I've got a 1979 Monark Magnum 16' 9" bass boat with a 1974 mercury 115h.p. inline six with thunderbolt ignition and all cylinders are between 115 and 120 p.s.i. compression and it has a 21pitch prop. The boat has a strange looking hull I don't know if you would call it a tunnel hull or a hydroplane or what ? The guy I bought it from claimed he had it to a best of 70 but It has a speedo with both mph and kph which I've gotten to a best of 50mph and that lines up pretty good with 70 kph so he may have been reading that it has no tach so I'm not sure what it's turning for rpm's. Anyways yesterday my brother in law drove beside me (about 50 yards out) and according to the digital tach on his jetski we were doing 62. So which is it ? A new tach and speedo will buy a lot of gas which this old motor likes the taste of.