Type 4-Stroke SOHC 2 Cylinder/4 Valves
Displacement 350 cc (21.4 cubic inches)
Bore & Stroke 59 mm x 64 mm (2.3 x 2.5 inches)
Full Throttle RPM Range 4,500-5,500 RPM
Rated Power 15 HP @ 5,000 RPM
Cooling System Water Cooled
Fuel Delivery 1 Carburetor
Ignition System PGM-IG
Starting System Electric/Recoil
Exhaust Through Hub
Lubrication Wet Sump
Trim Range 4 Stage 8°-12°-16°-20°
Tilt Range 72°
Gear Ratio 2.08:1
Then the process:
1) Install a tachometer
2) Run the boat with a light load and determine the full throttle rpm with the existing prop.
3) A change of 1 inch of pitch will produce a change of about 200 rpm
more pitch means less rpms, less pitch means more rpms
4) Buy a prop with the pitch that will give you 5200-5400 rpm at full throttle.
Find your year and model
then check the carb, block, head and exhaust part numbers
against the part numbers of the same year 20 hp Honda.
Those parts that have different numbers are the ones to replace.