Prop for 74 Johnson 50hp

[Jfj1972]

I have a 1974 50hp Johnson on my 16ft extra wide extra deep jon boat. Currently running a 15 pitch prop and only getting 4600 rpm wot and 26mph. Wondering if dropping pitch will increase speed. Anyone have a similar setup that goes faster? I'm thinking I should get 30-35mph out of this am I wrong?

 

[jimmbo]

Is the engine in good shape? Compression test? Is the spark advancing, and are the carbs opening fully? your engine has a WOT rpm range of 5000 - 6000. To get to the sweet spot near 6000, you would have to drop to a 10 inch prop, not realistic.

 

[Jfj1972]

Compression is 150 on both cylinders. Carbs are fully opening and the spark is advancing. I'm wondering if compression is too high? Blown head gasket possibly?

 

[jimmbo]

I think your tach and/or speedometer are not accurate. 4600 rpm with a 15, my calculations show between 20 - 22 mph. Depending on how heavy you boat is, I would expect about 25 - 27/28 mph.
If a head gasket were blown you wouldn't have much for compression. You may need to run a few cans of Engine Tuner thru it
You say the timing is advancing. but is it advancing to WOT spec?
The carbs are opening fully, is that using the remote control?

 

[Jfj1972]

Yes timing is advancing to wot spec of 19 degrees and yes carbs are opening fully using remote control. I'm using GPS for speedometer and tiny tach around bottom plug wire if it matters which plug wire.