1973 Johnson 25hp Top Speed Problems

[tin_boat]

I got a 1973 Johnson 25hp on a 14ft aluminum boat. The top speed is 15mph. This seems low to me. Does anyone know how fast these motors can go? Is there anyway to get more speed out of them? Thanks

 

[jimmbo]

There are many different 14 ft Aluminum Boats. A 25 might be able to push one(Empty Car Topper) close to 30 mph, while another(Runabout with 4 people) considerably less.

Transom and Engine are both same, either 15” or 30”?

Begin with standard engine checks:
Compression
Good Spark to both cylinders, meaning the Spark can jump a 5/16” gap
Points set correctly
Carb is opening Fully

Engine Specs
22 cubic inches
25 hp at 5500 rpm
WOT range 5000 - 6000 rpm
gear ratio 21:12

 

[tin_boat]

Thanks for the reply. Both the boat transom and the motor's leg are 15". I set the points. I did a compression check and both cylinders were at 110psi. Is this OK compression?

I have set the throttle linkage on the carb to open as much as possible.

The top speed was taken with a GPS with only myself and a full gas can in the boat. I have had a 9.9hp motor on the same boat with a top speed around 13mph. This is why I thought the 25hp should be going faster.

 

[jimmbo]

You mentioned you set/adjusted the Throttle Linkage, is the Spark timing advancing properly?

 

[racerone]

Might be running on 1 cylinder.----Verify good strong spark.----Could be an issue with a reed valve too , although rare.

 

[Roonhunter]

My 1972 25hp pushes my 14’ flat bottom xwide &xdeep 25-27mph with just me and a tank of gas if that helps

 

[tin_boat]

I did a spark test. Strong spark on both cylinders. Took it out on the lake again. Did a gps speed and still at 15-16mph. Maybe it's the prop pitch?
I'm thinking I might just sell this motor and stick to the 9.9. Its probably better for the small lakes I fish anyway.