Yes Dave,
The website is laid out a little funny. There is actually more than one
suggested cause. One of which is improper timing which has been suggested in
a number of other replies. I'll post it here for your viewing pleasure.
No rpm drop when checking magneto in Lycoming or Continental aircraft
engine
Warning - see a mechanic immediately - if the propeller is moved even
slightly the engine may fire causing injury

A drop in rpm is expected when one magneto in a redundant ignition
system is shut off. Should the propeller be moved by hand (as during
pre-flight or maintenance) and a functional "hot" magneto exists, the engine
may fire and cause injury to personnel.
1. Open magneto primary (P) lead causing hot magneto.

2. Magneto timing advanced beyond the specified setting.

3. Open in the grounding circuit of the feed through capacitor (Bendix
S-1200 series magnetos see Bendix S.B. 624).

4. Defective ignition switch.

5. Open magneto capacitor. For Bendix magnetos see Teledyne Ignition
Systems Critical Service Bulletin CSB641 or latest edition.




"Dave Butler" wrote in message
...
Rich Badaracco wrote:
See the following site:
http://www.sacskyranch.com/eng101.htm

Thanks. Is there something in particular I should see there? I didn't see
anything that got to the heart of my question. The P-leads must be OK
since the
engine stops when the switch is in OFF.

Dave

"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.






--
Dave Butler, software engineer 919-392-4367