9.9hp johnson to 15hp conversion

[numbers]

Be forwarned, I'm a newbie.
I have a 1981 9.9 Johnson and was wondering what it would take to convert it to a 15hp. The 9.9 just doesn't have the umph to properly plane a 14' aluminum with two guys.
Just curious if it is do-able and at what cost in time and $.

THx

 

[Chinewalker]

Re: 9.9hp johnson to 15hp conversion

Might be easier to buy a lower pitch prop - akin to downshifting in your car. A properly set up 9.9 ought to pop a 14-footer up with a couple guys and gear fairly easily.

 

[numbers]

Re: 9.9hp johnson to 15hp conversion

Thanks for the input.
What pitch of prop would be appropriate?

Any other takers on this.

Many thanks.

 

[Jerry_NJ]

Re: 9.9hp johnson to 15hp conversion

My sympathy to your question, as I understand (absent any details) that the 9.9 and 15 HP are the same engine save some carburetor and maybe ignition differences.... my be a simple as jets and throttle lineage.

When I asked today for help with the ignition on the 9.9 it was suggested I look in the OEM manual for my engine... that may be of help to both of us, but I'm trying to not have to buy a shop-level manual, I, like you, am looking for something one can do with only a "shade tree mechanic" expertise.

 

[bob johnson]

Re: 9.9hp johnson to 15hp conversion

the shade tree mechanics way of turning a 9.9 into a 15 hp involves selling and buying..one of each!! ha ha ha


I would think that if a 9.9 cant plane a boat the 15 would struggle as well..if it could even do it.

bob

 

[northwoodslivin]

Re: 9.9hp johnson to 15hp conversion

Pretty sure for your year of motor, all you need is a carb off of a 15. I think you are in luck and don't require shims for your year motor. But like others have said, the 9.9 should get a 14 on plane with a couple avg weighted guys, but a 15 carb will give you a bit more "omph" but with the expense of more fuel being burnt. I would try a higher pitched prop first, might also want to test your compression.

GL

 

[Chinewalker]

Re: 9.9hp johnson to 15hp conversion

I would try a higher pitched prop first...

GL

LOWER pitch, not higher...

Can't recommend a prop because I don't know what's on there now. Find out what you have and drop a couple inches in pitch. And, as NorthWoods pointed out, check compression - if the motor is tired, changing the prop or adding a 15hp carb won't help...

 

[bob johnson]

Re: 9.9hp johnson to 15hp conversion

seriously, if a 9.9 motor cant get a 14ft boat on plane with 2 guys...would a prop change work at all??

I mean if the boat didnt get on plane easy or took a little longer than the boater liked...maybe a lower pitch would help out

but if itsnt getting on plane at all....I am thinking 2 "of pitch wont be the answer!!!

something in the boat has to change or the motor isnt working right...

unless he is running an 11 or larger pitch.... he has a lot of ground to make up...

bob

 

[northwoodslivin]

Re: 9.9hp johnson to 15hp conversion

LOWER pitch, not higher...

Can't recommend a prop because I don't know what's on there now. Find out what you have and drop a couple inches in pitch. And, as NorthWoods pointed out, check compression - if the motor is tired, changing the prop or adding a 15hp carb won't help...

oops :redface:

 

[ChrisAG]

Re: 9.9hp johnson to 15hp conversion

my be a simple as jets and throttle lineage.

In converting from one HP to another, be it outboard motors, motorcycles, whatever, it is NEVER jets... ever.

Carburetors themselves are different (bigger throat diameter) in this case.

 

[numbers]

Re: 9.9hp johnson to 15hp conversion

Thanks for the input.
I've had the motor since new, and it doesn't have a whole lot of hours on it. Recently tuned-up by the mech. Runs great but like I say it can't quite make the change from coming up to plane to actually planing. Almost, but not quite
I'll talk to my mech. about prop pitch.

Your help is appreciated. Thx

 

[mchin]

Re: 9.9hp johnson to 15hp conversion

add a doelfin or some other hyrdofoil. The 9.9 to 15 hp conversion has been beat so many times over, it's exhausting. Do a search and pick a side. 15 hp carbs tend to fetch a pretty penny, so you may just be better off getting a larger motor. Also, check your compression and link and sync, in my mind a 9.9 should be able to get you up on plane. I run a 74 evinrude 9.9 and get up on plane no problem, with 2 people and way too much fishing gear.

 

[northwoodslivin]

Re: 9.9hp johnson to 15hp conversion

I know this may sound silly, and perhaps you have already tried this. But did you try taking your pin out of the transom bracket and putting it back a hole or two to see if this will do the trick with a heavier load up front?

 

[Bobschlemmer]

Re: 9.9hp johnson to 15hp conversion

In 1978 is the carb the only difference?

 

[dkonrai]

Re: 9.9hp johnson to 15hp conversion

In 1978 is the carb the only difference?

yes, correct the 15hp has a larger inside throat.

 

[Bobschlemmer]

Re: 9.9hp johnson to 15hp conversion

Thanks for responding

 

[jmendoza]

Re: 9.9hp johnson to 15hp conversion

Hmmm... my 9.9 will plane the boat with three people at 6800 feet of elevation, and it's a 145 pound 12 1/2 foot welded aluminum semi-vee type hull. I havea prop that is 2 inches smaller in pitch to compensate for the power loss due to altitude and load, this allows the engine to turn up over 5500 RPM at full throttle. I added a 15 carb to my 1975 9.9 and it gives me more top speed if the boat is lightly loaded, but thats about all. I did find out however that if the engine is one of the early models with points, like mine, setting the ignition timing properly, and checking the adjustment of the throttle cam makes a big difference in power and how smoothly the engine runs. Also, doing a de-carb with some Valv-Tect marine motor decarb will pep up a tired or weak motor, especially if it has been run on cheap oil.

 

[mikesea]

Re: 9.9hp johnson to 15hp conversion

http://www.leeroysramblings.com/Johnson 9.9_15.htm

Be sure to find the title"Difference between the 9.9 and 15.In your eng.its not just carb.Also,it should perform better as said.Check to see all your likages are woring right.This site that I set as a link will tell you everthing you ever need to know on that motor

 

[Bobschlemmer]

Re: 9.9hp johnson to 15hp conversion

Could u hook up a 1990 15hp carb to a 78 9.9

 

[Haffiman]

Re: 9.9hp johnson to 15hp conversion

If you want the torque to get on plane with load, leave it as 9.9 and simply pitch DOWN the prop. Most of the difference in the 15Hp is by increasing the rpm. If you want more torque, square out the intake ports with a Dreml and increase the main jet.