1989 johnson 150 only 4200 RPM

[robertdale]

I'm new to this forum but have been looking around for a week or so. I recently purchased my first boat it's a 1978 Glastron 17' bass boat. It is powered by a 1989 150 hp johnson. It runs very good. starts and idle fine. Runs well on top end with three adults it will get up to 52 GPS 4000 RPM. The problem is it only runs about 4200 RPM trimmed all the way up with 1 person. The prop is a SS Raker 14.5X24. Where should I start. I'm new to outboards but mechanically inclined.

 

[tashasdaddy]

Re: 1989 johnson 150 only 4200 RPM

sounds as if it's way over propped, should turn atleast 5000. the prop guys can help you. post here http://forums.iboats.com/forumdisplay.php?s=&daysprune=&f=35

 

[robertdale]

Re: 1989 johnson 150 only 4200 RPM

The previous owner installed an adjustable jack plate on this set up. When I trim it all the way up it seems to run better the farther up it goes no porpoising or bad handling. I may raise the plate to give me a little more trim.

 

[tashasdaddy]

Re: 1989 johnson 150 only 4200 RPM

you are overpropped. you are lugging the engine, reduce prop pitch, and let that engine turn where it is suppose to, or you will wear it out before it's time. with a jack plate you need a water pressure guage to make sure the engine is picking up enough water. get to high and you cavitate.