Convert 9.9 to 15hp ??

[Kymasabe]

IS it possible to convert an '82 Johnson 9.9 to a 15hp?
I need a little more than 9.9hp and it seems like 9.9's are everywhere but I'm having trouble finding a 15hp around.
Also, my 9.9 has low hours and runs GREAT and I'd rather not sell it and take a chance on a different motor.

The conversion however may be too difficult and/or too expensive to be worth it.

 

[CATransplant]

Re: Convert 9.9 to 15hp ??

Yup. Too expensive, unless you can find a donor 15hp of the same vintage. The early 9.9's, on the other hand, had only the carburetor as a difference. I believe the year that changed was 1977. Yours has several differences between the two horsepowers. Buying the parts new would cost more than the outboard is worth.

That said, 9.9s are very popular, and seem to sell for about the same price as the 15s in my area. I think your best bet will be to buy the larger and sell the smaller.

What's the maximum horsepower rating on your boat? 18 and 20 hp johnnyrudes are out there at bargain prices, if your boat can handle them.

Edit: Never mind. If this is your 12' Duranautic tinny, it's rated for 10 hp. A 9.9 should plane it right out and run it up to around 20 mph. I wouldn't put anything bigger on it.

 

[johnnyb563]

Re: Convert 9.9 to 15hp ??

I have done a TON of research into this. What I have found is that from 1974 to 1979 the only difference in the 9.9 and 15 is the carb. Therefore, those years are good for the swap. After that, they began to use different exhaust tubes and other components, so those become more costly to convert. I am in the process of trying to find a carb to try the conversion o my '74 9.9

I just need to get it tip top before I go through with it.

I'll let you all know how it goes.

Thanks

 

[johnnyb563]

Re: Convert 9.9 to 15hp ??

I had seen it all over the net. People saying on these forums that it can be done, or it can't be done, or it's too expensive.

I took a carb that I bought on Ebay from a 1976 15HP, and put it on my 1974 9.9HP. The engine finally runs like it should. Smooth power across the whole RPM range, and a definite increase in top end power.

I would say that the swap is worth while on 1974 to 1979 MY, but al my rsearch indicates that outside this MY range there are more components that are required to make that power.

It took a lot of tuning to get the 9.9 to run smoothly, and it never ran as well as it does after the carb swap. This proves to me that Johnson did infact "detune" the 15HP with the smaller carb to make 9.9.