The mystery of the compression-less Gold Star

Top half of a BSA CB32 Gold Star.

I've owned a CB32 Gold Star for longer than I care to remember. I say longer as in all the years of my ownership I've never gotten round to making it work properly. It's been a real heartache machine, there's not much intrinsically wrong with it but getting it working spot on has eluded me, and it's not through lack of effort. Time has come now that I'm fed up of seeing it on the bench and have decided to have a final push at getting it on the road for the next season.

The last big go at making it work involved sending the motor off to a friend of a friend for a rebuild. It came back all checked over but untested. The motor was installed back in the bike and started up. However there was a mysterious lack of compression. For various reasons disappointment set in and rather than get it sorted straight away the Goldie was put to one side again. Since then various vain attempts have been made at sorting the bike without stripping it down. But now the time has come and the top end has been stripped.

What has this top end strip revealed? Absolutely nothing. I had hoped that the valves would obviously be leaking, the piston rings misaligned or for there to be some other obvious cause. The valves seal perfectly, there is no obvious leak from the head gasket, the piston rings are fine. I had even wondered if too low a piston was fitted but it slides up to the very top of the bore at the top of the stroke and is a dome topped Omega which should give a ratio of something like 8.5 to 1. Not much to do now apart from re-assemble and hope it magically becomes better unless someone out there has any ideas... 

And the bottom half of a CB32....

Piston seems fine. It's new. No obvious signs
of piston rings not doing their thing. The valves
seal properly and the valve timing is right.

Shiny CB32 Omega piston.