1978 Mercury 70hp Worth Messing With?

[Berdink]

Not a robust lower unit for a 70 HP.----And 2 carburetors for 3 cylinders.----Not on my list of favorite motors.

Yeah, my research is telling me this same lower unit is used all the way down to a 30hp. And the 70hp was the biggest motor they used this lower unit on.
It does look really small.
.
Google keeps giving me different answers for this motors weight.
Sometimes it says 250 or so, sometimes it says 190-200.
I don't think it weighs anywhere near 250.

 

[racerone]

In my opinion it was designed ( in a hurry ) to compete with the hot selling OMC 3 cylinder motors of that era.

 

[Mc Tool]

In my opinion it was designed ( in a hurry ) to compete with the hot selling OMC 3 cylinder motors of that era.

Yeah ,Im thinkin the 2 carbs for 3 cylinders is a bit of a patch job using what was laying around at the time ....and probly same thing with the small-ish lower unit .

 

[jimmbo]

Yeah ,Im thinkin the 2 carbs for 3 cylinders is a bit of a patch job using what was laying around at the time ....and probly same thing with the small-ish lower unit .

It was that way because of the Design of the Reeds, Mercury had on their Production Inlines. In 76, merc did make a 650XS which had the same Bore and Stroke, and thus displacement, but it used an entirely different induction System, with Reeds on V-Blocks, and it had 3 carbs. It was rated at 90hp with an WOT range of 6000-7000 rpm. Still used the modified Crossflow Induction Merc called Direct Charge. I'm sure the Port Timing and Exhaust Tuning were different.

 

[Mc Tool]

It was that way because of the Design of the Reeds, Mercury had on their Production Inlines. In 76, merc did make a 650XS which had the same Bore and Stroke, and thus displacement, but it used an entirely different induction System, with Reeds on V-Blocks, and it had 3 carbs. It was rated at 90hp with an WOT range of 6000-7000 rpm. Still used the modified Crossflow Induction Merc called Direct Charge. I'm sure the Port Timing and Exhaust Tuning were different.

Thanks jimmbo . Guess it what one is used to seeing , all the motorbikes I recall had one carb per pot , and then I saw some 2 strokes feeding multiple cylinders from a plenum with one carb ( I guess a bit like a V8 with a 2 or 4 barrel carb on a distribution manifold ) ....so why not 2 carbs if it gets you to the required cfm .
Im coming round to the outboard way of doing things ......I mean they are pretty lazy compared to other 2 strokes but an under stressed motor is going to be both more reliable and durable . There is little point in racing out to spot X in a highly strung 2 stroke if it munches a piston or a big end on the way and you have to paddle home

 

[jimmbo]

The problem with the 2 carbs for 3 cylinders is that a 3 cylinder has 120 degrees between inhalations in the induction systems. With merc's usual 2 cylinders per carb, the flow was smooth and even as the cylinders combined, inhaled 180 degrees apart. With the 2 carb for 3 cylinders, each carb was feeding 1 1/2 cylinders, and sharing that 1/2 cylinder with another carb. That created some very uneven air flows through the carb, as sometimes it was only supplying to one Cylinder, but at some point it would be supplying for 1 1/2 Cylinders, then it would only be supplying for 1/2 of a Cylinder. The other Carb was facing the same thing. Calibrating the Fuel Delivery Curve of the Carbs was a real challenge for the Mercury Engineers.
It also made for some difficult Exhaust Tuning, as the resulting Mixtures to the Cylinders were different, which affected the Exhaust Pulse, and thereby the engine's power output.
These Engines had a very different Sound to them, than any of the other Mercury Engines of the Era.
Not the best engineering from Mercury

 

[Mc Tool]

the design of the plenum could have a bearing on the 120 deg intake pulses , being that one carb would be drawn on evenly its the physical location of two carbs in relation to the reeds that becomes an issue , however I suspect the Mercury probly didnt spend a lot of time or money worrying about that and just bunged em on where it was easiest
One could argue that 3 cylinders would have a more even flow thru a plenum than two, and get away with a smaller venturi sectional area for the same combined cylinder volume

 

[jimmbo]

Inline Merc of that lineage do not have a Plenum.
This pic is of the top two cylinders of a inline six. The above mention 3 would have another Reed block and Carb, between the 2nd and 3rd cylinder, not that main Bearing Block.

 

[Mc Tool]

With reeds on both sides or just one ?

 

[jimmbo]

Both Reed Blocks have reeds on both sides, which is how the center cylinder would draw from both carbs

 

[Mc Tool]

Both Reed Blocks have reeds on both sides, which is how the center cylinder would draw from both carbs

yaah I get it now

 

[Texasmark]

In my opinion it was designed ( in a hurry ) to compete with the hot selling OMC 3 cylinder motors of that era.

The OMC 3 was Loop Charged which made a BIG difference in fuel consumption plus a redesigned, streamlined, lower unit. In looking at later sales brochures I notice that the Merc. 75 has 3 carbs and is also Loop Charged.....don't know if the earlier model 70 with 2 carbs was a Looper or not.