Years ago, I appropriated my dad's old B&S-engined mower on which the engine would not start. I eventually discovered that the exhaust valve seat insert had detached itself from the head and was preventing the exhaust valve sealing properly. I imagine that I discovered this with a compression test, but I might just have stripped the thing down for the fun of it and then happened upon the problem (it was a while ago).

A compression test is easy and will allow you to rule in/out the problem I had.

I simply fixed the valve seat insert back in place with exhaust paste and the application of a blow torch, to cure the paste. It ran fine when put back together.

Regards.

Terry.