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Yeah, that would have helped.

That is the boats rated maximum capacity.

Im looking myself lol

Scour ebay for months at a time

Anything

Google "Leeroys Ramblings 9.9"; this is an excellent resource for anyone looking to do this. In short, you need to find a Johnson 15 hp carburetor (for this series) with a 7/8" throat diameter and change it out.

Chad Wassink. Thank you

You need a 15hp prop too or you will over rev.

@@brianchisnell1548 What is the stock size/pitch for the 15hp Prop?

From what I recall, the gas tank was full, so that is 38, I weighed about 210, the dog was 70, the motor is 67. That leaves 155 pounds for the two girls. The tag max weight includes the motor.

its a 1236 rated for 15hp (I have the same boat)

"technically" ... you would have to put the engine on a dyno to prove it wasn't a 9.9 unless you go blabbing on to the Parks and Wildlife guy about your modifications. This is not a visible modification unless your eyes are so good you can "see" 1/4" difference in the carb venturis on a boat on the water and even then "It was that way when I bought it" is a viable response and it's a lot more legitimate than just putting a 9.9 cowl sticker on a 15. Most Rangers aren't going to look up your Ebay history when giving you a sobriety test. Behave responsibly, and you probably won't get harassed in the first place. If you're that worried about it, don't do it.

I'm not sure what you are referring to. The boat is registered in the state of Michigan. The boat tag on the transom says the max horsepower allowed for this boat is 15 hp. I'm not aware of any other restrictions for the waters I am on.

I found a "tuned" exhaust from a newer early 1980's Johnson 15 hp motor on Ebay. I forget which year they changed to the better exhaust but I think it was the last few years they made this motor. I don't think it did much at all.

@@chadwassink7846 and it is alot of work to change out that exhaust tube too. lol

@@rrrttt1112 Yes, it is a fair amount of work, for no gain that I can see. It can probably be done in a couple of hours, but I don't think it's worth it.

Glad it helped.

The 9.9's seem to be more common. It is simply what I found for sale locally that was in my price range. I did not know anything about the 15 hp conversion when I bought it. I only found that out later, so I kept watching ebay until I found a 15 hp carburetor. To your point, I agree that just buying a 15 hp motor (if you can find one) is much easier.

good answer.

There are also lakes that only allow a 10hp maximum so it’s cool with the mod you can keep food speed fully loaded.

Acceleration may be a bit slower b/c you've sacrificed some low/midrange TORQUE with the bigger carb. IOW, it's the same thing that happens when you put heads with larger intake valves on a 4-stroke motor. You increase max hp potential but you lose air velocity at the lower end. As for top end speed gain it may seem modest but you're up against the law of diminishing returns - drag increases faster than speed. The real difference may be apparent with the boat heavily loaded; it won't LOSE as much speed with three people aboard as it did with the 9.9 carb.

When I mentioned that the acceleration was slower, I meant because of the higher pitch prop. I don't think i lost anything due to the shims and exhaust mods, but I can't tell that I've gained either. This video mainly showed the increase from just a simple carb swap, and the significant advantage was when under load.

What size prop did u go up to

What's the prop diameter and pitch

I think the venturi size is different too.

The throat size of the 15 hp carb is 7/8", for the 9.9 it is 5/8". You can see the difference looking down the carb. The 9.9 motor is basically a detuned 15 hp, primarily by using the different carb body. The extra mod's were used on later years of the 15 hp to try to match the performance of Japanese 15 hp motors (from what I have heard), but I did not see any improvement at all after I added these.

Jetting it richer with the same small venturi will just make it bog out! You need a carb with a bigger 'throat' for less restrictive breathing.

Those minor mods MAY show up on a dyno but maybe not on your boat where they may only mean a fraction of a mph increase.

I put a jack plate on a 12 vhull with a 9.9 Nissan;20.8 mph with me and dog.Helped a lot with spray back into transom.worth doing andeasy to do.

Keep your eyes peeled on ebay

Ebay

The test you describe is shown in the previous video (except for the oars), starting at 1:53. The result was 22.5 mph for minimal load. The gas tank shown on shore in that video is an extra I used for weight in the fully loaded test. The real gain from the 15 hp was with a fully loaded boat, which was only 13.4 mph with the 9.9. So from the 15 hp conversion I got a 60% increase when fully loaded, and an 11% increase when minimally loaded.

Chad Wassink Cool! Thanks!

Except a 9.8 merc is a different motor than a 6. At one time an OMC 40/50 and 60 horse were the same motors, carbs and exhaust tuners were the only difference.

Won't work. The OMC 9.9/15 powerhead is 13 cubic inches, enough to crank out 15 ponies. It's restricted by a small carb and a less efficient exhaust downtube on a 9.9. Removing these restrictions gets you the full power potential. The Merc 6 probably lacks the displacement to get you 10 HP.

So buy a bigger motor😂 you not going to get any kind of speed out a 2.5hp🤣