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Thread: Mercury 70

  1. #1
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    Default Mercury 70

    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

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    Sam Cullis Mark75H's Avatar
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    Default Plan C

    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)
    Since 1925, about 150 different racing outboards have been made.


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    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?

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    Sam Cullis Mark75H's Avatar
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    Default

    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
    Since 1925, about 150 different racing outboards have been made.


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    YARD BIRD
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    Default tripe.......er, triple

    Quote Originally Posted by Mark75H View Post
    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 .

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    Sam Cullis Mark75H's Avatar
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    Quote Originally Posted by Roy Hodges View Post
    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?
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    Since 1925, about 150 different racing outboards have been made.


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    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

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    Sam Cullis Mark75H's Avatar
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    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
    Since 1925, about 150 different racing outboards have been made.


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    Sam Cullis Mark75H's Avatar
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    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.
    Since 1925, about 150 different racing outboards have been made.


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    That Tohatsu guy. jeff55vDSH's Avatar
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    Default My 2 cents

    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.
    Jeff Yungen

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