In 1981, the 50hp was a 4 Cylinder.
First Questions.
How long have you owned this Motor?
Was it running Fine Prior to the 5 Mph?
What are the Numbers from a Compression Test?
Can it make a Spark Strong enough to Jump a 3/8" Gap, on all Cylinders?
Is Fuel getting to all the Carbs?
Are the Carbs...