1975 mercury 85hp max wot rpm?

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bayboat1973

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Hello I have a 1975 85hp mercury thunderbolt that I have revived from foot to stator, I had to totally build my own wiring harness as the original was deteriated beyond repair.... Man was this alot more work than i expected lol.
I am happy with most aspects of this nearly 40yr old motor but was wondering if anyone knew the max recommended rpm? I am currently trying to find the best prop for this rig and with the 13x 15 prop I can easily turn 5500 rpm and maybe 40mph. I think I may be better off with a 17 or 19. This is a light center console (1050lbs w/o motor gear and person)
Basically looking for recommended wot rpm but also any advise on prop pitch is greatly appreciated as well.
 
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Chris1956

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Re: 1975 mercury 85hp max wot rpm?

Actually that is better performance that I thought possible, with that rig. You might leave it where it is.
 
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bayboat1973

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Re: 1975 mercury 85hp max wot rpm?

Well I guess I'll deal with the prop if it is a normal wot rpm but sounds high to me. May could even push 6000 but sounds like a Indy car and don't feel comfortable twisting this old a motor that hard.
Well??? Anyone know for sure the max rpm?? Thanks for any input.
 
dwco5051

dwco5051

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Re: 1975 mercury 85hp max wot rpm?

Mercury manual for that motor shows recommended top end is 5500 rpm.
 
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oldman570

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Re: 1975 mercury 85hp max wot rpm?

If you have a good tach, and the motor is turning 5500 at WOT, then the 15P is OK to run. If it turns more than 5500 then I would try a 17P, and it should drop the RPM down a few hundred, and would still work. I did this to my setup and vented the prop and it works fine. Venting the prop will help get on plane quicker and not work the motor as hard at the lower RPM's. JMO
Oldman570
 
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bayboat1973

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Re: 1975 mercury 85hp max wot rpm?

Thanks everyone for your advice and knowledge, I think I'll find a 17 pitch prop and just use wich is best suited for the days purpose. I like having options so won't hurt to have a spare anyways.
But what is this about venting? Is this something I could do myself or option with new prop? You have me interested. What are the benifits?
Im running the tach that was on this boat 1999 with 50 Johnson. I also used the rectifier/regulator from the same Johnson. The regulator had its own tach wire so its a paired regulator/ tach from original equip. So I feel like its pretty dead on. Thanks for info love the site!
 
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oldman570

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Re: 1975 mercury 85hp max wot rpm?

Some new props are vented when they are made, and most are the stainless steel ones. You can vent your prop if so desired and it can help get the boat on plane somewhat quicker. There is a thread in the prop section here on Iboats Fourm that tells how to do the work needed. It is nothing more than drilling holes in the prop at the leading edge of the blades in the correct spot. If you look at the stainless props for you motor you can see one that is made for it. JMO
Oldman570
 
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