Low EGT at idle in one cylinder...

Very interesting story.

Would you like to tell the lower EGT value  2/4 side ?∆ 50°C above is acceptable at lower rpm.any how, Lower value of EGT indeicat  rich mixture. Or faulty slow running jet in carburetor.

The low #2 cylinder EGT could also be due to a lean mixture or an ignition problem and that cylinder not firing.  At this time the idle is so rough, 2.0 IPS according to my prop balancing equipment, that the float needle on the right carb, cylinders 1/3, will not stay seated and the float bowl is flooding and pouring fuel from the vent tube.  The left carb, cylinders 2/4 is not.  Both carbs have been rebuilt to factory new standards.  If the mixture were rich on the left side wouldn't it show as a low EGT for both the cylinders on that side?  That is why I wonder if there might not be an intake manifold leak just on #2 cylinder or since the compression in that cylinder is good there might not be a valve train issue.  Remember this is only happening at idle and in the low RPM range.  Once the engine RPM is brought up it seems to run fine and all the EGTs line up straight across.  

I just remembered an old mechanics trick for locating leaking intake manifolds.  With the engine running at idle you spray ether starting fluid around the suspected leaking area and the engine  RPM should pick up and run smoother at the point the vacuum of the intake manifold draws the ether in.  Something else, the carbs have been balanced using a dual pointer manifold pressure gauge, like from a twin-engine aircraft, and the needles move together throughout the entire RPM range.  That doesn't mean however that I didn't adjust the carbs to match each other including the possible manifold leak.  So they actually aren't matched at all but one side is working as it should and the other side is adjusted to compensate for the leak if there is one.  

Are the probes new? If not swap the EGT probes and make sure it is not a probe that is starting to fail, they have a life span of about 200 to 500 hours

Garrett,

Yes, they are brand new and were installed with the AvMap EngiBox.  I had thought about switching the wiring at the EngiBox to see if the low EGT reading moved with the connections.  I just haven't gotten around to that test yet.  Thanks for bringing that up.  

Hi Jeffery

My suggestion would be to test the valve spring tension, particularly on cylinder #2.
Recently I diagnosed a rough running early 912UL. Several of the valve spring pressures were about 50% below specifications.
Replaced four dual valve springs and that engine runs nice and smooth. 

Best regards 

John Baker iRMT