Run-up Roughness (Right MAG)
 

Hello,

Our Aviation Air Explorer Scout program has a C-172 M 1974 model. I am a CFI with the group and today during run-up did the traditional mag check. During runup the left mag small drop nothing unusual. Though, when we do 2 clicks left and check the RIGHT MAG (Serious engine roughness) and large decrese in RPM. Any thoughts, that can help me to discuss with our A&P/IA during normal hrs. on Monday? Do you think it is a bad set of mags or just one? I know every 500hrs they are to be inspected and/or overhauled.

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THANKS

I beleive that means you have a fouled spark plug.
Did you try throttling up and leaning out the mixture? That usually clears the plug.


Here you go I found something that may be usefull to you on AOPAs site: http://www.aopa.org/pilot/features/mos9702.html

Scroll down to "Bad mag? Don't taxi back yet."

The things listed on that AOPA site is exactly what I did. The issue remained after doing those steps twice. Also don't think it would be spark plug issue as the C 172 M A/C has 8 spark plugs which all were reinstalled as NEW June 2006.

Thanks,

Like puppyz said, most likely fouled plug. This happens if you fly around all the time without leaning the mixture. You get carbon deposits that prevent the electrode from firing properly. You can usually burn off the crap by running up the engine and leaning out the mixture a whole lot to increase the temperature enough so that it clears itself or if you have a lot of time to spend you can change plugs. If it is really really badly fouled you may not be able to burn it off in which case you are only left with the taking the plug out and changing or cleaning it manually.