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I have a 1970 Johnson 40hp motor that won't stay in forward gear when throttling up. As throttle is applied, the boat start to plane out then "let's go" of forward momentum and the engine rev's up.

The fwd/rev selector lever is all the way fwd when this happens, and I can regain "thrust" at lower speeds. Why am I lossing my power to the prop?

Is something a miss inside the lower unit?

Bryan

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May be a spun hub in the prop.

Do you have a spare prop you can try?

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Does it jump out of gear when you say it "lets go" or does it feel like it's slipping? Do you have to go to neutral and then back into gear again?

My first inclination is that your shift linkage may need adjusting. If that's not the problem, what joe says is what else I'd suspect.

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I know this is just "Guess" work but, I suspect that his prop is a bit worn and he's "Ventilating"- where the prop blows a big air hole or cavity around itself due to "slip", and she drops off plane and revs out. What does the prop look like (condition) ?

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When the shift lever is put into fwd gear, I can start moving forward, i then advance the throttle slowly, sort of babying it and at about 1/2 speed the engine revs up and I lose thrust. Pull it back to an idle and she grabs again.

Prop looks like it is in good shape, couple of dings but otherwise in good shape.

This was the first time i had the whaler out on the water since acquiring it. I had the carb bead blasted and cleaned up and rebuilt, new water pump installed as well. Engine was running rough on the water and using alot of fuel before I finally leaned her out via the high speed rich/lean adjustment knob. I could not get the boat to plane out previous to this, but once I leaned her out she jumped right up and flat our ran well for about 6-7mins before I felt the "prop let loose" and rev up. That's when the trouble started. I could idle around without trouble, but put any throttle to it and it would feel as if it was disengaged from the prop.

Joe - if the hub on the prop is bad, could I put it in gear and try to turn the prop by hand (engine off of coarse)? would that tell me if the hub was bad?

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For safety reasons we cannot tell you to try that.

I would try a different prop or have that prop checked out by a professional.
Get the dings fixed while you are at it.

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Props are funny, I've bent them back and not noticed anything different in performance, other times I've had prop's get small "Dings" in them that affect performance and vibrate because they are off balance.

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Well...I will go fetch me a new prop this weekend and have the old one checked out and repaired if it is bad (always need a spare anyways) I appreciate everyones advice and knowledge that is always displayed here at Whaler Central....Thank you...will post an update once i get the prop on and the boat in the water....I am iching to get going as I loved the way the little 13' Sport sat in the water last weekend...what a stable platform...I am impressed!!!

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New prop installed, old prop being refurbed for a spare. Put her in the lake and putted around for an hour per the prop guru...then gave her the throttle..and she jumped out of the water and off we went. What a great little boat.....had 5 people (wife and 3 small boys, and myself on board and she jumped up on plane and away we went. I am still trying to find the optimal motor angle setting (boy i wish i had power trim) to get the best performance from the ole johnson sea horse......still not idling good, but good enough to have some great days out on the lake.......Thanks for the input, as always, you guys are the best!!!

Bryan

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Fantastic news!!!

You may want to start thinking about rebuilding your carbs in the fall after the boating season is over.

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You can run some stabil in the gas. That should clean up the carbs some.

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I think in post 5 he said he had the carb bead blasted and rebuilt already. It may be something as simple as a low speed adjustment on the carb, or the problem may lie elsewhere.

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Your right Dave he did. Maybe he needs to readjust with the new prop.

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bhgran,

That's good news that the new prop works properly.
Let us know what was wrong with the old prop once you get the bill.

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sounds like the prop
when your getting at the point that it slips out of gear
try trimming the motor down a little