NEED INFO.

[REBOOT2704]

HELLO I HAVE A 1969 20HP JOHNSON SEAHORSE JUST WONDERING CAN I BEEF UP TO 25HP? MODEL #20R-69D. AND IF SO WHAT CARB WOULD I NEED. AND WHAT OTHERS WOULD I NEED. ANY HELP WOULD BE GREAT. THANKS.

 

[Scott Danforth]

welcome aboard

no need for all caps

look at the 20hp parts listing and the 25hp parts listing and see what changes

 

[racerone]

This topic and question will not go away. ---Takes more than a carburetor change.-----There were some internal changes. to make more power.----And I believe crankshaft was balanced to allow it to rev at 6000 RPM.-----The 18 / 20 HP was rated at 4500 RPM.-----The 25 was rated at 5500 RPM.

 

[F_R]

18-20-25hp motors had different combinations of pistons, connecting rods, crankshaft, and cylinder heads as well as different carbs. And no, you can't just slap a 25hp head on an 18 or 20. It's all or nothing. OMC saw you coming on this one and made sure it wasn't an easy do.

 

[jimmbo]

Reeds had some differences too.
This is one case/example where the are no differences in the Block/Ports, but that doesn't mean the Port Timing is the same between the engines