Engine Compression

[Thorfinn]

Chris Harris on Youtube demonstrates a leakdown test procedure for an airhead engine that looks like a good way to judge engine problems. He has several other repair procedures for airheads that were informative. Brace yourself for his salty language.
Wavy

I reckon that Mr Harris' videos are a major boon to us airhead types. The "Leakdown" test (using compressed air injected via the spark plug orifice) is a case in point - excellent idea!

And though his language be "salty", he's also very amusing!  

Recommended viewing, in my opinion.

[nhmaf]

Any restriction on the piston's ability to draw in air will reduce the overall compression PSI reading, so you really do want to do this with the carbs "off" when testing your engine for the most accurate results- as that is also the condition that BMW uses when it specs the "good", "average", and "poor" values.   Otherwise, your figures will tend to be lower - how much lower is variable/anyone's guess.