Propeller reving high for 25hp johnson on 14 ft Jon boat

[rlowhorn]

Just started using a 25 hp Johnson on a 14 foot Jon boat. The motor is running great but I cannot run it at full gas. At full gas it revs with no more speed. It almost like the propeller is out of the water. Is the boat to light for this motor? Can I do something to get the full benifit of the motor?

 

[Silvertip]

Re: Propeller reving high for 25hp johnson on 14 ft Jon boat

One of several things is going on. 1) The engine may be mounted too high so the prop "blows out" at high RPM. 2) The engine may be trimmed out too far. 3) The prop has a spun hub -- which means the rubber between the hub and outer portion of the prop is slipping. 4) The prop has too little pitch.

 

[rlowhorn]

Re: Propeller reving high for 25hp johnson on 14 ft Jon boat

I received this boat once from my dad after his death, so as you will see from my words and discussion I know nothing about this. I reworked the boat, by repainting and reworking the transom. I purchased this motor, which form best I can tell it is a short shaft. My understanding that is what this boat needs. The motor is old. 1975, but runs good. If seems to lean into the boat at the bottom, and does not lock down. When I put into reverse it tends to want to pull out of the water. The way the motor set on the boat, the prop is pushing at a angle downward (slightly). This is the first time I have had the motor in the water. what do you mean by trim?

 

[F_R]

Re: Propeller reving high for 25hp johnson on 14 ft Jon boat

Trim is the amount of tilt of the motor in relation to the boat. It is controlled by the tilt pin which should be in one of a series of holes toward the bottom of the stern (clamp) brackets. It sounds like that pin may be missing since you say it won't lock down..it locks down to the tilt pin. Tucked down too far like you describe can be dangerous as the motor's thrust tends to force the bow down into the water, sort of like stubbing your toe.

Check for prop hub spinning by making a mark on the prop nut and another one on the propeller, aligned with the first. Run the motor till you experience the suspected slippage, then stop the motor and see if the marks are still aligned. If they are not, the hub has slipped.

 

[rlowhorn]

Re: Propeller reving high for 25hp johnson on 14 ft Jon boat

Thanks. I believe my problem may be relative to this missing stern clamp. Where would I find one of these for a 1975 Johnson 25 hp motor? Also, where would find a good service manual for this motor. The serial number on this motor is E4223990 and the model number is 25R75B. Is it possible that the clamp is there and I just do not know how to lock it down?

 

[tashasdaddy]

Re: Propeller reving high for 25hp johnson on 14 ft Jon boat

rlowhorn, email me @ tashasdaddy@comcast.net i scan and send you a picture of the locking lever and tilt pin. make it easier to understand. request lock down '75 25hp. so i know what to send you. it's a good little engine, my dad had one on a 14' fiberglass boat. i you can send me a picture of you boat & motor, especially a side view of the motor in relation to the bottom of the boat.

 

[F_R]

Re: Propeller reving high for 25hp johnson on 14 ft Jon boat

Also, there is a complete parts list diagram at http://epc.brp.com/default.aspx?brands=ej&lang=E