My '66 has an engine in it of questionable lineage. 

At any rate, compression in #3 is very low, like 50, where all the others
are pretty reasonable, around 95 - 105.

I ran compressed air into the cylinder, and checked to see where it cam out.
(Cylinder was in firing position)

There appeared to be a draft coming out the exhaust pipe (uhoh), and perhaps 
a hint (I could be wrong about this one) coming out the carb.

I also took the valve cover off and there appeared to be a hint of air coming 
out of the exhaust pushrod tube.

I have a feeling none of the above is good:-}

What I was wondering, is given the symptoms, would a replacement head go a 
good distance to solving some of these problems?  Are heads usually replaced in
pairs?

Thanks,
...Markus

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'66 "rough looker, hackjob skylit" Westy aka "The Tardis"  
                        (british racing green/white)
'74  Thing              (pumpkin orange) 
'78  Bug 'vert          (black, white top)