" Charlie; I have a 1990 25 hp
" Charlie; I have a 1990 25 hp Johnson that the previous owner had converted to a 30 hp by a very reputable marina here in northern Illinois. I have a 16?Lund Explorer and am very happy with this boat/motor combination. According to Joe Reeves the 25 and 30 have the same block, cyl head, crankshaft, rods, pistons, leaf plate, leaf valves etc. The differences between the two are as follows:
The throat hole venturi on the on the carb and manifold is larger on the 30 than the 25.
30hp is hit at 5500 rpm. 25hp is hit at 5000 rpm.
The cam, which is attached to the magneto armature plate, is different. The 30hp cam is # 326876. The 25hp cam is # 334750.
The orifice at the extreme top of the carburetor is different. The 30hp is # 39 (.039) # 334568. The 25hp is # 30 (.030) # 317473.
The high speed jet, located at the center bottom portion of the float chamber is different. The 30hp is #65D (.065) #327541. The 25hp is #51 (.051) #331567.
I hope this information is helpful to you. It is my understanding that trying to increase hp is usually not a good idea, however I think in the case of this particular motor it works well. "