# 86 9.9 Johnson upgrade advice



## BiteMyLine (Sep 7, 2006)

I'm considering taking my 1986 Johnson outboard and upgrading it to a 15hp carb to increase horsepower. I just put this motor on a 14' flat bottom and wouldn't mind having a little extra out of the motor.

Is it worth it? Do I need the spacers? Anybody have any suggestions of a good website for sales and/or installation procedures? 

Any advice or horror stories about this conversion would be greatly appreciated.


----------



## huntindoggie22 (Mar 28, 2008)

Unless you can find a carb cheap it's not worth the extra money. You only pick up 2-3 mph with the 15hp carb. To answer your other question if you do go that route anyway all you need is a 15hp carb and 15hp exhaust tube. Nothing else.


----------



## bountyhunter (Apr 28, 2004)

I,ve been told you have to change the reed valves?? never done it ,but it was a dealer mech that said this ,any body know ?


----------



## Lewis (Apr 5, 2004)

Absolutely just the carb will work wonders on that motor. No further mods needed. I have a 1989 9.9 Johnson that I changed just the carb. The boat was a heavy14.5 ft. Sea Nymph that would not even get up on plane. Adding the 15 hp carb I picked up at least 10 miles per hour and it will flat out get on plane quickly. No need to change reed valves or exhaust. The difference was night and day with the 15 hp carb. Very reliable too. I have been running this setup hard for 20 years.


----------



## Lewis (Apr 5, 2004)




----------



## Lewis (Apr 5, 2004)




----------



## Lewis (Apr 5, 2004)

You will notice that the throat(butterfly area) of the 15 hp carb is much larger.


----------



## huntindoggie22 (Mar 28, 2008)

Lewis said:


> Absolutely just the carb will work wonders on that motor. No further mods needed. I have a 1989 9.9 Johnson that I changed just the carb. The boat was a heavy14.5 ft. Sea Nymph that would not even get up on plane. Adding the 15 hp carb I picked up at least 10 miles per hour and it will flat out get on plane quickly. No need to change reed valves or exhaust. The difference was night and day with the 15 hp carb. Very reliable too. I have been running this setup hard for 20 years.


The reason you picked up 10 mph is because the 15 hp carb gave you just enough to get your boat up on plane thus getting it on top of the water. This will not happen with every boat. The 9.9 carb top rpm is 5700. The 15 hp top rpm is 6000. Not a big difference but that being said if the extra 300 rpm is enough to get you up on plane, whereas the 9.9 didn't then you could pick up a little bit more speed but as stated previously this will not be the case with all boats.


----------



## Lewis (Apr 5, 2004)

Huntingdoggie, with all due respect, I think you are off on your 300 rpm increase. The operating range for a 9.9 is 4500 to 5500 rpm whereas the 15 hp is 5500 to 6500.
*The outside visual dimensions of both carburetors appear the same, unless you look INSIDE the throat.* *If you look in from the rear, you will have to position the throttle plate to a horizontal position to be able to see the throat very well. * *The 9.9 hp carburetor’s INTERNAL THROAT is .625 dia.*, * 15 hp* * internal throat is .875 dia. up until late 1987 when they went to a plastic topped carburetor. NOTE this is not measured from either the from or back but the internal throat that you can see on the LH photo below. The outlet or rear throat dia. is the same on both to match the manifold port dia. The idle jet venturi system is also slightly different between the two. The part number for the the pre 1987 9.9 hp Carburetor Assembly is #388936, while the 15 hp Carburetor Assembly is #338273. This increased throat dia. and larger main jet allowed it to breather better and to pull more fuel in, increasing the RPM about 1000 RPM, hence more HP. *


----------



## Lewis (Apr 5, 2004)

Check out Leroy's ramblings.....The guy knows his stuff. http://www.leeroysramblings.com/Johnson 9.9_15.htm


----------



## Lewis (Apr 5, 2004)

People can argue this until they are blue in the face, but this video example is damn near exactly what performance gains I have enjoyed out of my carb upgrade for 20 years. Yep, boats on plane go faster...lol


----------



## huntindoggie22 (Mar 28, 2008)

I'm not off on my 300 rpms. My dad and I have been working on these motors for years. Between the 2 of us we currently have 13 of them. I don't care what the sticker says or what you may read online about the rpm operating range but I'm giving you first hand info coming from these motors. We have a digital tach that we put on them when we test then before selling so the info I'm giving is not coming from something I read.


----------



## Lewis (Apr 5, 2004)

We'll agree to disagree then. My info is not only coming from something I've read. It's coming from my experience on my Evinrude 9.9 that the carb change made all the difference in the world. Just curious on your digital tach rpm readings. Are you taking these readings after changing to a 15 hp carb? Just wondering, if to you it's not worth it to change carbs how many motors have you have actually done a before and after carb upgrade and rpm testing on? I have worked as a machinist/mechanic for over 35 years. About half that in the engine assembly field with lots of dyno testing, so I more than understand engine rpm. All that I can say is that in my case I picked up way more than 300 rpm and a boat that would not plane with one guy in it. After the carb change it planes out with 2 men and all their gear and has for many years.


----------



## steelneyes2 (Jul 19, 2011)

Well, aside from the current pissing match, may I suggest a prop change before doing the carburetor? The carbs bodies for many of the older OMC motors are NLA and you can pay dearly for some of them depending on the model. One guarantee is you will burn a lot more fuel which may or may not be a concern. A prop change will be necessary for optimum performance even if you do recarb anyway.


----------



## Popspastime (Apr 1, 2014)

Here's an idea... because I was going to do the same with a Merc and before I was done I cold have (which I did) invest in a larger motor. Sell the 9.9 and take that money and the money you would have spent and buy a larger motor. Your NOT going to gain what your thinking...


----------



## pymybob (May 28, 2004)

steelneyes2 said:


> Well, aside from the current pissing match, may I suggest a prop change before doing the carburetor? The carbs bodies for many of the older OMC motors are NLA and you can pay dearly for some of them depending on the model. One guarantee is you will burn a lot more fuel which may or may not be a concern. A prop change will be necessary for optimum performance even if you do recarb anyway.


Bridges Boats in Cuyahoga Falls recommended this same course of action for my '82 9.9 Johnson. It's not a cheap upgrade if you go with a quality prop brand like Solas which will be my next upgrade.


----------

