Had compression problems on an engine I sold.
I had it on my Sprint for about a month - before I took it apart for the restoration - and it was working perfectly.
I sold it and even installed it on someones Sprint and even then it was working fine.
Few months later, he called me up and asked me if I could have a look, as it was going on 3 cylinders.
Everything was fine with the sparks and carbs, but it was still going on 3.
I made sure all the sparkplugs were working, and removed the cables one by one to find out which one was not responding.
What I found was that cylinder 3, even though it had spark, it wouldnt make any difference with spark cable on or off.
That is a clear sign of no compression...
I drove the car about for a couple of days, trying to thrash it from time to time, and then it started working normally.
Took it for a nice blast up the mountains and back and everything was back to normal.
The car has been working fine ever since