I have a 1984 Johnson 4.5hp on my Sandpiper. When in neutral, I can rev the outboard to its maximum RPM but when in gear, it seems incapable to reach max RPM. The boat is 1200 lbs and the OB should be quite enough. What's wrong?
What's wrong with my OB
- Thread starter JacquesR
- Start date
The pitch of the propeller. Different boats produce different drag according to their displacement and the shape of their hull. A properly pitched propeller should allow the engine to reach maximum rated RPM and not to go over. Outboards are sold with a general pitched propeller which may work better on some boats than others.
We called them speed props or power props... anything with a prop can have blade pitch, blade depth (chord), and length changed to bias toward power or speed. We used to play with props on our RC planes. A power prop could let us climb vertically at low speed with a “40” sized plane and a “40” engine, but the engine would never get near full RPM. A speed prop would require a much longer runway to take off, and the vertical, low speed climb would be nil, but the plane would hit a higher speed, and the engine would hit max RPM. Get too biased in either direction, and you suffer too much degradation.
The same will go for your outboard. Depending on the prop, your motor will not hit max RPM while pushing your boat, but it will have the power you need.
The same will go for your outboard. Depending on the prop, your motor will not hit max RPM while pushing your boat, but it will have the power you need.
Good feedback from @agprice22 (still flying RC with my son now since he was 6 years old) and I also don't understand do you lack power under load like @jviss asked? I had 2 6hp Tohatsu outboards, a regular one for my 12' flat bottom fishing boat and a sailpro for my 25' O'Day. The regular one was very fast to get the little fishing boat on plane, the sailpro isn't fast and really isn't much different from mid throttle to max for speed, just more torque to push the larger displacement hull.
Sounds like it isnt making enough power to turn the prop under load. That motor is almost 30 years old. Probably is in need of some head work. Or a good tune, at least. Is this a "new to you" motor, or have you had it awhile? As in, whats its history?
Its new to me. I had it checked. Compression is a bit low but otherwise everything checked out. It seems to me that even with less than ideal compression, it should achieve max revs.
If the compression was good, then Id check prop pitch and carb. Since its a new to you motor, I would check the internet to see what the recommended prop is. This website has a prop guide that may help if you dont have the manual: https://www.rubexprops.com/ You can at least see if your prop is close to being the right size. Id be surprised if its been changed unless the original was damaged. And then why get a different pitch? The most common issue, in my experience with small motors, is the carburetor. The jets are very small. I assume when you rev up the motor in N you dont go wide open throttle to get max RPM. Im not sure how yours is jetted, but its probably not on the high speed jet when testing in N. When you test under load and you have the throttle wide open a clogged jet or incorrect mixture will cause low power. Also check the fuel filter. A decades old filter will prevent full fuel flow and rob power.
I had the carb redone last fall after getting the new boat. The motor had a tendency to flood, so I figured a carb rebuild was in order. Yes I rev the motor to max rpm in neutral, so I don't think the carb is the issue, but I'll have to check the prop to see if original.
But the throttle is not open all the way in N, correct? While in Neutral you should normally get a motor to max revs with about 25-50% throttle. When in gear you may need the maximum throttle open position to get max rpm. The fuel demands are quite different. Thats why a clogged fuel filter or main jet could be limiting your max power. Having the carb "redone" can mean a lot of things, and is possible to even cause dirt to get in the carb during the rebuild if the tech is not careful. Ive done it myself. Just trying to explain and help. Hard to diagnose on the internet.