I have tested compression w/ throttle propped open and closed and noticed no difference in readings, but open is smarter. When I measured 90 psig in one cylinder of my 1969 slant, it soon started showing a clean spark plug (raw, unburnt gas). I removed the lifters and push-rods in that cylinder to keep the valves closed and ran it for a year or so until I swapped for a rebuilt long block. When I pulled the head, that cylinder was perfectly clean inside (i.e. no combustion) and the cylinder walls were noticeably scuffed. If I recall, it was cyl 5, which also seems to be the one that likes to put the rod thru the side of the block, at least for several members here. Coincidence?
