I recently purchased a 1987 Larson 17 foot runabout with a 1983 Johnson v4 115.  This is my first boat. I’m in love with the classic boat but my issue is it will not go over 8 mph. I have found a bad coil which I replaced. New prop. Compression reads 103,105,110,114. I ran a whole can of sea foam in 1 gallon of gas which helped the idle a lot. Idles great and runs like a raped ape on muffs. Once placed in the water it won’t get out of its own way. Will rev high with the idle control but once it’s in gear it will not idle up much at all. The tach does not work but I would take a guess as to it will not rev past 1500 or 2000 rpm. I get 7-8 mph wether alone or with the family. Nothing more. I’m at a loss as to where to look next. Any help would be much appreciated.