70 Johnson !!!!!! Rpms

[yellowfever]

Hi I just got a 1978ish 70 Johnson. What rpms will that motor make power to? And r there any tips to making it better? Thanks

[terry taylor]

Is the compression good, With good oil up to 6,000 should be reasonable. With an older engine it.s best to make sure all parts are operating properly and close to maximum ,that in itself is as important as hp. gain thanks.

[yellowfever]

I cleaned the carbs and started the motor. It run great and has good compression. What can I do to make it faster

[terry taylor]

Put in on a light boat and run a 24 raker thanks/.

[Mark Poole ModVP]

All of the 49 cu. in. OMC motors from the 65 thru the 75 are tough engines. They respond very well to shaving the head to increase compression. If you want to do some mild internal mods, keep it simple, raise the exhuast ports .050" and have the head milled .050" also. You will need to run premium fuel of course. I have cut the head on these engines as much as .080" for racing but this requires that you use a dremel tool and cutter bit to re open the water channel that runs around the head. Cutting the head that much shallows the channel so much that it has to be deepened again. One more trick, see if you can find the flywheel off of a 1972 65. It will work with your ignition and it is a lighter flywheel.

[yellowfever]

I sorry my motor is a 81

[powerabout]

the 75 had bigger carbs so you could swap them

[yellowfever]

What about tuner, timing, jetting

[Mark75H]

You can't do much timing and jetting until you have decided on prop and set up. Even then, it would be individual testing that results could vary from motor to motor, same model boat and same model prop. Adjustments that precise don't translate from another's experience; comes from your own testing ... lots of it.

[yellowfever]

The motor is going on a 15ft archer craft. With a 4 inch set back. Lowwater pickup

[yellowfever]

Boat has a 3 inch wide pad and NO hook. The boat ran about 40 mph with a classic 50 merc and a sh•Ty prop

[Fast Fred]

never could get the 65hp flywheel to fire on time with the new style power pack, only the old style with all. the screws . it fired too early on the new style pack. i got a fue 20" 75hp's, all have the same tuna as the 70hp. them 75hp carbs, and a head clip should spin-it-up some more. there is a notch in the exhaust manifold (middle plate) only on the 75hp. think that about covers it for factory go fast stuff. maybe some exhaust bolt on stuff, Runny , maybe spelled it wrong,
makes more power, not sure about the others, have not seen any that send the exhaust back down the mid like stock, just dumps out the side, loud, not every one likes loud, i'm ok with it.

you have to go inside it to make more power than bolt on stuff.:cool:

[powerabout]

FF how about a 15" tuna in the 20" housing??
Anyone ever tried that?

[yellowfever]

Or maybe cutting the tuner down?

[Fast Fred]

i'm sure some one has tried it, think you will need all your low end for that big foot. puttin a 56er with a 20"mid on a Carlson Glasstron , about 15', went with the 20"mid tuna. solid mold SST60 copy, with finger ports. was thinkin 58mph, maybe 60.:cool: