Mercury 70

[Plan-b]

Hello,

I am wondering if anyone could help me out, I have a 1980 mercury 70 and can’t find too much info about these motors. What I have found is that the top hole has a tendency to go lean. Is there any way to fix this problem? Does anyone know the rev limit of this motor… how tight can I spin it before it throws a rod? Does changing to a fiber reed increase performance or is it just for a safety factor? Are there any easy ways to make this motor faster? I am new to the two-stroke world… so can you please tolerate me being naïve.

Thanks

[Mark75H]

The 3 cylinder Merc is not a candidate for hot rodding. The limiting factor is the 2 carbs and 2 reed cages. Fiberglass reeds won't make it any faster. Merc made a hi-performace version with 3 carbs and better reed cages but that does not overcome the motor's short comings. Later Merc triples have a different crank with a different firing order to reduce their bad vibration at high rpm (I don't know what year the change was made)

They do not have a mixture problem with the top cylinder, they have a cooling problem. Later motors came with a vent at the top of the block to release air and ensure full water filling ... early motors should have this retrofitted

You can spin the motor as fast as you wish without worrying about throwing a rod, if you can stand the vibration.

The best thing you can do for more speed is to trade the Merc for an OMC triple (I'm a Merc fan ... thats a 6 cylinder Merc in my signature)

[7500Blizzard]

Not saying this motor doesn't have its problems. But how is 2 carbsand cages any worse than 2 carbsand cages for a 4cyl or 3and 3 for a six?

[Mark75H]

I never said it was worse on the triple.

Its the choke point for all of those motors; the triple has a way around it, but has its other issues which I think are mostly related to being a big bore deflector motor. With the blind bore non-removable heads, deflector pistons and internal reed cages none of them are ripe for hot rodding

[Roy Hodges]

I never said it was worse on the triple.

Its the choke point for all of those motors; the triple has a way around it, but has its other issues which I think are mostly related to being a big bore deflector motor. With the blind bore non-removable heads, deflector pistons and internal reed cages none of them are ripe for hot rodding.......................................
.................................................. .................................................. .....................................
Well,............ your S I X cylinder has non removable heads, deflector pistons and internal reed cages ,also .

[Mark75H]

Well,............ your S I X cylinder has non removable heads, deflector pistons and internal reed cages ,also .

Yep makes it a real pain to make much more power than came out of the box. If you had a shop do the work I've done to increase the power from 60 to 88 hp, it would probably cost $4,000. You can usually buy a pretty good used 100 hp motor for $4,000. You could certainly buy a used OMC triple, soup it up some and have cash left over.

Anyone know where I can find 2 more sets of these?

[Plan-b]

Mark75H,

Going form 60 to 88hp is something like a 47% gain that’s not too shabby at all... I was thinking around a 15-25% gain in power form this motor would be more then enough.

As it stands now the motor is in really good shape so I would like to keep it that way... So I would like to know how to keep the top hole form burning up? Do you know the part number for the retrofit kit?

What year and model 3cyl OMC is the best to mod?

Thanks

[Mark75H]

I think the only way you will see any gain with the Merc triple is to put the 3 carb front on it ... but that basically involves finding one ... most likely on a complete "650XS" motor ... so you'd just be trading


The 75 hp 49 ci and 70 hp 56 ci OMC's overlap from 86 to 88, so any triple after 88 is going to be a 56. Before '86 all you need to look for is a 75hp. The short shaft "Stinger" and "Hustler" models have a special skinnier gear case with faster gears. That's where I'd start. Probably a 75hp Hustler, since most people seem to be willing to pay a lot more for the orange paint on the Stinger.

You could go the 56ci route if you'd rather look for newer motors and more cubic inches to start with.

For $1,000 Ed Runne will put on his custom exhaust that made the 850cc Mod class jump from 85 mph to 95 mph overnight on the 49ci motors with stock carbs ... but I don't know how well it fits under the stock cover or if anyone has even tried putting a cover over one.

The top end steam vent is just drill a hole in the top of the water jacket, tap for 1/8" pipe and screw in a fitting to drain away the water, route it down to the bottom cowl and out. Pretty low tech; after people saw what was in the kit they stopped buying them and just made them. If its still running it might have it already :)

[Mark75H]

The reason I did the stuff to my poorly designed for hot rodding motor is to conform to 850cc Mod racing rules. If I wasn't restricted by the rules, I would have just bought the biggest motor that would float on the back of my boat.

[jeff55vDSH]

I agree with Sam.
I think the single biggest problem with the 3cyl Merc service engine is cooling. Be sure to have a good water pump, thermostat (if you use it at all), and have the cooling vent at the top of the block installed.
What is the HP rating limit on your boat?
Any hotrodding gains will be small and perhaps barely noticable in a service application.
I'd look for the biggest engine that is legal for the boat.
I hot rodded my family ski boat by installing a bigger motor than what it was rated for and then putting engine covers on it from a lower hp motor so it would be "legal" if the coast gaurd stopped me. But, I know I could have been caught if they'd have run the serial numbers. ;) I guess it's hard to recommend that somebody do what I did. :cool:

[Fast Fred]

Some NOS wake it up:eek::cool:

[Mark75H]

Explosions tend to wake up the neighbors, too :eek:

[Fast Fred]

if it's in good shape, ya can dust it 10 donkeys per hole:eek:

[Roy Hodges]

Some NOS wake it up:eek::cool:........................ .................................................. .................................................. ...................................
I think i mentioned it before; TOM (Ireland ) sent out a nitrous kit to P J (Paul Bender) in late 70's or early 80's for S E boats 75 horse EVINRUDDERS . Several tried it , made their boats go like stink. No trouble,as far as I know . But, a mild kit , only 30 additional H P . But, out of the corners, came on like jack the bear .

[Roy Hodges]

if it's in good shape, ya can dust it 10 donkeys per hole:eek:......................................... .................................................. .............
.................................................. .................................................. ...................................
I think I gotcha on the Evinson Johnrudes , BUTT ! Could Mk 75H get 10 donkeys /hole on his 6 hole muck 75 ? or woodit "Blow" ?

[Mark75H]

I think I gotcha on the Evinson Johnrudes , BUTT ! Could Mk 75H get 10 donkeys /hole on his 6 hole muck 75 ? or woodit "Blow" ?

Too bad the bottle is not legal in Mod. 10 donkeys per hole on the 6 would probably be possible on an unmodified muck 75, but I'm pretty sure it would bust my fully modded motor

[Roy Hodges]

Too bad the bottle is not legal in Mod. 10 donkeys per hole on the 6 would probably be possible on an unmodified muck 75, but I'm pretty sure it would bust my fully modded motor............................................. .................................................. ...............
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Is yer fully modded motor ORIGINALLY a genuine MK75H , or a fishing Mk75 dockbuster ?

[Roy Hodges]

Is it possible to bore out the original sleeves from 2&7/16" TO 2&9/16" ? the Mk75 to Mk78 size? A local guy here claimed his was - I was very skeptical. Lon Stevens told me that about .050" was practical .Correction, I just remembered that he said you could get away with boring out about 1/2 of the sleeve thickness , but that .050 pistons should be available to save old worn out blocks .

[Mark75H]

Is it possible to bore out the original sleeves from 2&7/16" TO 2&9/16" ? the Mk75 to Mk78 size? A local guy here claimed his was - I was very skeptical. Lon Stevens told me that about .050" was practical .Correction, I just remembered that he said you could get away with boring out about 1/2 of the sleeve thickness , but that .050 pistons should be available to save old worn out blocks .

Why not just use a Mk78 block? They are essentially the same other than the transfer passages and post 1960 Mk75 replacement blocks were based on the Mk78/700 block (same transfer passages) with smaller sleeves. The only difference was the bore and the text on the side of the motor.

I'm not certain anyone has a block with an original Mk75H serial number. The one that Ron Collins had might be one, Bud Clark might have one, but I've never had a serial number reported to me from an actual block from the right series.

[Roy Hodges]

I have NO need for ANY 6 cylinder motor. I am just asking because of my curiosity. I am always wondering "what if ?'. I knew of one mk 75H motor. Stevens had hopped it up . way back in the 70's . I remember he did NOT like the port timing of the Mk 55h & Mk75H . He did like the Mk 58 port timing .

[Plan-b]

Some NOS wake it up:eek::cool:

Fast Fred,

While i do indorse the use of N2O, The problem for me with spraying is that now to get full benefit of the N2O would to prop accordingly to ether situation and that cant be done… so I will have a slug when I am not running spray because now the prop is too big… or in the situation of running spray while propped to be not spraying, I will end up out of the power band. Unless the button is always pressed down. :)

Good if your running the 1/4... Bad idea if you want the power all the time... especally at $5.25 a pound.

[Plan-b]

I agree with Sam.
I think the single biggest problem with the 3cyl Merc service engine is cooling. Be sure to have a good water pump, thermostat (if you use it at all), and have the cooling vent at the top of the block installed.
What is the HP rating limit on your boat?
Any hotrodding gains will be small and perhaps barely noticable in a service application.
I'd look for the biggest engine that is legal for the boat.
I hot rodded my family ski boat by installing a bigger motor than what it was rated for and then putting engine covers on it from a lower hp motor so it would be "legal" if the coast gaurd stopped me. But, I know I could have been caught if they'd have run the serial numbers. ;) I guess it's hard to recommend that somebody do what I did. :cool:


I like the idea :) ... thats why that 70 is already on the back of the boat... as it's only rated for a 50:(

[Plan-b]

Maybe I will Build an air box and attach to it a leaf blower… something like 4 psi of boost at 900 CFM… boost reference the fuel… and see what happens...

[Mark75H]

I have NO need for ANY 6 cylinder motor. I am just asking because of my curiosity. I am always wondering "what if ?'. I knew of one mk 75H motor. Stevens had hopped it up . way back in the 70's . I remember he did NOT like the port timing of the Mk 55h & Mk75H . He did like the Mk 58 port timing .

Mk58 and fishing Mk75 porting are the same ... also the same is Mk55 fishing porting

I have one Stevens block, definitely higher porting on the exhaust; might be std fishing porting on the intake

[Mark75H]

Maybe I will Build an air box and attach to it a leaf blower… something like 4 psi of boost at 900 CFM… boost reference the fuel… and see what happens...

48 ci ÷ 12 (to change to cubic feet) = 4 cubic feet;

times 6,000 rpm

6,000 X 4 cubic feet = 24,000 CFM ... the Merc will suck the leaf blower in, or at least being connected to the leaf blower will be a major restriction to the Merc

(if you did boost the motor, the fuel would be automatically boosted by the increased pressure in the crankcase which operates the fuel pump)

BUT ... supercharging a 2 stroke does not increase cylinder pressure in most instances because unlike a 4 stroke, the transfer (or "cylinder intake") port closes first while the exhaust port is still open - releasing any pressure increase to the exhaust pipe.

So, even a 60,000 cfm leaf blower won't help beyond some slightly improved intake and cylinder scavenging

[Fast Fred]

the leaf blower thinkin will just lean it over. could go to alcohol and splash in some Nitro methane, not like it's guna blow the head off:eek:

doublein the hp with NOS, works but is short lived:eek::cool:

[Plan-b]

I Know it has no outboad in it ... just kinda fun to watch and think...


http://videos.streetfire.net/video/ab544e3b-5dff-416d-9cfd-a1ad23ca9564.htm

[Plan-b]

48 ci ÷ 12 (to change to cubic feet) = 4 cubic feet;

times 6,000 rpm

6,000 X 4 cubic feet = 24,000 CFM ... the Merc will suck the leaf blower in, or at least being connected to the leaf blower will be a major restriction to the Merc

(if you did boost the motor, the fuel would be automatically boosted by the increased pressure in the crankcase which operates the fuel pump)

BUT ... supercharging a 2 stroke does not increase cylinder pressure in most instances because unlike a 4 stroke, the transfer (or "cylinder intake") port closes first while the exhaust port is still open - releasing any pressure increase to the exhaust pipe.

So, even a 60,000 cfm leaf blower won't help beyond some slightly improved intake and cylinder scavenging

Mark 75H,

The cubic conversion you stated is incorrect 1 cubic foot = 1728 cubic inches. So 50 cubic inch = 0.028 935 185 2 cubic foot … and this is assuming a 100 % cylinder fill, we now can use your formula related to volume consumption via rpm.

0.028 935 185 2 X 3 (# of cyl) = 0.0868055556 cubic feet

0.0868055556 cubic feet X 6000 rpm = 520.8333335 cubic feet

so at 6000 rpm assuming the engine is 100% efficient in cylinder fill it will use 520 cfm

24 000 cfm will suck a person who is sitting in the boat into the motor… as this as much cfm as a small wind tunnel.

But I do understand why adding positive pressure to a two stroke will not provide a benefit of increasing cylinder fill… Thanks for pointing that out.

like i said before i dont know much about two-stroke engines but this rule will apply to any engine...

:)Its best to learn form the guys who race, because they are the ones who know how to make it work!!!!!:) And that why i am part of this forum.

[Frank Volker]

The cubic conversion you stated is incorrect 1 cubic foot = 1728 cubic inches. So 50 cubic inch = 0.028 935 185 2 cubic foot … and this is assuming a 100 % cylinder fill, we now can use your formula related to volume consumption via rpm.

0.028 935 185 2 X 3 (# of cyl) = 0.0868055556 cubic feet

For a 50 cu in engine, the total swept volume is 50 cu in / revolution. Each cyl volume = (total displacement) / ( cylinder count ).


0.0868055556 cubic feet X 6000 rpm = 520.8333335 cubic feet
so at 6000 rpm assuming the engine is 100% efficient in cylinder fill it will use 520 cfm.

Total = (50 cu in * 6000 RPM) / 1728 = 173.61 CFM.
Per cylinder = 1/3 total = 57.87 CFM.
If 2 carbs, 86.8 CFM through each.


But I do understand why adding positive pressure to a two stroke will not provide a benefit of increasing cylinder fill… Thanks for pointing that out.

Actually, it could help. Especially if you maintained the A/F ratio. A ton of factors come into play--port timings, reed type/stops, and especially the exhaust system, since the excess blow-through air/fuel would most likely change the exhaust system temperature, thereby changing the effective tuned length.


like i said before i dont know much about two-stroke engines....

Me neither, but I'm always willing to learn. :)

Frank

[Plan-b]

Frank,

Thank you for pointing out my mistake… I made the calculation for displacement of 50ci in each cylinder….

So what it should look like was this….

0.028 935 185 cubic feet 2 X 6000rpm = 176.6111 cfm

same as what you got:cool:

[John (Taylor) Gabrowski]

I went through screwing with half a dozen of those 3 cylinder 2 carb blocks and 4 of them did that to the top cylinder destroying the piston and the head every time no matter how much extra cooling we fed in at different points or took out of others spots knowing it had that problem. The last two complete blocks (no carbs or ignitions) and all their spare long block parts (no carbs or ignitions either, they are worth something) from the other 4 went into metal recycling (my neighbors took them in and bought beer with the proceeds) this last spring as even the dealers didn't want them back even though they were overhauled and fresh!

To begin with when I asked my marine dealer friends for these engines to test for FE, they thought I was nutz immediately. They told me I was wasting my time from square one. They gave me complete powerheads for free, take all I wanted!! They told me if a person wanted to loose customers you sold them a Merc or Mariner 65 3 Holer 2 Carb and they the customer would be back for warantee until it ran out which made them madder still and then you lost them as a customer permanently. They were that bad a motor! A very ill concieved deflector using dated technology well past its design use.

One dealer had a complete Merc 3 holer with a for free sign laying by his inlet road for 3 years and no one took it! Finally a grader wrecked it when it tried to make it part of a snow bank clearing the road in winter!

Enough said. :mad:

[Mark75H]

Darn, I was asleep when I did that math, wasn't I?

OK, the leaf blower can blow into the 49 ci motor, but it is still just going to blow out the exhaust ports. You will get may be a 5 hp power increase due to improved scavenging. About enough to justify carrying the leaf blower in the boat, but probably not enough to justify the 10 or 15 hp more gas that you will be using in the outboard alone.

[Mark75H]

A very ill concieved deflector using dated technology well past its design use.

Bad execution for sure, but the technology was not dated at that time. The 70 hp triple used Merc's "Direct Charge" deflector technology which was state of the art in its day.

[Fast Fred]

it's guna dry out the bowl and Nose over:eek:

[Mark75H]

It won't dry the bowl ... the fuel pressure is a function of crankcase pressure on motors with crankcase actuated fuel pumps. I thought the OMC's still used them.

[Fast Fred]

Stinger 75 uses a vac pump, sst60,CC, electric pump, when the air speed gets to a certain point in the venturi the specific gravity goes out the window,:eek:

[Mark75H]

I didn't know anyone was talking about SST60's

Specific gravity doesn't enter into it.

There are a lot of motors that have successfully used "blow thru" carb supercharging. Studebaker and Ford used it as far back as the 1950's. Lots of turbo applications used it in the 1970's

[Fast Fred]

Studebaker 2 stroke:eek: no. step up the fuel volume. we could do a Demo with your Moda:eek:

[Roy Hodges]

That method works, but it does NOTHING for fuel EFFICIENCY . The carb or injector feeding INTO the intake of the Supercharger can actually increase fuel efficiency , as well as make a big boost in power . I always liked the idea of a centrifugal supercharger or turbocharger which had an aircraft carburetor (adjustable fuel mixture ) mounted to the intake side of the charger, naturally having a pyrometer in the the exhaust to read the exhaust gas temp. And before someone complains that it's too complicated, NOTHING worthwhile is easy !

[Fast Fred]

a power valve that closed the top of the exhaust window just before the intake windows closed on the up stroke, then you would have somethin.

otherwise it's NOS, no pressure change inside to blow the charge out the exhaust.

only with injection that this is true
Specific gravity doesn't enter into it. :eek::eek::):cool:

[Mark75H]

A valve like that would not be called a "power valve" it would be a "sleeve valve". If you are building something as monsterously complicated as a sleeve valve motor there are lots of possibilities, but nothing remotely related to Plan-b's aspirations. It would require a completely new motor and I agree, you would have something with immense power potential.

[Fast Fred]

are not all "sleeve valve" modas 4 choke? like the Aero, i think thats it,:cool:

[Mark75H]

with a supercharger they don't have to be, do they?

[Fast Fred]

there was one, but the valve was in the head, single valve, on the intake side.
Lotus.

[Fast Fred]

i was thinkin, why don't we just inject the air and boost in to the cyl with the fuel at the head. then thares no valve drive or extra movin parts. maybe shrink the intake ports, cuzz just be movin oil for the most part.:cool:

[Mark75H]

i was thinkin, why don't we just inject the air and boost in to the cyl with the fuel at the head. then thares no valve drive or extra movin parts. maybe shrink the intake ports, cuzz just be movin oil for the most part.:cool:

It can be done. The main reason it is not done that way in practice is the device to do it is the same size as the rest of the engine.

[Fast Fred]

hey plan-b,
theirs a 70CES copy up for grabs over at the scream n fly, :cool: