Mark your wheel with a felt pen (alcohol to erase) and place something like 4 oz opposite the center of the mark, but moved about 8" forward in the direction of rotation. Then do the other wheel. Back and forth. The marks getting longer means you're getting there. Eventually you're just chasing your tail and are done.
If you have both fore and aft plus up and down movement, your cast in counterweights aren't opposite the piston. Blindly adding or subtracting to the counterweight isn't going to do it. The Mr X method will allow you to figure out where to put the weight to correct for the whole mess, if you can take the time to do it methodically. The Mr X method requires movement- so putting your base on some carpet/rubbetr scraps or such to allow movement is needed.
Mr X's method does work well, without removing the flywheels, but it takes a LOT of starts and stops and time.
38AC's method is now my 1st choice.