Merc 7.5 and 9.8

[mk30h]

These little engines had the same bore and stroke as the KE4 and latter Mark 7. Now I know the blocks were entirely diffrenent, but I find it amazing that the dimensions are exactly the same.

One question I have is why did Merc produce such a little engine, when just a few years earlier they had made the KG7 and latter Mark 10 , which were both 10 hp (while the Mark 10 was anyway). I would suspect that a number of Merc customers would have been dissapointed with the 9.8 hp.

While I had a 73 9.8 in my youth, I did love the engine on a 12ft alumium boat- I could even get air with it- although that's not saying much, I could do the same with my Mark6 before I blew the gears.

Any idea why Merc built this engine instead of a larger cubic inch model like the new 9.9 or even ?the KG4 or Mark 10 and early Merc 100.?

Would a old squat OMC 9.9 blow the doors off these little beasts?

[Mark75H]

The 9.8's and 9.9's were built in response to horsepower restrictions on many lakes. Some outboard brands had as little as a decal set and throttle set screw holding the carb from fully opening to reduce a 15 or 10hp to 9-point-something


no, 0.1 hp won't make any difference. Lower unit & prop dynamics, but not 0.1 hp

[David_L6]

Years ago in Louisiana you didn't have to register a boat that had a motor under 10hp.

[Markus]

Johnson used to offer a GT10 that was supposedly .1 hp stronger than the standard 9.9 and had a different paint scheme.

Does anyone know if there were any differences between the engines?

[mk30h]

Actually, I know that .1 hp exist only in a marketers imagination for that magic under 10 hp limit. Merc even made a detuned 7.5 (11 cubes) 6hp for those under 6hp limits- long after they stoped building the Merc 60.

I was more interested in how these engines actually peformed- I may have been wrong but OMC pre 73 did not have thru hub and I think it may have been a 9.5 - it had about 5 more cubes than the Merc 9.8 - so I was wondering how they compared. ( I was biased towards Merc in my youth) .

was the 7.5+9.8 really just a modern Mark 7?

[ggarland]

My Dad bought me a new 7.5 back in 1971, my very first Mercury........ and I still have it. I ran the balls off it on a little Sea Flea and a number of other boats and it never, ever let me down. I got clocked at 33 mph on that Sea Flea, on radar, and I could easily beat up on the 9.5 OMC's of that era, they never stood a chance. I always wondered what a 9.8 would have been like on that boat. It still runs great and there have been three generations using that engine up until now, and I suspect there will be many more as well. I want to be buried with that engine! It is an old family friend, and a trusted companion.

[staylor]

At one time ('75 ?)I had a Merc 7.5, followed a few years later by the 9.8. The carbs on both engines opened all the way, differences appeared to be in the reed cages. To make the 6, you'd buy a kit from Merc which included a carb restrictor plate. I remember pulling the carb off my 7.5 hoping to find a plate to remove and make it a 9.8- but no such luck. I used these little motors on rental fishing boats, the 7.5 was as fast as the "stumpy" OMC 9.5 built pre 1974 and the 9.8 was definitely faster by 2 mph or so; a lot when you're in a boat that only does 16 mph or so. But all of this changed when OMC brought out their 9.9/15 hp motors in 1974. These were not "stumpy" and were real screamers- the 15 was rated at 6000 rpm. These engines were around 13.2 cid and were available with a racing gearcase for APBA stock outboard racing A class. I traded my Merc 9.8 for a new 15 Evinrude, and found the OMC was a lot quicker. In fact, it was almost as quick as an old KH7 Green Top "Cruiser" I restored and was running at the same time. The KH7 was a mating of the KG7 Hurricane powerhead with the lower unit of the Mark 20 and was one hot "fishing" engine in its day (1952?). After a few years of building the 13.2 cid engines, OMC went to a 16 cid powerhead, dropped the rpm down by 1000 or so a made the engine into a clunker. Today I'm running the late model Merc 15 and it will eat the remaining late model 2 cycle OMC 15s for breakfast. On a 14ft, 200 lb fishing hull I'm running 25 gps.

[bearclaw]

After cutting my teeth on a LOUD 4hp. air-cooled McCulloch that permanently impaired my hearing in my right ear , and a 7.5 McCulloch that liked to run on one cylinder most of the time, my dad too bought me my first Merc, a new 1970 7.5. Ran it on my Minimax and could almost keep up with the 20 hp. Checkmate Playmates and GW Invaders. Bought a neighbor's '63 9.8 after that, and was able to blow by 'em then. God those were good motors. Guess that's why, out in the garage behind the big boat, I've accumulated three 9.8's (a '63,'69 & '70), and two '66 6hp. (those little twins ran like sewing machines). A couple are pristine originals, the others are in restoration. Now, as soon as I get those plans for a Minimax...

[MOTIONLINE]

Can anyone tell the real difference between a 7,5 and 9,8 (year around 1975 with fuelpump incorporated in the carb)?
Tried to have a look but couldn´t find anything exept perhaps
the porting??? The reeds look exactly the same to me!!!

[staylor]

Had a 7.5 and a 9.8, the 9.8 ran a slightly richer main jet and if I recall the reed stop settings were slightly different. Both idled about the same. I never pulled either engine apart, so I couldn't check the porting or reed cages. The 9.8 ran about 2 mph quicker on my fishing boat. Similar today with the 16 cubic inch 9.9 and 15 Mercs. I run a 15 and it's definitely quicker than the 9.8.

[ggarland]

Different carbs, different ports. The 7.5 was the lugger of the two, with a bit better torque.

George

[MOTIONLINE]

I have noteced that the ports are different but what about
the carbs, different venturi or just jets?

[ggarland]

Carb throat size and jets. If you compare carbs side by side you will see that the 9.8 throat is bigger, and the carb is a different part number. The larger carb and bigger ports give more ponies at top end but less torque at lower speeds, just like any other engine.

George

[Explorer 1]

I know this is an ancient thread, but trying to find some help. I need a used powerhead for a 1978 Mercury 9.8. I have found a 1975 and a 1976 that are available. It appears from what I have found that the 74 through 77 powerheads are the same, but the 78 is a different part number. The crankshaft number is the same for pre 1978 and the 1978. Does anybody know if the 75 or 76 powerheads will work or what the difference is. I already bought a powerhead that was newer than 78 thinking it would work and it did not, so I don't want to buy another one that will not work.

[bernie]

We built the block, crankcase, crank, rods, reed blocks, flywheels, gears and gearcase, propshafts, and other parts for the 7.5/9.8 at Merc Plant 4, me being the head of the inspection department from '70 to '76.
We made these parts for US, Canada, Australia, and Belgium assembly plants. Quantites of 1000 blocks per day! Thats 2000 rods, 1000 of everything else. We used machines known as Wismatic multiple drill and tap machines, we could drill 5 sides of a block is 25 seconds and tap it in 13! While all this was going on, we were also building propshafts, rods, flywheels, misc. aluminum parts, etc. for other models like the in line 4 cylinder mid range and the in line 6's, and Mercruiser parts! Three shifts and all day Saturday for years - truthfully, I enjoyed it until something went wrong (like gearcase shift cams with the reverse track too shallow) and then on paper our plant got charged for the disassembly and rebuild time) and made our plant manager look bad - he didn't like that at all! On that shift cam deal, the responsible foreman and myself had to report to him and show him what went wrong, before we did we made sure we had corrections already in place. We still got a handful of bad cams thrown at us from across his office! He was definitely the bull of the woods hand picked by EC Kiekhaefer!