Seeking information about older 2-stroke Mercury outboards

[FoolishOne]

I would like to learn more about the 2 stroke Mercury outboards available in the 80s and 90s. I have no specific application really but I'm interested in 3 and 4 cylinder outboards under 300#.

What can be said about the reliability of these motors?
Which of these engines share bore and stroke?
What are the commonalities and differences between the powerheads? - What changed from year to year?
I believe oil injection was employed at some point in this time frame. Can this be eliminated in favor of using pre-mix?
How easy/hard is it to find parts for these motors?
I've been told that there was shift in rating HP from the crank to the prop leading to motors being shifted down the model lineup. Does anyone know when this occurred?

Probably more questions as I keep looking around and reading.

 

[jimmbo]

In the early 80s there were the 4 cylinder 44 inch blocks and the 66.6 inch blocks, the 3 cylinder blocks were 49.7 inches

In the late 80s the 3 and 4 cylinder loopers were introduced

Prop Rating Started in 1982 on all 6 cylinder engines, in 84 on the 66.6 inch 4 cylinder, by 86, Mercury was pretty much Prop Rating all engines

Oil injection(auto premixing), in was introduced in 1985 in the V6s. A very lousy system(Auto Blend) was added to the inline 6s and lower HP engine in 86 or 87. The 3 and 4 cylinder loopers came with an integral Auto Premixing, similar to the V6s
The System can be disabled, but why, it is a very Robust System. The Exception is Auto Blend, those should be unhooked, emptied and thrown as far from the boat as possible


Common Bore and Stroke:

49.7”(3 cyl), 66.6”(4 cyl), and 99.8”(6 cyl) all have same bore and stroke, however, that does not mean they all have the same pistons.
In the late 80s the 3 and 4 cylinder Loopers had the same bore and Stroke
The 44”(4 cyl) shared bore and stroke with the 22”(2 cyl) engine

Often there were 2 or 3 engines in the line that shared the same Displacement. This does not mean a change of Jets will make a lower HP model into a Higher HP. Even changing Carbs rarely did it. Porting, exhaust, induction were more often different

 

[FoolishOne]

Thank you for the information!

My thoughts on oil injection are basically it doesn’t matter how good the system is everything breaks eventually so fewer parts are generally better. In this case it seems like the only downside is mixing gas.

Based on your post it seems like Mercury was basically adding cylinders with the 22 and 44 ci both displacing 11” per cylinder and then the 49.7 , 66.6 and 99.8 ci each displacing roughly 16.6” per cylinder.

Now looking at a list of models from 87 I see that the 45hp was a 4 cylinder while the 50hp was a 3cyl. Would I be correct in saying the 45hp is the 44” 4cyl and the 50hp is a 49.7” 3 cylinder?

On that same list I can see motors up to 90hp were 3 cylinder. Would the blocks be interchangeable between these three cylinder motors? For example could I use the block and rotating assembly from a 50hp to replace a block and assembly for a 90hp?

You mentioned the pistons were not the same across motors. I’m guessing here but since there’s no valves to worry about and the major difference is going to be compression ratios? Or is it a matter of deflection angle? I would imagine that all looper pistons are compatible with one another but not with earlier models?

Thanks again! I haven’t wrenched on a two stroke since the 90s dirt bikes. Big learning curve coming into boats and outboards just trying to figure out what is what when I’m looking at the ads.

 

[jimmbo]

The 49.7 block was introduced in 72 as a 65hp, in 73 they changed the Block(same displacement, but added a one inch hole in the piston skirt, which basically added another transfer passage between the Crankcase and Combustion Chamber. These Power Port and Power Port Pistons were added to the 66.6" block upping the HP from 80 to 85. It was also part of the 99.8 cu" 150hp. That was the main difference when I said the Pistons were not the same. The 49.7 engine eventually became a 70, then in the mid 80s when the engines were prop rated, it was rerated as a 60, and as a 50. These 3 cylinders weren't one of Merc's better ideas, 2 carbs being shared by 3 cylinders.

Looper pistons may or may not interchange on looper engines(I was never Interested in those engines to bother looking it up


In the late 80s the 3 cylinder loopers appeared, they were bigger blocks and shared little if anything with the smaller blocks,That includes Midsections and gearcases, it was released a 75 Hp and as a 90, and probably an 80 as well. A 4 cylinder version was offered as a 100 hp, 115, 120, and 125. Personally I don't like those engines

The 44 cu" block was first made in the mid to late 1950s, it was rated as a 40hp, 45, and finally 50 hp. In the mid 80s it was relabeled Classic 50, Classic 45, and was prop rated as a 45 and maybe a 40.

To add another twist, from 1939 till 1968, alll Mercury Engines had been Crossflow designs, then in 1968 or 69, when the 66 cu" block was introduced as an 80hp. they had what Mercury called Direct Charge. OMC had just introduced their 49.6 cu" Loop Charged 3 cylinder. Merc answer was a Variation of Cross Charging, but to make it sound like an Engineering Triumph, it was call Direct Charge. Over the next couple of years, all engines(2, 3 ,4, and 6 cylinder) based on the '16.6 cubic” cylinder got a new block and Piston to make it Direct Charge. The 44 cu" block remained a Crossflow as did the smaller one and two cylinder engines.

 

[racerone]

-----" shear proof drive " was another engineering triumph.-----Instead of a 29 cent pin , folks now have to install new props after finding the bottom of the lake.

 

[jimmbo]

-----" shear proof drive " was another engineering triumph.-----Instead of a 29 cent pin , folks now have to install new props after finding the bottom of the lake.

Instead of a Shear Pin, Mercury is currently using a Shattering Hub system along with its Shear Proof Drive

 

[FoolishOne]

The 49.7 block was introduced in 72 as a 65hp, in 73 they changed the Block(same displacement, but added a one inch hole in the piston skirt, which basically added another transfer passage between the Crankcase and Combustion Chamber. These Power Port and Power Port Pistons were added to the 66.6" block upping the HP from 80 to 85. It was also part of the 99.8 cu" 150hp. That was the main difference when I said the Pistons were not the same. The 49.7 engine eventually became a 70, then in the mid 80s when the engines were prop rated, it was rerated as a 60, and as a 50. These 3 cylinders weren't one of Merc's better ideas, 2 carbs being shared by 3 cylinders.

Looper pistons may or may not interchange on looper engines(I was never Interested in those engines to bother looking it up


In the late 80s the 3 cylinder loopers appeared, they were bigger blocks and shared little if anything with the smaller blocks,That includes Midsections and gearcases, it was released a 75 Hp and as a 90, and probably an 80 as well. A 4 cylinder version was offered as a 100 hp, 115, 120, and 125. Personally I don't like those engines

The 44 cu" block was first made in the mid to late 1950s, it was rated as a 40hp, 45, and finally 50 hp. In the mid 80s it was relabeled Classic 50, Classic 45, and was prop rated as a 45 and maybe a 40.

To add another twist, from 1939 till 1968, alll Mercury Engines had been Crossflow designs, then in 1968 or 69, when the 66 cu" block was introduced as an 80hp. they had what Mercury called Direct Charge. OMC had just introduced their 49.6 cu" Loop Charged 3 cylinder. Merc answer was a Variation of Cross Charging, but to make it sound like an Engineering Triumph, it was call Direct Charge. Over the next couple of years, all engines(2, 3 ,4, and 6 cylinder) based on the '16.6 cubic” cylinder got a new block and Piston to make it Direct Charge. The 44 cu" block remained a Crossflow as did the smaller one and two cylinder engines.

I was under the impression the 3 cylinders were some of the better designs since the exhaust tuning was so much easier to pull off successfully. Two carbs sounds like they were trying to find a compromise between a big single (which would probably not run well at lower rpms) and triple carbs.

Using the info from Nada guides I made this spread sheet which compares the models of the 1980s based on lbs / hp and hp/cylinder. The higher hp 3 cylinders stand out in terms of lbs / hp.

 

[jimmbo]

To due to the design of the Reeds on most Mercury unlined, a carb is shared by two cylinders. Merc did make a 3 carb version of that displacement, the 650XS. Rated at 90 hp, it was only good for very light boats, usually racing hulls.