1975 25HP evinrude full throttle problem

[Michael77]

I have a 1975 25hp Evinrude. It runs really smooth and starts right up but there is one problem, at full throttle sometimes after a couple of seconds and sometimes maybe after 15-20 seconds the engine sounds like its revving really high almost as if its in neutral but its not jumping out of forward gear. Then i just have to move the throttle down just a little and it stops revving. I just put in a brand new clutch dog and forward and reverse gear. I also have a new prop on and i marked the hub to see if it was moving but its not. It runs fine at about 85% throttle but once i go for full throttle it seems to rev high and then i have to back off the throttle. Any suggestions anyone?

 

[cajuncook1]

Re: 1975 25HP evinrude full throttle problem

I am certainly no expert in this area, but did you change the pitch size of your prop? ( You stated you got a new propeller ) If your pitch size is too small for your recommended rpm range, you may be over the max rpm range of your motor. The reason I say this is because you mentioned it runs and idles fine, but when you open it up and 15 to 20 sec pass, its starts to rev high (That would the time frame of maximum planing speed and upper rpm range at WOT. You also mentioned that you marked your hub and there was no slipping noted.

Or did you adjust the height of your motor on your transom, causing the level of the propeller to be slightly higher and causing cavitation and the motor thus rpm-ing much higher?

My 9.5hp and 15hp have shallow water setting on the tilt of motor and my brother didn't realize this and took off wide open throttle (WOT) and I immediately recognized the the higher rev sound and looked back to see the outboard was in the shallow drive setting. My brother did not lock the motor in place and when he went in reverse at high speed the motor pop up a little out of the water and settled in the shallow drive setting.


Oh, well that is my guestimation. There are much more knowledge folk on this well established forum.

 

[Michael77]

Re: 1975 25HP evinrude full throttle problem

I actually have a feeling that the prop is coming out of the water. Because like u mentioned about the engine sitting on the transom, i recently rebuilt the back of my transom and fiber glassed a bunch of layers so do you think that extra fiberglass would raise the engine enough on the boat to where the prop would be sitting a little higher then normal causing it to pop out of the water at full throttle.

 

[bignefan]

Re: 1975 25HP evinrude full throttle problem

definatley sound like your prop is cavitating, did you change the pitch of the transom?If it was going into neutral the boat would stop does that happen? The change in weight and pitch( prop and transome) could cause prop cavitation

 

[Michael77]

Re: 1975 25HP evinrude full throttle problem

No the pitch should be exactly the same and the boat is still moving when it happens. I believe the prop is coming out of the water. I think there is an adjustment on the back of the engine to change the angle slightly of the engine?

 

[Vanover]

Re: 1975 25HP evinrude full throttle problem

I would agree sounds like the prop is not getting enough water around it while accelerating or on plane. I would lower the trim on the motor, or take it down one hole if it is a manual adjustment.

 

[Michael77]

Re: 1975 25HP evinrude full throttle problem

OK thank you. I am going to try to lower the trim to the first hole and see what happens.