Hi Flingwing, I'm new to this forum so forgive me if someone has already answered. What you describe is most probably either a burnt valve or a valve kept open just enough to not allow full compression. If it was rings the oil you put in would have sealed them and brought your compression up higher, the fact that it didn't and you can hear air out of the carby suggests an inlet valve. If it's a new build you could try tapping the valve stem gently or backing off the valve adjustment 1/2 to 1 turn and see if that helps. Cheers