Well I may be wrong but I've seen this before. The rear ''bearing'' (crank shaft) is knocked out of shape biting on the crank shaft. If you loosen all bolts on the engine block, the pressure is released and the crank can turn again. If that is the problem sorry to say that the engine is Kaput.
I have a few raw engine sitting away in boxes with this particular symptom. Sadly not much can be done.

C

Crank going out of alignment?
seen it with rebuilt cranks and dropped valves

Sean

Well! I've just had a cylinder head off one side. The valves look OK and nothing blocking the cylinder, I suspect I'll find the same the other side and it's something to do with the crank shaft as Viking suggests.... I'm going to tow it somewhere more comfortable to work on before I investigated further.

As pleasant as it is I've spent as much time chatting to people on my street as dismantling things all sorts start coming out of the woodwork and thread drift happens in real life!

friend had the same problem (completly off topic! but when he was building a boat in his front garden, as he lives on a camp site and loads of torists stopped to watch him work and talk to him and its taken him 15 years to build not even a shell!

anyway back on topic

good luck hope you sort it!

as were all stabbing in the dark now,

have you taken the cover off the points box, the bob weights and the carrier have been known to self destruct at stupid rpm

Sean