The exhaust pipe has to be totally clear / clean . That's why the use of oven cleaner or even drain cleaner .
Go ahead and pull the cylinder and head , along with the piston . Remove all carbon . That includes any blow by on the piston and it's likely there will be build up in the ring landings . Remove the rings , carefully , and use a pick or small screwdriver to clean the landings .
Clean the cylinder ports of any build up .
Pull it back together and do a compression check . 100 pounds would be the low end and a signal that it needs rings , at least .
Hold the throttle wide open while kicking multiple times to get an accurate reading .
"How does [compression ratio] translate to PSI"
This , from reference.com :
To calculate the pounds per square inch (psi) from the compression ratio, one needs the compression ratio and the atmospheric pressure measurement. Then, the first number in the compression ratio is multiplied by the atmospheric pressure, then divided by the second number in the ratio.