This is an old SI, I will try to copy it here:
SUBJECT: Magneto Drop-off
MODELS AFFECTED: All Avco Lycoming Opposed Series Aircraft Engines
TIME OF COMPLIANCE: During Pre-Flight Inspection
Preflight inspection of the aircraft invariably includes
a magneto drop-off check to determine the operating
condition of the ignition system. This test is
accomplishedby switching from both to either the right
or left magneto and noting any appreciable variance or
loss in RPM. Excessive loss, or variance in loss of
RPM warns the pilot of some defective part in the ignition
system, usually a spark plug or a magneto.
Limits for magneto drop-off (loss in engine speed
when magnetos are switch from "BOTH" to either
"RIGHT" or "LEFT") is indicated in most Avco Lycoming
publications in terms of maximum permissible
drop in RPM from a STATED ENGINE SPEED.
This method is satisfactory for most normal conditions
but does not permit any latitude in unusual circumstances
of temperature, humidity and engine wear.
Therefore, a method of checking magneto drop-off,
based on engine manifold pressure, will be found satisfactory
under more diverse environmental conditions
than the method based on engine speed alone. Consequently,
the following procedure is recommended for
all Avco Lycoming opposed series aircraft engines and
will be adapted in all new editions of operators manuals.
1. Warm up the aircraft in accordance with manufacturers
instructions.
2. If the aircraft is equipped with a controllable
pitch propeller, set blade angle at minimum.
3. Set throttle to produce 50 to 65% rated power as
indicated by the manifold pressure gage.
4. Turn magneto switch from "BOTH" to "LEFT"
and note drop in engine RPM. Return the magneto
switch to "BOTH" position.
NOTE
Operation on one magneto should not exceed
10 seconds.
5. Turn magneto switch from "BOTH" to "RIGHT"
position and again note drop in engine RPM.
6. Drop in RPM (magneto drop-off) must not exceed
200 RPM for helicopter engines and 175 RPM for all
other models. Also, the difference in drop-off between
both magnetos should not exceed 50 RPM.
7. In the event magneto drop-off persistently exceeds
maximum permissible, an inspection of the
engine should be carried out to determine the cause.
Common causes for excessive magneto drop-off
include incorrect grade of fuel; fouled or incorrectly
gapped spark plugs; incorrect timed magnetos; incorrect
fuel/air ratio.
MAGNETO CHECK BY POWER REFERENCE
Max. Drop-Off
All geared and direct drive engines. .....
All helicopter engines . ......
*175 RPM
200 RPM
MAGNETO CHECK BY ENGINE SPEED
Engine Speed
All geared and direct drive engines
All helicopter engines
(2000-2100 RPM)
(Rated RPM)
Max. Drop-Off
175 RPM
200 RPM
