1958 johnson 18hp wont plane

[henry954]

ok guys, got it on the water today . got a 14 ft alumacraft jon boat. Motor is dialed in . she revs hard idles perfect but when I go full throttle I don't go any faster than 5 mph. Im posting some pics. its either the prop (there are no sharp edges) or the gears ? again motor is revving the problem isn't there . you can feel she has power. but she wont plane and is very slow. could the prop be the problem being so rounded off?

 

[nwcove]

Re: 1958 johnson 18hp wont plane

Probably a spun prop hub.

 

[jonbellamy]

Re: 1958 johnson 18hp wont plane

Is it revving up when you punch it full throttle? Or is it not getting high rpms?

 

[henry954]

Re: 1958 johnson 18hp wont plane

it is revving up. the motor has more than enough power to pull this boat. it just is not hooking up. going to check this prop hub

 

[henry954]

Re: 1958 johnson 18hp wont plane

can someone recommend a good prop for this motor, And a place to source it from. Nothing fancy. I don't have a press to remove this hub . might as well change it .this prop is toast anyway.what pitch

 

[nwcove]

Re: 1958 johnson 18hp wont plane

did you mark the prop and hub and run the boat?
if so, and the prop hub is spun, check the salvage yards in the top secret file or.........
http://www.marineengine.com/newparts/part_details.php?pnum=OMC0763583&ptype=&Engine=&Model=

 

[64osby]

Re: 1958 johnson 18hp wont plane

If it's not a spun prop, check to see you're getting spark on both cylinders. It will run on one and be a dog.

 

[henry954]

Re: 1958 johnson 18hp wont plane

not yet. but I put the prop on a vise and the hub seems to have some slop in it.

 

[nwcove]

Re: 1958 johnson 18hp wont plane

if you didnt confirm it to be spun, dont go out and buy a prop. as osby said, those motors will start and run quite well on one cyl, but just wont get up and go.

 

[henry954]

Re: 1958 johnson 18hp wont plane

ok guys made two punch marks.one on the hub ,one on the prop. tested and no movement whatsoever.so that's not the problem. I cant believe its running on one cyclinder. both plugs are new and they look identical after running for some time. im getting spark on both plugs.

 

[phillnjack2]

Re: 1958 johnson 18hp wont plane

what size is the pitch of the prop and diameter ?
maybe the prop was for a heavy boat with a small pitch etc,if so youl never get high speed.
also is the shaft length correct ? i.e shortshaft engine on a longshaft boat ?

just a couple of thoughts.


phill

 

[henry954]

Re: 1958 johnson 18hp wont plane

furthermore just went downstairs, started it and took turns pulling spark plug wire while running and when u pull one it runs on the opposite no problem, but they r both firing fine. what about the lower unit. can something go wrong in there. what height does the prop have to be in relation to the bottom of the hull. is this an issue

 

[henry954]

Re: 1958 johnson 18hp wont plane

phil, ok boat is an alumacraft jon boat. 59 inches at its widest in the back 14 feet long and twenty inches high at its highest point. im going to say prop is 8 inch diameter pitch I don't know its not on the prop.

 

[henry954]

Re: 1958 johnson 18hp wont plane

oh the shaft on the motor is 32 inches from bottom of skeg to bottom cover of motor long shaft right?

 

[nwcove]

Re: 1958 johnson 18hp wont plane

are you testing for a spun hub in a barrel or on the water?

( as far as long shaft/short shaft goes......measure from the transom clamp to the anti vent plate above the prop.....from where you measured 32" it would make it a short shaft tho)

 

[henry954]

Re: 1958 johnson 18hp wont plane

im testing in a river nearby. this pic shows the prop shaft is pretty much level with bottom of hull. is that bad.

 

[nwcove]

Re: 1958 johnson 18hp wont plane

wow!!! what is the measurement on the transom height ???? sure looks like to much stern ,even if you trim the motor in to its last notch! cant imagine the prop being able to get good water with the motor that high.

 

[henry954]

Re: 1958 johnson 18hp wont plane

20 1/4 inches

 

[henry954]

Re: 1958 johnson 18hp wont plane

what can I do to mitigate this.

 

[nwcove]

Re: 1958 johnson 18hp wont plane

you either need to find the parts to convert to long shaft ( not so easy) or do some careful measurements and cut the transom down and seal it up. or buy another boat that is made for a short shaft motor. jmo