20hp johnson need help please

[RooSmith]

i have a early 70's johnson 20hp tiller hander not sure of the year but it doesn't have points, just got the boat, i have had the carb cleaned, foot oil replaced, dumped out old gas put new mix in, and new spark plugs, i posted on here before and someone said to check the drive pin and i did, and it was in 3 parts so i replaced that, then took the boat out just after that and still it runs great at idle but then when i give it gas there is lots of vibration, and it will not go very fast at all, it revs up then slows down, revs then slows down is not very drastic revving but its noticeable, the motor is on a 14ft malibu boat, i was told by a boat mechanic that the prop blades are slightly bent on the ends but not much, if anyone could help me out it would be greatly appreciated, thank you

 

[Daviet]

Re: 20hp johnson need help please

Some thoughts:
Will pumping the primer bulb when the engine is running poorly help, maybe fuel pump weak.
Dropping a cylinder under load.
Could be some trash in the high speed jet.
Prop hub could be slipping.
Bent blades on the prop causing cavation.

 

[RooSmith]

Re: 20hp johnson need help please

what could cause dropping a cylinder under a load?

i just had the carb cleaned

and i was told the prop is damaged so maybe it is the prop but would that cause vibrations in the motor?

and what kind of prop do you think i should get for this boat it is on a 14ft malibu boat

Thank you for all the help

 

[Daviet]

Re: 20hp johnson need help please

I guess if you know the prop is damaged replace it or have it rebuilt and see if it solves your problem. There are probably several different props available, but I would stick with the size you have if it worked for you in the past.

 

[RooSmith]

Re: 20hp johnson need help please

is there a way i can test to see if the prop hub is slipping?

 

[Daviet]

Re: 20hp johnson need help please

You can try drawing a line on the prop to match the direction of the drive pin, take it out and run it till it acts up and check the mark, if the mark no longer matches the drive pin you have a spun hub.

 

[RooSmith]

Re: 20hp johnson need help please

i know i probably sound dumb but the hub is part of the prop? will getting a new prop solve this? and how does this happen to it just outta curiosity?

Thank you for all your help i appreciate it a lot

 

[Daviet]

Re: 20hp johnson need help please

If you look at your prop where the drive pin goes through you will see that there is a brass hub that goes over the propshaft and the drive pin and propshaft align to install the drive pin. Now flip the prop over and look at the back side, you will see a rubber bushing between the brass hub and the prop itself. If the rubber is old or if you hit something hard the power from the engine will spin the brass hub inside of the prop. Normally the problem is something like "the engine revs up but the boat slows down". The hub can be replaced by a prop rebuilder.

 

[RooSmith]

Re: 20hp johnson need help please

ok thank you for all the help i REALLY appreciate it