67 Johnson 40HP looses thrust at high RPMs

[JKSander]

This motor runs fine at low RPMs but if I go past half throttle motor races away and thrust goes away. Please help!!

 

[Xcusme]

Re: 67 Johnson 40HP looses thrust at high RPMs

Welcome to iboats!

Sounds like you have a spun prop hub. Remove the prop, using a waterproof wax pencil, mark a line across the inner and outer surfaces of the hub. Replace the prop and take her out for a spin on the lake. Remove the prop again and inspect your line. If the line has shifted, you have a spun prop hub. Most marine dealers can have the prop re-hubed and you'll be back in business.

 

[F_R]

Re: 67 Johnson 40HP looses thrust at high RPMs

Spun prop hub immediatly comes to mind. But then I think, hey wait a minute! Is it electric shift? If it is, that's a whole different ball game.

 

[JKSander]

Re: 67 Johnson 40HP looses thrust at high RPMs

I am new to small boat motors, if it was a ship I could fix it. If it is a spun prop hub do I just remove the foot and take that to a marine dealer?

 

[F_R]

Re: 67 Johnson 40HP looses thrust at high RPMs

If it is a spun prop you just take the prop to the shop. They will send it out to a prop shop for rehubbing or sell you a new one, whichever is more practical. Or you can deal with the prop shop directly if they are local to you.

You haven't answered the question: Is it electric shift?

 

[JKSander]

Re: 67 Johnson 40HP looses thrust at high RPMs

Not sure what you mean by electric shift. The lever next to the throttle lever shifts the motor from forward to reverse by a cable that pulls or pushes the lever on the motor.

 

[F_R]

Re: 67 Johnson 40HP looses thrust at high RPMs

OK, that is not electric shift. It's the prop hub slipping. Nothing else in there can slip without making a horrible grinding noise.