1979 Johnson 70HP compression?

[mainer4]

Hi, my engine has 90PSI compression in all three cylinders, give or take a couple. I was reading a comment on a 1987 version of the same engine and the comment included the fact he was seeing 150PSI for each cylinder. I bought my motor used and have run it for several years now with no real problems on my 16 ' Starcraft aluminum runabout. I went from a Big twin 40 to the 70 and was surprised that the boat speed did not increase that much (from about 24MPH to 29MPH at WOT). Now I am wondering if that may be due to low compression. What should the compression be on one of these motors in reasonable condition?

 

[flyingscott]

I would check compression with a different gauge. Reasonable compression would be about 120ish for your motor. What Starcraft runabout do you have. What RPMs are you running? You want to be around 5500 rpm for that motor. I had a 1978 Starcraft SS 160 with a 1979 70 hp that did about 30 MPH. When I put a 1986 70 hp Evinrude on I went to 39 mph. These speeds were GPS measured.

 

[racerone]

My tester would show 140 psi.-----So find another gauge.