no RPM drop on mag check

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)

Bigger bore cylinders (like an IO-360) should show even more mag drop
than small cylinders (like an O-300). It takes slightly more time for
the flame front to get across the cylinder head. Something is wrong
here.

wrote:
Bigger bore cylinders (like an IO-360) should show even more mag drop
than small cylinders (like an O-300). It takes slightly more time for
the flame front to get across the cylinder head. Something is wrong
here.

Thanks everyone. Despite gorgeous weather here, I haven't had a chance to try it
again lately.

Partly based on the feedback from this group, I went back to my partner who
manages the maintenance and asked him to investigate it a little more. He says
he tried it and got a 25 RMP drop on each mag. I haven't been able to check it
out again myself, but I'm wondering why he sees a 25 RPM drop and I saw nothing.
Anyway, that's where it stands for now. I'll let y'all know if I learn anything
more. Thanks for all the information and advice.

Dave