1984 Johnson 4.5 under load

[JacquesR]

Hi all.

My new 1984 Sandpiper 565 came with a 4.5 hp Johnson Seahorse. I had the motor checked, (compression is a bit low) replaced the impeller and changed the oil in the foot. The OB runs great in neutral but under load, I can't get the full RPM. My boat is 1200 lbs and the OB should be plenty powerful. Suggestions?

[Sparx]

Time to clean the carburetor .

Sounds much more daunting than it really is.

[JacquesR]

I had the carb rebuilt last fall. When I got the new boat, the motor had a tendency to flood. It's been running great (max RPM in neutral) but as mentioned, I can't get max revs under load. I don't know if the prop is original. Will check. Don't know what else it could be.

[a64pilot]

Well low compression reduces power significantly, but if your trying to get max RPM while pushing a sailboat, that is not happening, to start with it’s not likely a high thrust motor, meaning the prop is and will always have too much pitch, best you can do is go for max number of blades with the lowest pitch made and fit the largest diameter possible, but it’s still not going to turn high RPM, and don’t try to make it, You’ll overheat it at least, stop adding throttle when speed only increase a little bit, that may be half throttle or so, but holding it wide open while it’s overloaded is not good for it.

It’s just not made for max thrust at very low speeds, it’s made for a small, much lighter, faster boat, like a Jon Boat. It works pushing a bigger, heavier boat of course, but just don’t overload it.