This engine is a BT95.
It has a single top landed piston ring.
If it was going ok on its last outing and now has no compression, which I presume you have established, it is probably that the ring has decided to stick in the piston groove.
You can check this by bringing the piston to top dead centre and trying to move the piston with your finger in a rotating manner.
The ring, which is quite visible, should stay expanded to the cylinder while the piston moves about.
If you are unsure as to what you are looking at, lift the barrel carefully off the piston, checking that the base gaskets don't tear.
Don't let the piston or conrod bang agaist the crankcase.
Now you have the piston exposed you can check properly that the ring is free.
If it is stuck it needs to be removed and cleaned or replaced, as does the piston.
I use Nitromors paint remover or barbeque cleaner to soften the carbon as it's much more friendly to the metalwork than going at it with emery paper.
Heating the ring and piston can soften it enough to carefully ease the ring off the piston.
It probably stuck after you came in and you may have been running a bit rich.
I used to run our BT85 rich to make it last.
Why not give me a ring as I offered?
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