Mag drop

Now that I am using the NGK plugs, I will definitely be changing them every 100 hours.

Although the rpm drop and the differential are within spec, it is disconcerting to have things change all of a sudden. 

I will rebalance the carbs and see if that helps get things back the way they were for the first 290 hours on the engine!

Good news!  I think that the problem is solved! I went to the hangar today and removed the rest of the plug caps, trimmed the plug wires, and screwed them back on with a dab of dielectric grease on the end of the wires. I also cleaned the inside of each plug cap with a q-tip that was damp with a little DeOxit solution. I then wiped off any residual solution from the inside of each plug cap with a with a dry q-tip and reinstalled. After I started and warmed up, I did the runup and ended up with smaller rpm drops than I have ever seen, around 80 rpm! After that success, I checked the carb balance and made a tweak to get the CarbMate smiley face all the way to the finest resolution! Couldn’t be happier!

Thanks to everyone for the tips and suggestions. I can’t be sure but I think the issue was the inside of the plug caps being dirty (maybe with carbon from arcing???).