[1858]

What should the compression be on a 225 shortly after a rebuild? How about 60,000 miles later? How about 120,000?

 

[RRich]

Do it warm, with all plugs removed, block throttle all the way open, do it for 3 or 4 pulses - all cyls the same number of pulses - else it's not accurate.
Between 110-150 -- most important it must be within 10% from highest to lowest.

 

[1858]

My 225 with the bad cam that I just removed had 64,000 miles with compression between 130-135.

The replacement engine has 150-155.

Does 130-135 indicate that the other engine would probably do well with a cam only, not worth an entire rebuild? It ran strong until the miss started when the valve quit opening due to the lobe rounding off on the cam. It was on the #6 cylinder, is that common to these engines? Seems like I've see other similar posts.

 

[RRich]

A compression test will often NOT FIND A BAD CAM.
The engine's turning so slow at cranking speed that air gets in anyway. A bad cam limits valve opening, not closing.

I actually did a compression test on something that an intake pushrod had completely fallen off - and compression tested fine. Apparently the suction on intake stroke was enough to open the intake valve enough to let air in to compress. Had me going for awhile!

Best way to verify a bad cam - look at the valve movement compared to the others. Even then, make sure it's the cam before pulling everything apart. It's possible a bad lifter - they can get badly worn - become "squishy."

Easy way -- if you can reach down through the head holes with a lifter puller, (or remove the intake manifold) pull out the lifter and a good one next to it - put the known good one in the bad hole - rotate by hand and measure lift again.

Generally when you have a missfire, start with ignition, check for vacuum leaks, then compression, - if still misses but above is fine, then look at valve movement.

Whenever a new cam is used, use new lifters - prevents that problem at 50,000 miles.
And very very important, use the new cam break-in procedure even if the old cam and lifters were used and the lifters were put back on the same lobes (that's extremely important that they go back where they came from - mixing them almost garantees failure.)