Causes of 2100 RPM engine vibration.

[AL in NY]

I have a very loud and noticable engine vibration at 2100 RPM's which I can accelerate through and the engine returns to running smooth a glass. This vibration is not as noticable in 1st or 2nd gear, but increasingly noticible in 3rd and 4th(F150 OD trans).

I have just really started putting some big mileage on "my" new engine recently, that is how I figured out it was engine vibration and not engine knocking. I have been fighting a rear main oil leak since last spring which limited my driving until now. I made the trip to the 2012 New England Meet last weekend and put almost a 1000 miles on my 30 CCPU. Made the entire trip without any major problems, just had the drivers side window freeze up on the trip. But this trip really highlighted the vibration problem. Had noticed it before doing local driving, but didn't figure out it was engine vibration until this long trip.

My engine has a counter-weighted crankshaft drilled for pressurized oil and was balanced by Rich Falluca at Antique Engine Rebuilding. The flywheel was lighted to 34 pounds and balanced a few years ago by Bob Bouldin in Belfast, NY with another crankshaft. The rods are Rich's inserted rods and the pistons are Pete's modern Chevy style pistons made for the Model A.

My questions (finally) are: Is it possible I have some sort for crankshaft harmonics occurring at 2100 RPM's as a result of not having all my bottom-end parts balanced together? Has anyone else seen this type of vibration with a counter-balanced crankshaft?

The main and rod bearing clearances were checked in June and found to be .0015" on all. I have execellent oil pressure, 20 lbs at idle and 35 lbs above 1000 RPM's. The engine performs beautifully with plenty of power and it idles quietly at 400 RPM's. Right now I am trying not to run the engine very long at that RPM, but I am considering pulling the engine at the end of the driving season and having all the components balanced together.