So I have a question about doing a runup on a C-172 with a Lycoming 180 horse engine. It involves the following service instruction and concerns doing a runup and checking the mags. Service Instruction 1132B There is a debate between the IA and the Instructors on the line. Currently/previously we would start and warm up in a normal manner and taxi to a runup area. Once there we would obviously bring up the RPM's so that we could check engine instruments, smooth run, and the magneto drop off. In the past we have had problems with drops greater than 150 rpm's and we would return and squawk the aircraft. The mechanic we have has found this service bulletin and has presented it to us with an unusual procedure. In his opinion we should, once in the runup area, do the following (very simplified) procedure: Increase RPM to 2000 lean mixture for peak RPM retard throttle to 2000 rpm check mags I have the idea that this is not correct because we are changing the test to receive the results we require. To me the service bulletin gives a procedure to leave the mixture full rich, increase the RPM to 1800 (2000 max), and then check the mags. If during the initial test you get a drop of greater than 175 and no more than 50 between the two then you should continue to the leaning procedure. Once you do this you should see a drop of less than 175 and 50 between and smooth operation you may continue the flight. My questions would be as follows: What procedure would you interpret in SI-1132B If you see a drop of greater than 50 RPM in the initial mag drop, would you continue to step E(1)(d)? We do typically see high density altitude on a regular basis during the summer months but usually don't have problems with the mag drop. We have had in the past problems with certain aircraft having a high or inconsistent mag drop during the runup but it may be isolated to specific aircraft during the warm months here. I should note that if we see a bad drop we would typically do a spark plug clearing procedure but would still see the same or greater drops on the mag check. Any suggestion or comments would be extremely helpful. I am trying to find ways of making suggestion to the IA without stepping on his toes and letting him "own" the solution for this but I am finding it difficult to come up with suggestions due to my lack of knowledge. Thanks!