Convert a johnson outboard from 20hp to 25hp?

Question

i heard that older model johnson outboards could produce more power by either changeing the carb to a larger size or maybe by making the hole in the intake larger. any body know anything about this? can it be done on a 1969 20hp ?

Answer 1

Change the cover or stickers to 25hp as the difference is so small you couldn't fell the difference> Except fuel usage>& maybe 1 mph faster top speed only> The gears are the same RPM the same>

Answer 2

There are a number of different HP models that share common engine blocks.  Most require, at a minimum, new carb jets, intake manifold and cylinder head.  Spend a couple thousand, gain about 1/2 mile per hour.

Otherwise, enlarging the carb & intake will just give you a richer mixture.  Increased fuel burn per hour, more unburned fuel dumped in the lake, cooler combustion temps, fouled plugs, less power, not more.

Answer 3

texas is right --trade that 40 yr old motor in for what you want. You could change up a lot of motors with a few parts, but consider what you would spend on parts and what the 20 is worth then shop for a motor you can live with. I've done it but I had a pile of castings and boxes of parts from the orphans (abandon motors on my door in the AM) And with free parts it wasn't worth the trouble. The difference between a 7.5 and a 10 honda was the cam. There was no detectable change in performance!