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My 1985 Johnson 25hp rapidly increases RPM and loses forward thrust at certain point while throttling up (very consistent point on throttle). I assumed the old prop was slipping and it is currently at shop. While waiting a friend let me use a prop from his 1983 30hp. The two motors use the same prop. It slipped again. Not sure if his old prop which hadn’t been used for about 3-4 years was slipping also or I have a different issue. This motor was purchased and installed by a shop in Detroit back in 85 as I was told by previous owner (I have original receipt). This shop has a high quality reputation. I did however confirm the cavitation plate is inline with the keel. My boat sat for quite a few years however visually I don’t see anything wrong with the keel. Also there is no grinding sounds in lower gear case. If my prop comes back from shop and does not correct the problem, does anyone have any ideas for things to check?
JeffA wrote:
My 1985 Johnson 25hp rapidly increases RPM and loses forward thrust at certain point while throttling up (very consistent point on throttle). I assumed the old prop was slipping and it is currently at shop. While waiting a friend let me use a prop from his 1983 30hp. The two motors use the same prop. It slipped again. Not sure if his old prop which hadn’t been used for about 3-4 years was slipping also or I have a different issue. This motor was purchased and installed by a shop in Detroit back in 85 as I was told by previous owner (I have original receipt). This shop has a high quality reputation. I did however confirm the cavitation plate is inline with the keel. My boat sat for quite a few years however visually I don’t see anything wrong with the keel. Also there is no grinding sounds in lower gear case. If my prop comes back from shop and does not correct the problem, does anyone have any ideas for things to check?
Is there a transducer or other element on the hull that could be pushing air bubbles towards the prop? I have had the prop "slip" on one of my boats when my child tried to climb out of the water using my transducer as a step (while the boat was at anchor,) turning it partially sideways thus pushing air bubbles towards the engine and causing the propeller to ventilate.
Edited by drewread on 11/10/21 - 5:29 PM
Drew Read
Thanks for the reply. I noticed it is angled down a little. I will adjusted to where the flat bottom sid of the transducer is parallel with the bottom of the hull.
