Propeller won't spin?

[whitnes10]

So the other day my prop stopped spinning on my 1979 70hp Mercury. Full throttle still doesn't spin, but the engine sounds as if it should be going full speed.

I did some research and first thing was broken shear pin. So I took off the prop but did not see a shear pin or where it would even go. Does anyone know if/where the shear pin should be?

Any other things to check and problem shoot this issue?

 

[Chris1956]

No shear pin on those props. They have a pressed in rubber hub. Make a mark on the prop hub and prop housing that match. Now run the motor and see if the marks move. That indicates a bad prop hub.

Normally you would get some speed from a prop with a bad hub. At least idle speed.

 

[tacx]

As Chris has said, even with a spun prop hub the prop will usually turn some. You could have a more serious issue.

With the engine in neutral the prop can be turned by hand . Put the engine in gear and you should not be able to turn the prop. If you can turn the prop by hand with it in gear, you may gave a broken gear train.

 

[whitnes10]

Ok so I have tested what you mentioned. When the throttle is in gear, the propeller does spin by hand. It also "clicks" as it is being spun. Not sure what this means, I'll be taking it out of the water here soon to take a closer look.

 

[Chris1956]

Prop will click(rachet) in forward gear, when turned clockwise. That is normal for Mercs of that year. It should not turn CCW in forward,

 

[whitnes10]

Which it does not turn ccw when in gear. So what are your thoughts?

 

[Chinewalker]

Could be several things. Spun prop as mentioned above. Broken driveshaft, which could give you the issue you've described, particularly if it broke above the water pump, in which case the impeller would give some friction to your prop spin tests. If you can turn the motor over using the propeller, then that negates the driveshaft bring broken - do so ONLY with the spark plug wires remove from the plugs. I've also come across a few where the crankshaft splines have been rusted out, to the point where the driveshaft splines spin inside the crankshaft without engagement. Usually caused by previous servicing of water pump not including a new o-ring and fresh grease in the splines...

 

[flyingscott]

I will go with broken driveshaft