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
there are also a few slight changes depending on year, what year motor do you have?
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
intake and exhaust ports on 9.8 are larger and at a different centerpoint than the 7.5.
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?
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.
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
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