fixed as soon as
possible. No mag drop on runup is UNACCEPTABLE. You need to know both
mags are working. You could have timing issues, bad mags, bad ignition
switch. The suggestion about the EGT guage is good. EGT will go up
when the engine is running on one mag.

Dave Butler wrote in message ...
Our Mooney just came out of annual and now the tachometer needle doesn't move at
all during the magneto check as you switch through R, L, BOTH. The engine does
stop running with the switch in OFF.

I asked my my partner who manages the maintenance relationship to look into it
and he contacted the IA who did the annual. The answer I got was:

"I discussed mag drop problem with the IA: he complied with the mag AD during
the annual stating that he also timed the mag and set the points and would
expect very minimal RPM drop. He also tested the P lead and found it normal. The
only confirmation is turning the key off and the engine should stop, I guess
that was done. Looks like a non issue."

This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual magneto",
two magnetos in a single housing.

I will check the POH to see what it says about RPM limits during the magneto
test. I'm not where I can access the POH right now.

My question is: is it OK to not have any RPM drop at all when switching from
BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up with any
reason it's a-bad-thing.

Thanks.

No mag drop on runup is UNACCEPTABLE. You need to know both

You can wait until it gets dark. Pull the airplane by a mercury vapor street
light. Try to set the RPM at 1800. If the prop appears to stop at 1800 RPM your
tach is right on the money.

Run the engine until the prop appears to stop like if a timing light was
shining on it.Do your mag check. The prop should appear to turn to the left
because the RPM dropped. If it does this you should be able to tell if the mags
are independent of each other.

This takes some precision throttle movements to get the initial 1800 RPM
setting, but in the end the whole process is self explanatory.

This is also a tach check. The prop should show some motion change at 600 -
1200 - 1800 - 2400 RPM. Three blade props might or might not work like this,
logic says they will but at a different RPM.

Hope this helps.

Just My 2¢ Worth

WARREN1157 wrote:
No mag drop on runup is UNACCEPTABLE. You need to know both


You can wait until it gets dark. Pull the airplane by a mercury vapor street
light. Try to set the RPM at 1800. If the prop appears to stop at 1800 RPM your
tach is right on the money.

Run the engine until the prop appears to stop like if a timing light was
shining on it.Do your mag check. The prop should appear to turn to the left
because the RPM dropped. If it does this you should be able to tell if the mags
are independent of each other.

This takes some precision throttle movements to get the initial 1800 RPM
setting, but in the end the whole process is self explanatory.

This is also a tach check. The prop should show some motion change at 600 -
1200 - 1800 - 2400 RPM. Three blade props might or might not work like this,
logic says they will but at a different RPM.

Thanks, Warren and everyone! To summarize: There could be a problem with the
switch or the p-leads, but since the mags were just timed, it seems likely the
timing is off. Thanks for the mercury vapor light tach check suggestion, Warren.
That should be a more sensitive test than looking for small movement on the
analog tach. The consensus seems to be this is a symptom that should not be ignored.

(assuming the timing is wrong): the danger is (1) reduced power output? (2)
detonation? (3) inability to test magnetos at runup. Is that the kernel of it?

Thanks again to everyone. Dave

Dave Butler wrote:

(assuming the timing is wrong): the danger is (1) reduced power output? (2)
detonation? (3) inability to test magnetos at runup. Is that the kernel of it?

Reduced power, increased EGT, and increased risk of burning exhaust valves. Inability
to test mags comes way last. Risk of detonation is actually decreased (probably
eliminated).

By the way -- if my IA gave me the reply that he "would expect very minimal RPM drop"
after timing them, I would take it elsewhere to get the timing checked.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

"G.R. Patterson III" wrote in message ...

By the way -- if my IA gave me the reply that he "would expect very minimal RPM drop"
after timing them, I would take it elsewhere to get the timing checked.


Excellent point. Most of the smaller ContiLycoSaurus engines
should show a minimum 50-75 rpm drop. This should be noticable just
by the sound. If the mag drop is not perceptable, that's a bad thing
and the IA should know that.

John Galban=====N4BQ (PA28-180)