Reply To: 1975 Johnson 15R75C 15 hp

[David Stanton]

The white prop blade shape is more of a weedless design.  If the pitch is the same, the black one will likely be a tad faster.  Pitch is hard to compare by eye when the prop blades are shaped different.  They came new with a 10 inch pitch prop.

1974-1976 models have points and condensers under the flywheel, and a pulse is generated that is applied to the external ignition coils that step the voltage up higher yet for spark.  No electronic powerpack, as the points control when the spark occurs.

The high speed jet in the carb is fixed – no adjustment. The knob on the front panel is the low speed carb needle.  If that low speed needle is adjusted too lean, it will bog an maybe kill if you open up the throttle quickly.  Usually have to find best lean idle setting and then advance the knob about 1/4 turn richer to avoid the bog on acceleration – I turn them richer 1/8 turn at a time until the bog is gone.  Need to have it on a boat or a test wheel to adjust for bog elimination.

Max RPM range for 1974-1980 is 5500 to 6500 if you have a tini-tach or equivalent to measure RPMs under load.    In 1981, a tuned exhaust horn was added  inside the exhaust housing, which added another 500 RPM to these numbers and got the HP to 15 at the prop shaft instead of at the powerhead.  This was done to compete with imports that measured HP at the propshaft.

Spark plug gap is .030.  On 1977 and newer the gap is .040.

The 1974-1976 15s didn’t have a telltale cooling stream, which was added in 1977, but there are youtube videos showing how to add one by drilling and tapping the cylinder’s water jacket.

There is a thermostat on the cylinder head just below the cylinders.  In the mid to late 1980s it was moved to the top of the head.

I have never had to dig into the magneto on a 1974 -1976, so can’t offer more help there.  I have six  similar 15 HPs that I got from a failed resort, but all are between 1979 and 1991.

Dave