1984 cmc Speedster Low rpm miss. 75 lb compression #2 Cyl

So the cylinder isn't moving too when that piston goes up and down - is it?  If it varies only thing it could mean is worn rod or main bearing --- or a connecting rod about to break or piston cracking at the rings. Can you pull that cylinder and check for looseness of that piston and connecting rod assembly.   Strange that #2 is the low one but # 1 has the varying compression ratio.

A compression check is half the information you need. You should have done a leakdown test to figure out where the leak is: intake valve, exhaust valve, or rings. Leakdown testers are very inexpensive these days. I got one on Amazon for $30.

Did you dump oil in the cylinder and do a compression check again? If you didn't, you should have. If the reading goes up it usually means rings.

What Danny said x 2...Since you now have it somewhat apart and feel the valves are good ... it has a known issue , IHO at this juncture tear it down inspect and rebuild it as needed

I’m with Wolfgang, if the deck height is changing on one cylinder, then you really aught to follow Alan’s advice and do a teardown to see what’s loose; wrist pin or loose connecting rod cap or fractured/missing rod bearing.  Whatever is going on ain’t good.

Thanks every one. I will tear it down and see if I can figure out what is really going on. 

Mike 

The good news is that they’re really simple engines that come apart/go back together really well.  Grab a Bentleys or Chilton’s manual, or Tom Wilson’s book, “How to Rebuild your Volkswagen Engine off of Amazon and follow that for the rebuild (personally, I like Tom’s book)

You shouldn't need a gasket between case halves unless it has been “O-Ringed” between the case halves.  If it has been, get on here and you’ll get half a dozen different ways to seal the o-ring space.    Or, just shoot the groove with Permatex and rave on.

Also remember that these are NOT “American” engines.  They were designed and assembled by OCD German people.  That means that if you feel you need to apply force to get things to go together, something’s wrong.  They were designed to fit properly together without forcing anything.  

Good luck, and keep us posted.  I wanna know what’s going on in there, at least!

Didn't think of it before, but did you put a nut on a head stud to hold the cylinder down Rustrod?

That's what I was thinking - that much variation and the compression would be off.  Low compression in only 1 cylinder - sounds like a crack in that head, burned/non-sealing valve (could be cam or rocker arm too), or broken rings.  Assume other cylinder's compression was 120 ish.  Before cracking case - I'd clean that head really well.  If nothing there - pull the cylinder and see if play in that piston/rod assembly.  Also, check the oil for shiny metal particles,

Thanks for all the information.  The compression was low because the I did not have my gauge seated.   The deck height difference was the cylinder or barrel . Honed the barrel all good there.  The miss was a cracked plug. Oh. Well now I know how to remove tin, heads, and cylinder barrel's.  Thanks again Mike