The Motor Has An Ignition Misfire Around 1,000 RPM

 I went out yesterday to go flying, got to the end of the runway to do my run-up check and found a noticeable misfire around 1,000 rpm.  It wasn't noticeable at idle nor at 1,700 rpm for the magneto check.  Obviously I decided not to go flying.

Today I went back out with my infrared thermometer.  My plan was to run the motor to determine which magneto was missing by running the motor at 1,000 rpm then switch mags to see if there was a noticeable difference.  The motor started fine and there is a pronounced misfire when running on the right magneto.  The left magneto seems fine.  The right magneto was the one I had problems with earlier in the year.

I kept it running on the right magneto until the cylinder head temperature reached about 200 degrees F.  Then I shut the motor down and quickly opened each cowl access panel.  Using the infrared thermometer I checked temperature of each cylinder.  The expectation was that if the problem were a misfiring plug that cylinder would be cooler than the good cylinders.  No such luck.  The 2 front cylinders were about 20 degrees cooler then the 2 rear  The front and rear cylinders were almost the same left and right.  Clearly not a single misfiring spark plug.

I went ahead and pulled the spark plugs for the right magneto, the easy ones on top of the cylinders.  The first thing was a visual inspection to see if they looked different.  The color was the same normal tan color.  Next I used a 10x glass and checked the plugs for any lead fowling.  None was visible and since they are new plugs they really looked good.

The last thing I did was to very carefully check the electrode gap and make minor adjusts to assure each electrode was precisely between 0.015" - 0.018".

The plugs were reinstalled and torqued.  The motor was restarted.  The problem is still the same.  Clearly something has changed in the magneto.  Before pulling it I wanted to re-read the service manual to see if checking timing, etc. might give some insight as to what to look for in the magneto.  I also need to look up what parts I've already replaced.

Hopefully tomorrow I'll get back out, do any checks, and remove the magneto.  we'll see what I learn once it's off