How to convert 7.5 hp to 9.8 - vroom vroom

[gomofast]

I have an '81 7.5 merc. How can I convert it to the output of the 9.8? I've heard you can swap the carb or perhaps re-jet the carb, but I haven't gotten a definitive answer. Has anybody done this?

BTW, I'm a noob on here, so go easy on me!

 

[green4themoney]

Re: Convert 7.5 hp to 9.8 - vroom vroom

Re: Convert 7.5 hp to 9.8 - vroom vroom

to switch carbs will only make it run a little richer...... technically speaking it cant be done unless you find a 9.8 engine block and switch it into the body, i dont know why you would want to tho they are close enough in speed/ horsepower

 

[gomofast]

Re: Convert 7.5 hp to 9.8 - vroom vroom

Re: Convert 7.5 hp to 9.8 - vroom vroom

to switch carbs will only make it run a little richer...... technically speaking it cant be done unless you find a 9.8 engine block and switch it into the body, i dont know why you would want to tho they are close enough in speed/ horsepower

Oh really? I thought the blocks were the same.

I have a 14 foot meyers aluminum boat and the motor just barely gets it up on plane, and it's kinda hard to maintain. I just need a little extra ooomph ya know? But the small local lake that I put it on has a 7.5 hp max, so I can't go to a bigger engine. I was hoping to just upgrade mine a bit.

 

[Fuzzytbay]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

Get the parts manuals for both motors, then see what parts you need to upgrade it, if its possible.

 

[green4themoney]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

we had a discussion on this before try the search function. .... there are not 2 many differences but there are some that beef up the 9.8 a little higher then the 7.5 but there is more to it then just a slightly bigger carb

 

[green4themoney]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

you know i jsut looked at the technical specs for the 7.5 and the 9.8 and they are vitually the same, only real difference i found was the jets in the carb, other specs such as reed lift/bore/stroke/P.skirt size, ring size, Blocks Cubic inches were virtually the same, you could try a 9.8 carb of the same year and might actually get a little more spice out of it, that is, if it fits which it should ive seen a few vids of it on youtube and never really thought much of it but now im checkin out the specs and thinking it might just be do-able

 

[green4themoney]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

there are also a few slight changes depending on year, what year motor do you have?

 

[7920hpMercury]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

intake and exhaust ports on 9.8 are larger and at a different centerpoint than the 7.5.

 

[gomofast]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

there are also a few slight changes depending on year, what year motor do you have?

Well, my manual has a copyright date of 1981, but that doesn't necessarily mean that's the year of the motor. Is there a way to tell by the serial # perhaps?

 

[gomofast]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

you know i jsut looked at the technical specs for the 7.5 and the 9.8 and they are vitually the same, only real difference i found was the jets in the carb, other specs such as reed lift/bore/stroke/P.skirt size, ring size, Blocks Cubic inches were virtually the same, you could try a 9.8 carb of the same year and might actually get a little more spice out of it, that is, if it fits which it should ive seen a few vids of it on youtube and never really thought much of it but now im checkin out the specs and thinking it might just be do-able

Yeah, I have the original manual and all of the specs in there seem to indicate that they are identical. There can't be to much difference between them.

 

[gomofast]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

intake and exhaust ports on 9.8 are larger and at a different centerpoint than the 7.5.

Is this true for all years, or just specific years?

 

[Laddies]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

Why does this goofiness continue to go on weekly, the two powerheads are different, if you really want to know buy a service manual and go to a Mercury parts site and you will see that the 7.5 an 9.8 HP engines each have their own cyl. block it is denoted by different part numbers and the 7.5 has Comet cast in it and the 9.8 has Rocket, the difference is the port timing and compression ratio. Both engines use the same crankshaft pistons and rods, even the reeds and reed blocks are the same, the reed stops are set at the same height. The difference in the carbs is the main jet. I read all the time on this site that the 9.8 uses a larger main jet, that is not even the truth since 1972 the 7.5 uses a .040 and a 9.8 uses a .039 main jet, if you want a 9.8 Mercury save you money and buy one or take your block to a capable shop and have it ported and the compression chambers crushed. Then buy a smaller main jet.

 

[d.boat]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

Well, my manual has a copyright date of 1981, but that doesn't necessarily mean that's the year of the motor. Is there a way to tell by the serial # perhaps?

top of this very forum: "What year is my Mercury out board"

 

[green4themoney]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

Why does this goofiness continue to go on weekly, the two powerheads are different, if you really want to know buy a service manual and go to a Mercury parts site and you will see that the 7.5 an 9.8 HP engines each have their own cyl. block it is denoted by different part numbers and the 7.5 has Comet cast in it and the 9.8 has Rocket, the difference is the port timing and compression ratio. Both engines use the same crankshaft pistons and rods, even the reeds and reed blocks are the same, the reed stops are set at the same height. The difference in the carbs is the main jet. I read all the time on this site that the 9.8 uses a larger main jet, that is not even the truth since 1972 the 7.5 uses a .040 and a 9.8 uses a .039 main jet, if you want a 9.8 Mercury save you money and buy one or take your block to a capable shop and have it ported and the compression chambers crushed. Then buy a smaller main jet.

my 1970 9.8 has "lightning" cast on the side of the block im assuming from the old "super lightning" block 10 hp engines, ive also seen a 7.5 with comet and lightning embossed on the side of the motor, i dont know why this would be, i know they are referring to the old "comet silent six" but it does make sense there would be more changes, even tho there are many similarities, it has to make the extra horsepower, and this from common knowledge most likely cant be achieved by more gas flowing into the cylinder, if you went up a jet size due to elevation or just cause you thought it would give you more horsepower shouldnt alone enough to make the extra horsepower...... but again they are still close enough that you shouldnt want to ahve to change them, i should have stuck with my original arguement on simply finding a 9.8 block

 

[gomofast]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

Why does this goofiness continue to go on weekly, the two powerheads are different, if you really want to know buy a service manual and go to a Mercury parts site and you will see that the 7.5 an 9.8 HP engines each have their own cyl. block it is denoted by different part numbers and the 7.5 has Comet cast in it and the 9.8 has Rocket, the difference is the port timing and compression ratio. Both engines use the same crankshaft pistons and rods, even the reeds and reed blocks are the same, the reed stops are set at the same height. The difference in the carbs is the main jet. I read all the time on this site that the 9.8 uses a larger main jet, that is not even the truth since 1972 the 7.5 uses a .040 and a 9.8 uses a .039 main jet, if you want a 9.8 Mercury save you money and buy one or take your block to a capable shop and have it ported and the compression chambers crushed. Then buy a smaller main jet.

Oh snap. Okay, so different blocks, different jets. Too bad, woulda been nice to just swap the carb.

 

[gomofast]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

my 1970 9.8 has "lightning" cast on the side of the block im assuming from the old "super lightning" block 10 hp engines, ive also seen a 7.5 with comet and lightning embossed on the side of the motor, i dont know why this would be, i know they are referring to the old "comet silent six" but it does make sense there would be more changes, even tho there are many similarities, it has to make the extra horsepower, and this from common knowledge most likely cant be achieved by more gas flowing into the cylinder, if you went up a jet size due to elevation or just cause you thought it would give you more horsepower shouldnt alone enough to make the extra horsepower...... but again they are still close enough that you shouldnt want to ahve to change them, i should have stuck with my original arguement on simply finding a 9.8 block

Yeah, I was hoping for a cheap, easy upgrade, but oh well. Thanks for the info guys.

 

[Laddies]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

G4 your right on some 110 and 9.8 there was Lightning cast in the block also the 6 HP were marked Comet.

 

[hibbert6]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

Don't blame you for trying, though. Last year I saw an ad for (Suzuki?) outboards that had all the specs for all their O/B motors. There were, I think, 4 motors, from about 125-250hp if I remember correctly, that had the EXACT same specs - displacement, bore & stroke, compression,etc., but various HP ratings and huge price jumps from one motor to the next. I sure would like to know how they get double the HP from the same size block!

Dave

 

[green4themoney]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

heres a pic of a 7.5 with "comet" and "lightning" .... makes me wonder what they were doing.....

 

[shaner1]

Re: How to convert 7.5 hp to 9.8 - vroom vroom

you know i jsut looked at the technical specs for the 7.5 and the 9.8 and they are vitually the same, only real difference i found was the jets in the carb, other specs such as reed lift/bore/stroke/P.skirt size, ring size, Blocks Cubic inches were virtually the same, you could try a 9.8 carb of the same year and might actually get a little more spice out of it, that is, if it fits which it should ive seen a few vids of it on youtube and never really thought much of it but now im checkin out the specs and thinking it might just be do-able

I used to have a 9.9 Johnson, and the manual was the same as the 15hp. The only spec difference I found was the compression. I thought of shaving the head, but ended up trading it for a 20hp yamaha.