Picked up a 7.5 and a 9.8... how to tell which is which?

[LakeFever]

Found a great deal on two super clean motas for the 12' kids boat from my favorite kind of seller; the kind that takes things apart and cant get them back together lol. So I got one running well out of the two and some parts on order for the other but I cannot see how to ID the differences between the two powerheads? They both look the same to me, whats the trick?

[25two.stroke]

It depends on what parts were together. The carb bore is diefferent as I recall and all the lowers I ever seen on 7.5s had 2 blade props and the 9.8s had 3 blades. I'm sure those could all be changed though.

Pistons/rods/crank are the same on the 7.5/9.8 not sure about block porting.

Compare the dia. of the intake on the front half of the block where the carb bolts up, that may reveal that the 9.8 is a bigger dia. and the 7.5 is smaller...then you would know which block is which and put the carbs the correct blocks.

Fuel pumps, electronics and all that were the same on the 7.5/9.8s if the motors were the same years. Back then they were changing the ignition systems every year or two, fuel pumps changed slightly and ridges and stuff in the bottom of the pan changed which would give away what year it was...even though all the cowls would drop on.

[LakeFever]

He had pulled apart the carb on one and the electrics on both and got lost trying to connect em back up. All these parts looked the same so I just grabbed the cleanest of the pile and stuffed them on one and its running great. I need a water pump and a carb kit for the next one which are on order. I have a two blade and a three blade. I'll look at the carb throats see if I can spot a difference, thanks

[25two.stroke]

Yeah I'm pretty sure that 2 blade came on the 7.5...and not many people do prop swapping on the little motors...its more of a you get what you get for max rpm and speed. So I would say its safe to assume that 3 blade goes on the 9.8 and the 2 blade prop lower goes on the 7.5. Electrics are the same if they are the same year.

[LakeFever]

The two blade is a 9 pitch and the three is a 7 pitch. Im thinking it was the two blade on the 9.8 originally makes more sense being higher pitch no? I had a Chrysler 9.9 many years ago that had a two blade, that thing was the fastest 10ish hp motor I've ever had by far. We'll have to see how these little Mercs do I had a sport twin that my Dad bought new in 72 that has since died. It got tired and the compression is way down. Im gonna rebuild it one day

[Jmr1974]

I’m no expert but I bet the only difference in the two motors is in the carburetor and timing. I had a late 90’s 25hp tohatsu and all I had to do to make it a 35hp tohatsu was buy a $3 gasket and change the timing a little, cheap 5hp gain. I’m betting every part on the 7.5 merc and the 9.8 merc are interchangeable if the two motors are relatively close in the year they were made.

[Jmr1974]

Whoops, I meant to say 30hp not 35hp.

[keefallan]

I would check into carb sizes. They added larger carbs on powerhead platforms to make more power..........sometimes different heads.

[LakeFever]

I cant get my fat hands into the runner to measure anything and Im not about to take it back apart just to measure so its gonna have to be old school testing.... a race lol. Once I get the parts in to fix the other one up i'll run em both same day same prop same tin boat and which one runs harder is getting labelled the 9.8

[22R]

I have a 1968 Mercury 9.8 and it has a two blade and I am working on a 1975 7.5 for a buddy and it has a 3 blade..



22R

[LakeFever]

Heres the beast lol. The two blade prop was mashed up pretty good so I got out the spool gun and the flap disk and gave it a 20 minute overhaul. I used a piece of poster board to make a template of the pitch and another to the shape of the better of the two blades and welded/ground to a 9/10 fishin mota grade or a 5/10 scream and fly rating. Way better than it was

[LakeFever]

Got it. The side of the block says "lightning" on the 9.8 and the 7.5 has a sticker over top of the lightning that says "comet"

[Forkin' Crazy]

7.5 and 9.8 were pretty much the same, less carb size. Just like the 9.9 and 15 OMCs. I've put several 15hp carbs on 9.9s and turned them into totally different motors. Some say the tuners were different, but the main difference was the carb size. Same diff between an old 150 to 175 cross flow... 200 to 225 loopers. Like going from a 2 barrel to a 4.