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1970 20hp JohnsonPerformance parts

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cjinpa

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" Is there such a thing? I kno

" Is there such a thing? I know they make hop-up parts for the newer 15s and what not, but is there really anything out there I can find to up the performance of a 1970 20hp? The motors going ona 10ft. thrillcraft and I'd like to keep the weight down as much as possible. I originally ran a '56 merc mark 55a 40hp which was scarry fast, but lack of available parts(it died), and the sheer weight of the motor on the back of the boat, has me looking in this direction. The only reason Im going with this particular motor is because I already have it, and its only a toy in a long list of toys so I'd rather not go out and spend the money on a newer model. Unless of course anyone knows where I can find another late 50s early 60s super thunderbolt merc, and maybe a quick silver 1:1 lower unit. Any help would be greatly appreciated.

Thanks a heap,
-Pat "
 
The 20 can be made into a 25h

The 20 can be made into a 25hp.You need the carburetor and cylinder head off a 25hp 1969 to 1977.Crankshaft would not hurt either.
 
" Hi Pat,
The primary diffe

" Hi Pat,
The primary difference between the 22-cubic inch 20 and 25hp was the opening on the reed-stops. If you can find a blown up 25hp powerhead, you can swipe all the "good" parts and make your 20 a 25hp. The crankshafts are the same. To my knowledge, there are no real high-performance props for that motor. I think there may be an older style 2-blade bronze wheel from Michigan that might give a little more bite. If I think of it, or if you e-mail me to remind me, I'll look it up in the Michigan catalog. Bronze would also be easier to have a prop guy work on and cup, etc. Be careful in how high you jack the motor on the transom, as the water intake is in the exhaust snout under the cav plate.

Care should be taken to make sure the motor is putting out all the rated power - piston ring grooves are free of carbon? Compression is good? Full travel on the throttle linkage? Carburetor is properly jetted? Timing is set at .020", exactly 180-degrees apart? There's some tweaking you can do within the bounds of your motor without tearing it up.
Also, INSTALL A KILL SWITCH! You can use the two wires from the base of the magneto plate that hook up to the kill button in the belly pan.

- Scott "
 
" Scott,
Thanks for all the

" Scott,
Thanks for all the great info, I just got on the michigan wheel website and ordered a catalog, whether it'll be in there or not, who knows. If you would happen to get an oppertunity to look that up though, tha'd be terrific. I actually ran a michigan 2 blade bronze on my merc. Thanks again for your help,
-Pat "
 
my parts book gives part# 315

my parts book gives part# 315286 for crankshaft on 20r69 thru 73 and #383182 for crankshaft 25r69 thru 76. They look the same but-----
 
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