Yes, compression differences can contribute to rough running. One way I learned to check on such problems was the original cylinder balance test Sun Electric Co. reccomended. The engine was run on only two cylinders, like #1 and #6 on a small block Chevy, for instance. Those cylinders are exactly 360 degrees apart in the firing order. The manifold vacuum at 1,500 RPM was recorded for that cylinder pair, then the next set of companion cylinders, 8 & 5 would be tested, then 4 & 7, then 3 & 2. A noticeably lower vacuum reading would indicate trouble in one of the two cylinders being tested. That procedure worked better than shorting out one cylinder at a time and recording RPM and vacuum drop. Yes, an engine WILL run on 2 cylinders!
Another test is the cylinder leakdown test, where a pressure regulator is used to set 100 PSI against a dead-ended air line, then the air is fed through a nipple adapted to a spark plug shell into a cylinder at top dead center with both valves closed, and the crankshaft locked. A pressure reading is taken. Let's say the cylinder retains 92 PSI. The leakdown is 8% for that cylinder. By listening at the intake, exhaust, and oil filler, and looking for bubbles in the radiator, the leak can be isolated very accurately.
All these tests can be done in a relatively short time, with little or no disassembly of the engine. Sun's slogan used to be "We Test, Not Guess!"
Jerry
