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Bad RPM Drop? Check Plug Washers

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I think this is common knowledge among mechanics. But for unknowledgeable commoners like myself it was baffling. All summer I have had a 200-400 rpm drop when checking the left mag. No amount of clearing or lean taxying could manage it. After flying, the drop was 60 on both sides. But once shutdown, the subsequent taxi would yield the same results.

I was sure this wasn’t a mag issue. I cleaned the plugs to no avail. I then swapped the plugs from left to right. I noticed that the plug at the top of number 3 had an extra washer stuck to the port. Hmmm. I pried it off and hoped against hope that this was the source of the problem. I have only managed two flights since but both mags now drop 60 whether cold or hot. I have been told since that this sometimes happens with bad CHT probe installations (not my case) because the extra little distance away from the piston makes the plug fire cold and subject to fouling.
 

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