1975 Johnson 70hp

S

saltchuckmatt

Commander
Joined
Jul 19, 2019
Messages
2,898
Neil1982 said:
Just an update. I have cleaned the carbs, and I am firing on all cylinders. I replaced all the coils as well. I was able to get the tach dialed in, and I was running too low of RPMs when in gear after a little research. Now I have all cylinders firing, and I am running about 850 RPM in gear in the water. Not sure if anyone knows how many MPH I should be able to do in a 16' StarCraft aluminum boat, but I can only get about 30 MPH out of the boat. Not sure if that is where it should be or if it is low.
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Should go faster....what RPM's are you achieving?
 
R

racerone

Supreme Mariner
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Dec 28, 2013
Messages
38,926
One one of those should go 35 MPH.----That is with a motor that is producing it's rated power.----Using a 17" pitch prop?
 
N

Neil1982

Cadet
Joined
Jul 29, 2025
Messages
9
racerone said:
One one of those should go 35 MPH.----That is with a motor that is producing it's rated power.----Using a 17" pitch prop?
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yes 17" prop. I can get 35MPH out of it if I set the trim rod up further but not in my regular running position.
 
flyingscott

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
8,232
Youre speed is normal for that motor and set up. Your motor is powerhead rated , realistically 60 at the prop. I ran that same combo and I would recommend a 15" prop to get closer to 5500. 32 was the best I ever got out of it. If you want significant speed increase get an 86 or newer 70 hp.
 
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A

airshot

Vice Admiral
Joined
Jul 22, 2008
Messages
5,834
jimmbo said:
There are much better Snake Oils than SeaFoam. Combustion Chamber Cleaner does actually dissolve Stuff. SeaFoam, just makes a lot of Smoke
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Have pulled down engines after Sea Foam and it has always cleaned the combustion chanbers very well. Have used it to soak old small engine heads, they came out clean as a whistle. Their may be other products, but Sea Foam has always did just what it advertised for me.
 
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