'84 Mercury 7.5hp won't accelerate

I'm having trouble with my 84 Merc 7.5 gaining speed. The first time out this year was great/no problems. Today it started immediately, idled fine, took off fine but wouldn't pick up speed... Motor raced and water "peed" out faster but no acceleration in forward or reverse. Between the 1st and 2nd time out I had tilted the boat at 45 degree angle to scrub and hose out, I guess the prop must have dug into the ground... I changed the lower unit fluid, but this didn't fix my problem. Help, what's my next step to correct my problem?

Re: '84 Mercury 7.5hp won't accelerate

I would eliminate a prop problem first. It could be slipping. The prop is a slip fit with a rubber bushing between the prop blades and the portion that slides into the spline on the output shaft. Put the engine in gear and see if you can turn the prop by hand. If you turn it the wrong way you will have a clicking noise. If you turn it the correct way it should lock up with the spark plugs in as it will be under compression. If you can turn it, then the prop is bad. If it locks up then the prop is good and you have other problems. But if the engine revs up OK and the boat doesn't move I would believe that it is the prop or a lower unit problem.

Re: '84 Mercury 7.5hp won't accelerate

Sounds exactly like a spun prop hub to me.

Re: '84 Mercury 7.5hp won't accelerate

Please understand I'm a boating rookie... When I'm behind the boat/motor in the forward gear I can easily turn the prop clockwise and hear a "clicking" sound. While in forward gear if I turn the prop Counter-clockwise I get resistance and hear sounds in the upper engine area. Should it be impossible for me to turn the prop in gear? I don't know how much pressure I should feel and don't want to break something.

Re: '84 Mercury 7.5hp won't accelerate

#33scoresagain said:

Please understand I'm a boating rookie... When I'm behind the boat/motor in the forward gear I can easily turn the prop clockwise and hear a "clicking" sound. While in forward gear if I turn the prop Counter-clockwise I get resistance and hear sounds in the upper engine area. Should it be impossible for me to turn the prop in gear? I don't know how much pressure I should feel and don't want to break something.

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If you can turn the prop counter clockwise and hear sounds in the upper end then the prop is probably bad. You engine should have almost 90 lbs of compression on each cylinder and for you to turn it by hand with the plugs in would almost be impossible, unless you are a really strong weight lifter. Pull the prop off and take it to a marine dealer to have them check it.

Re: '84 Mercury 7.5hp won't accelerate

Thanks for the info guys! I'm not sure these details matter, but I looked up my seriel number "4746724" and it looks like the motor is a 1977 and I found the original Operation and maintenance manual that covers models 200-110-75-45 and printed in the lower right corner of the manual it shows C-90-78326(6/77). So I'll assume it's not a 1984 unit, but a '77.
I took the prop off and tried to turn the spine with my fingers, I obviously didn't have any leverage, but it didn't move. Looking at the prop I noticed a groove had be worn into the prop base! It seem the prop was pushed or riding on the lower unit housing. The rubber inside the prop seems to be in good condition, but maybe when he had the boats weight on the prop I affected/moved the rubber causing the prop wear and bending the blades or something. Unfortunately, it's Memorial Day and I can't get to a prop store.

Re: '84 Mercury 7.5hp won't accelerate

I doubt that you would be able to turn the prop blades by hand with the prop off even if the compression fit was bad. It would have to almost fall apart for that to take place. I still think it is the prop. You may have spun it when you tilted it and the 45 degree angle. If there was a groove worn into the lower unit as you indicate then I have to believe that the prop hub/blades are loose as they should not move at all if the compression fit is correct.

JMT

Steve