which 9.9 engines can be converted to a 15?

[krawler]

Here is a web site that will cover just about everything you ever wanted to know about Johnson/evenrude 9.9's

https://www.leeroysramblings.com/Ramblings.html#OMRA


More on the carb's

https://www.leeroysramblings.com/Johnson_9.9_carb-fuel_pump.htm


Enjoy

 

[aaronjbrowning]

thanks for the info! It appears as though you can not make it a 15hp. Did I read that article correctly? Thanks again.

 

[Johny25]

No you did not read it correctly, yes you can and as I have stated before I have the same EXACT motor and did the conversion with good results. Are you not reading our posts because you ask the same questions even after they are answered? Maybe a video will convince you better :lol:

https://www.youtube.com/watch?v=V7rP8oOcAVQ

 

[aaronjbrowning]

I have read everything that was posted including what you said hence the acknowledgement I gave you. The last post mentioned an article which I felt was confusing therefore sparking my question.

"Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet. All you accomplish is that the engine is being fed too much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end."

I'm here to learn so I ask lots of questions. If that bothers you please ignore the thread.

 

[Johny25]

"Do not think that you can up the horsepower by simply modifying a existing 9.9 carburetor by replacing the main jet with a 15hp main jet. All you accomplish is that the engine is being fed too much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end."

I am not sure if you are quoting this statement or asking a question but here is the answer: No you cannot just replace the jets to get more HP. The 9.9 carb has a much smaller venturi which will not let enough air in to accommodate the extra fuel flow of larger jets. Look back at the ramblings article where it shows the carbs side by side and you will see the venturi of the 15 is much bigger. Also the tuned exhaust of the 15 is required to reach the 15hp.

 

[Johny25]

I'm here to learn so I ask lots of questions. If that bothers you please ignore the thread.

You have asked the same questions even after they have been answered on more than one occasion hence the frustration with your post. Nobody is here to give you bad info, and most of us are giving you knowledge we learned first hand by doing it. You know "been there done that" sort of thing. I have walked several people through this HP upgrade on these J/E motors so I know what I am talking about, not to mention I have done this to THE EXACT SAME MOTOR YOU ARE LOOKING AT and it is in my shop right now 50ft away from me.

 

[bobschlemmer]

So changing the carb makes it faster how does it idle? I have a 78 an thinking about doing it too

 

[krawler]

One thing you have to keep in mind is, that the 9.9 and 15 are essentially the same motor. Converting from 9.9 to 15 by changing the carburetor is not going to make the boat jump up on plane any faster or provide anymore performance in the lower rpm band. The only difference will be on the top end. A 9.9 will achieve about 10 hp around 5000 rpm's and 15 hp at around 6500 rpms. The bigger carburetor allows the engine to reach those higher rpm's therefore achieving up to 15 hp.

Let me give you an example. If you had a 14 foot aluminum boat with a 9.9, two big guys, trolling motor, batteries and more gear than they really need. Running at full throttle the boat reaches let say 16 mph's at a rpm of 3800. If you were to switch over to the 15 hp carburetor you wouldn't see any differences in the speed. The motor is not able to reach the top rpm's. You would need to either lose some weight or change the prop to a lower pitch and then the motor might be able to reach those higher rpm's. Now, if your boat is able to reach a speed where the rpm's are around 5000 than the bigger carburetor would help to achieve a little more top end which would translate into more speed.

So if you read the article it talked about how one would use their motor. If putting around a lake and trolling is all you do then a 9.9 would be a better choice as it has a better/smother idle. Where as the 15 hp is used more for high speed runs.

 

[Johny25]

Most of what you just said krawler is dead on. But I will interject that the modification also increases and improves mid range horsepower not just top end. If your boat is just a little underpowered or you have just a little to much weight in it to plane this mod will put your boat on plane. This is exactly why I did it and it put my boat on plane and got me to about 16-17mph as opposed to not planing at all due to excess weight after putting a heavy floor modification in my boat. And it stands to reason since it is sucking far more fuel and air at ALL rpm ranges that it has more HP than the 9.9 in all RPM ranges. It is just more noticeable the higher the RPM becomes. The OEM book states 5500rpm for the 9.9 to produce peak HP and 6250 for the 15 to produce peak HP so they are only 750rpm's apart at peak HP not 1500. I would bet a good chunk of change the 15hp is kicking out about 12+hp at 5500rpm versus the 9.9 but I don't have a way to test prop HP

Holeshot will not have a noticeable change, that I did notice. But mid and top end was far different.

 

[bobschlemmer]

I wanna thank u for ur example so the best thing to do is get me a tach n see what she really doing first because I troll a lot n I also cruise to get to a spot

 

[Johny25]

Well a tach won't do much for idle. The idle speed of the engine will not change by going to the 15hp carb since both are recommended 625rpm at idle plus or minus 25-50rpm. So unless you change props your idle speed will be the same. The change in idle will be HOW it idles. The 9.9 idles much smoother and after the upgrade you will notice the engine will spit or chug more. Also it has a tendency to load up if ran at low idle for long periods. So if you troll a lot and want a nice smooth idle stick with the 9.9

 

[Johny25]

So changing the carb makes it faster how does it idle? I have a 78 an thinking about doing it too

I will also mention that you can put a tuned exhaust from an 81' and later 15hp and get more out of your engine. The 15hp in 78' was most likely not putting out 15hp without this. Remember OMC did not start prop rating HP on there motor's until 85'

 

[bobschlemmer]

So if I put the tune exhausted in n didn't like the 15 carb do u know how itt would react with the 9.9 carb

 

[Johny25]

Nope I don't know. I do know that the tuned exhaust will make the engine much louder. The tuned exhaust is shorter and does not seal down to the lower like the 9.9. This is to create better exhaust flow which intern makes the engine louder.

 

[bobschlemmer]

Thanks johny25 you've been a lot of help

 

[Johny25]

Thanks johny25 you've been a lot of help

The 15hp tuned exhaust can be a hard part to find sometimes, a lot of people like doing this mod and pick them off ebay fairly quick when they come up. I often wondered if I could have just cut off my square exhaust and got the same result?

 

[Pappy]

There will be zero "seat of the pants" differences in idle quality or trolling quality between the two different carbs and exhaust configurations. That engine was one of the best idling and running engines in both configurations ever built. Both of the carburetors are designed to run with the throttle blades closed completely and the idle calibration hole (air hole) through the throttle blade supplies the air. The engine knows zero diffence between the two carburetors at idle.

 

[bobschlemmer]

Quick question guys would a 1990 carb work on a 78 ?