I just picked up '69 Honda Trail CT90. Went through carb, valves, ignition timing. Compression is 138PSI. The manual says it should be around 171PSI. This bike got only 6000 miles on it but hasn't been ridden for more than 20 years. I read somewhere that piston rings can stick to the piston and causes low compression. Is it likely? If so, is there a way to free them up without tearing into? Seafoam in the crank case, gas tank? Diesel fuel?