How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Tan Your Own Question
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11071
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
Type Your Boat Question Here...
Tan is online now
A new question is answered every 9 seconds

My 1978 20 hp started to slip and rev up while under load.

Customer Question

my 1978 20 hp johnson started to slip and rev up while under load. made it home on strong idle. what would i look for?
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

I would check the hub of the prop see if you have a spun hub. Remove the prop and check the back side of the prop you will see slivers of rubber peeling from the hub. The other thing you can do is put the engine in gear pull the spark plug wires off then try to spinb the prop while the flywheel is locked in position. If electric start you can stick a screw drive between the starter gear and flywheel to keep engine from turning.

You have one of 2 things the spline in the crank shaft is stripped from age and rust or the hub in the prop is bad.

Now I don't have a listing for a 20 hp in 1978 so could I get the engine model number?

Post back with findings.