Thanks for the reply. My engines are IO-550's rated at 300 HP. To clarify things more, I notice at 2200 RPM the right engine RPM drops normally, but the left engine has to be around 2400 to note any drop. This happens even doing a runup after an hours flight. The mags drop normally at 1700 RPM, and the feather check works at 1500 RPM.
My guess is that the left engine prop governor could need adjustment.
As a side note, have you checked if the left prop will feather in flight and come to a full stop? I had a Baron where there was no RPM drop at 2200 and when I feathered the prop in flight, the prop would still rotate and not come to a complete stop.
The problem turned out to be in the governor.
If you have unfeathering accumulators that work, you can easily check the feather angle of the blades on the ground. To accomplish this, set RPM to 1500 and pull the prop control to the full rear stop. As the RPM drops thru 1000 RPM, pull the mixture to idle cutoff. Now you can check the angle of the stopped prop. When you are finished, just push the prop control back to maximum RPM and the prop will rotate out of the feather position.