1968 Johnson 9.5 HP free spinning

[kojack65202]

So I like these older motors because even an idiot like myself can generally figure out how to resolve issues, but I want to know if there is something I can do to maximize the output of this motor. It seems like I get about 6hp before the drive shaft starts free spinning without the prop turning. The rubber on the propeller isn't bad because it is a new propeller, so I get to limp along. Secondly, what determines whether they used the non thru hub exhaust and pin drive or spline drive? Was it for lower horse power engines only?

 

[Chris1956]

The prop should not slip while the motor is spinning. If it does, you either have ventilation or a bad prop hub. A lot of those older motors used a drive pin. The drive pins can break, and the prop still has a rubber hub that can break, both allowing the prop to slip. Ventilation is uncommon on those low HP motors but will occur when air is drawn into the prop, from above the waterline.

Drive pin props do not have or need splines.

 

[jimmbo]

The 9 1/2 was always a Drive Pin non-thru hub system. Sometimes new props have bad hubs. If the hub isn't bad, the prop could be experiencing ventilation. Since you were wondering about Drive Pin vs. Splined, and Thru Hub vs. non-thru hub, is there a chance you have an incorrect prop?

 

[kojack65202]

Chris 1956, I know that drive pin and spline are different systems. I was curious if the drive pin were on smaller hp engines only or if I will find the system on 15 and 20 hp motors also.

Guessing now that it is not the prop at all because this is the second prop I have used and had the same problem with.

 

[kojack65202]

How could I tell if the prop hub was bad? As far as correct prop, I'm using the recommended prop by the fella that I bought it from. It's his business not mine.

 

[kojack65202]

So, if I'm running along with an open throttle for a few minutes it will start to free spin (no forward momentum). If I back off the throttle a little bit it will re engage and continue forward. Higher RPM's makes it slip. Is that a more useful explanation?

 

[jimmbo]

Sounds like ventilation. The engine might be mounted too high on the transom, but that usually results in loss of bite as the boat gets on to plane.

What kind of boat is this being used on? Pics from the side of the boat at the transom, taken level a the height of the boat bottom, might shed some light/info

 

[Chris1956]

Pin drive props were not confined to low HP motors. I had a pin drive prop on my '59 50HP Johnny. They were probably on the early 60's larger HP motors, as well.

 

[jimmbo]

OMC used the Pin Drive on the 1966 - 68 100HP outboards, and on the Electric Shift Stern Drives, which went up to 235 or 240 HP