Cavitation 85n hp johnson

[dridog]

"I recently bought a boat that

"I recently bought a boat that has a 1977 85 hp johnson. It runs great but when i go past one fourth of the throttle the engine races to high rpm, so I have to back the throttle back to 1/4th. Why does it do this.Someone told me the prop is the wrong one It has a 11 x 17 pitch What size prop should this engine have on it"

 

[spicerak]

"try to go up two pitches,what

"try to go up two pitches,what are the rpms running at, u dont want to go over 5500 rpm"

 

[cehmass]

"are you sure the prop is not

"are you sure the prop is not slipping on its hub.
prop size for your particular applicatiion is not known,certain things need to be accounted for
max rpm,what you do with the boat/pulling skis,tubes etc/do you want out of the hole faster
or a little more speed."

 

[david_r]

"sounds like a spun prop......

"sounds like a spun prop......you can have it rehubbed for a lot less than a new one.........what i would do is check it and if it is bad have it fixed then if you reprop you can have it as a spare.

do this test



http://forums.iboats.com/attachment.php?attachmentid=17835&d=1221224083"

 

[dridog]

how do you check for spun prop

how do you check for spun prop. The explanation david real linked me to. says to make a mark across the prop using cotter pin as a guide. Then what?

 

[cehmass]

"you can run it in the water w

"you can run it in the water with the mark or /with a piece of wood between the blades to stop it from turning,
then put a socket on the flywheel and carefully
apply pressure it should not turn if good"

 

[bobbyc]

Propellors are available on th

Propellors are available on this site at a pretty good price...Click on the "propellor" section on this site for approximate propellor size and price.

 

[david_r]

"sorry don my thoughts dont al

"sorry don my thoughts dont always make to the keyboard.

you can do it either way charles suggested......i would do it in the water, but i like to drive my boat every time i get a chance.

but cavitation at 1/4 throttle........ i would put money on the prop being bad----unless your motor is outrageously too high."

 

[dridog]

ok guys thanx. I will try the

ok guys thanx. I will try the test tomorrow. If it is spun is there a hub kit and if so can an average mechanical person do the job or does it need to be done at a shop?

 

[cehmass]

"hub kits shoulb be available

"hub kits shoulb be available here or any dealer,
try a good machine shop it will have to be pressed in.."

 

[dridog]

"ok Im really confused now. I

"ok Im really confused now. I di the test with the board stopping the prop as I turn the flywheel. The flywheel turns and the marks stay aligned. I pulled a plug out and confirmed that the piston is moving, so the flywheel is not slipping. What does this mean."

 

[david_r]

you are supposed to put a sock

you are supposed to put a socket on the prop nut and the board wedged in between the prop and the lower unit--turn the prop nut and watch the marks......then you can see if the rubber hub is bad.

 

[dridog]

"ok thanx, but should I be abl

"ok thanx, but should I be able to turn the flywheel with that board in there?"

 

[david_r]

yep. especially if your motor

yep. especially if your motor is in neutral.

now if you put it into gear the prop will turn as the flywheel turns but not at the same speed due to gear ratio.

 

[jwbmarine]

"u have to have eng in gear,bo

"u have to have eng in gear,board,restraining prop,then try,..to turn flywheel..."

 

[david_r]

"why use the flywheel?... more

"why use the flywheel?... more leverage than at the prop nut?

my vote is till for puttin it in the drink

...........more fun less work!"

 

[cehmass]

"if it is in gear with prop re

"if it is in gear with prop restricted turn flywheel. ether way i guess he will find out.
just run it in the drink

"

 

[jwbmarine]

"either way wks just lovely,..

"either way wks just lovely,..."