Re: 1980 mercury 20 hp thunder bolt
A compression test will tell you. Comp's should be even and about 110PSI or better. Uneven Comp's in excess of 5 PSI could be sticking rings or a sign of wear. These Mercurys have a history of long life. I have just finished refurbing two 1975 models a they checked out good. Pay attention to the impeller and lower unit oil, use a good quality 2 stroke oil and put Stabil in the fuel. Failures are most often due to bad impellers, so get it replaced, clogged carbs which cause a lean and poor oil condition. So have the carb cleaned.
Stabil will lessen the risk of a clogged carb.
