1993 75hp vrs 90hp Mercury

7920hpMercury

Petty Officer 2nd Class
Joined
Mar 13, 2009
Messages
111
Considering buying a 1993 75hp Mercury but really want a 90hp. From doing a couple of searches, looks like carbs are different for the 75 and the 90. I would guess the porting is different too, similar to the old 7.5 vrs 9.8 Mercury. So the intake and exhaust ports would be larger holes and on a different centerline for a 90 vrs the 75. So, I would guess converting a 75 to a 90 by only installing the 90 carbs is not going to totally work, correct??

And as a practical matter, on a light weight boat, would the 75 be close in mph to the 90 anyway??

Much Thanks and all thoughts would be appreciated!
 

yzf125

Petty Officer 2nd Class
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Apr 20, 2009
Messages
103
I have always read that the carbs are the only difference, As the power heads are the same. You could always look up the part numbers on each to see for sure though.
 

Tloz

Cadet
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Aug 11, 2015
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6
I have a 1991 75hp that is badged 90hp..lol....says 90hp on the cowl but I recently purchased a remanufactured powerhead for it and discovered that it was in fact a 75. Came out of the factory a 75.

From what ive read (am I am by no means an expert) is that the jetting and timing are the only difference between the 75ho and 90hp for my year (1991) and some other years too.

Im having an intermittent issue at the moment with my engine currently (runs for a few min and then cuts out for afew minutes...over and over). When I get it sorted and broken in, I plan to try out the 75hp to 90hp "conversion".
 
Last edited:

ondarvr

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Apr 6, 2005
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11,527
Jetting is NEVER the only difference, the jets are different because there are other differences that require different jeting.
 

Tloz

Cadet
Joined
Aug 11, 2015
Messages
6
Jetting is NEVER the only difference, the jets are different because there are other differences that require different jeting.

What about jetting and timing? Larger jets and advanced timing. That's all that I have found thats different so far.
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
Have you checked the exhaust tuner? Just like cars, HP is achieved with cam, compression and carburetion. Two strokes have the same requirements but since there are no valves and cams, the exhaust tuner does some of that work. The WOT rpm is 5500 on the 90 whereas it is 5250 on the 75. With the gear cases being the same, there would be very little increase in speed given both motors are using the same prop. The 90 may however be able to twist a prop with slightly more pitch.
 
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