BF50 RPM Issue

[cczajkj]

The RPMs on my BF50 swing drastically at times, sometimes when I adjust the trim, other times when we are just running normally. This is on a 20ft pontoon and we seem to have more occurrences when we have heavier loads. I have also noticed when I open up the throttle, the rpms stay high until I back off the throttle a bit, and then when I throttle up, the rpms settle down.

Any ideas ? I did change the plugs and filter this Spring, otherwise nothing has changed.

Thank you!

 

[dark_star]

Sounds like cavitation problem. If you trim up too much the prop gets air time and with no resistance it can free spin
causing the motor to rev up.

 

[cczajkj]

Thank you for the reply --- some more info --- The prop is still underwater when this rpm revving up occurrs, and can happen at any speed.

 

[jgmo]

Hi,
Sounds like you could be losing your prop hub. As the rubber bushing starts to fail, it can lose "traction" under load and then "catch up" again as the load changes. Your description is of a "classic symptom" scenario of this.

A quick way to test it is with the boat and or motor out of the water, remove the engine cover so that you can observe the flywheel. Disable the engine from starting by pulling the kill switch key and then rotate the prop slowly by hand in the "forward" direction while someone shifts the gear selector to forward until the transmission is in forward gear and is "locked".

With the outboard in forward gear, try to move the prop forcefully by hand (it's best to wear gloves here) in the forward direction and see if you can get any rotational movement. The prop should not rotate at all without the flywheel rotating too. I doubt that you could turn a 50 hp by hand using the prop but I don't know how strong you are.

Any movement of the prop blades without movement of the flywheel indicates that the hub has is failing or has failed.

This isn't the end all, be all, high tech friction hub test but usually works. Just be aware that the hub could pass this test and still be failing. Again, it usually depends on how strong you are.

Another way to tell is to simply remove the prop and look at the hub. It's the rubber sandwiched between the prop's splined inner metal that slides on the shaft and the outer metal of the prop. A bad hub usually looks "not right" but sometimes this becomes a matter of experience to identify the failure.

Good luck and be careful.

 

[cczajkj]

Thank you, we will give it a try -- is this a costly repair -- any idea ? something I can do ? I am not a mechanic, changing oil/plugs is about the extent of my skills, but always willing to try.

 

[jgmo]

If the hub is bad, you will need a new propeller. They run around $100 or so. Checking and changing one is a simple task that is usually much easier than changing the oil or the spark plugs. You simply pull the cotter pin at the end of the prop shaft, remove the nut and washer that retains the prop and then slide the prop off of the shaft.

And, it is a chore that I encourage every boat owner to master because taking the propeller off and putting it back on is the only way to make sure your prop shaft is free from being entangled in fishing or anchor line.

Mono fishing line wraps around the prop shaft and just kills the seal that keeps lube in and water out of your gear case. If you allow that to happen, THEN you will have a costly repair.

I advise people to remove their prop and the thrust washer behind the prop to check for line wraps every two or three trips. More frequently if you operate on a lake or any water that is heavily fished.

The thrust washer behind the prop needs to be put back with the large flat surface toward the gear case so pay attention to it's orientation when you remove it. Pay attention to it's condition too. That surface that contacts the gear case needs to be smooth and flat. If not, replace the thrust washer.

Also, the prop usually slides off of the shaft easily but sometimes the thrust washer can be difficult to remove. If so, I can walk you through getting it off but be advised I have to cut them off from time to time. In which case, I use a Dremel type rotary tool and a carbide cutter wheel. A real pain but better than risking damage to your gear case.

If you need to replace your prop, buy a new prop, a new thrust washer a new nut and a new cotter pin. That way, you'll be doing the job right.

Get er done!