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Speed Test - Johnson 9.9 to 15 hp Conversion
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- เผยแพร่เมื่อ 7 ก.ค. 2012
- The motor now has a 15 hp carburetor. This video documents the difference it makes to a Johnson 9.9 motor. It is a pretty easy modification, if you can find a carburetor. For the original motor's results, see my previous video. Both tests are using my 12' jon boat.
The main benefit of the 15 hp modification was seen when the boat was fully loaded. I have also put a tuned exhaust on since this video was posted, which made no top speed difference at any weight. It does seem like it accelerates quicker though.
That bad boy is running strong. Great post.
Thank you for this video. Helps me.
I bought it off Ebay for about $100. They usually go for about $125 or so. Sometimes you have to wait a few months for them to come up. Make sure you are getting the true 15 hp carb, it has a 7/8 dia throat, not a 5/8 dia like the 9.9. I originally bought one that was a 9.9 that was sold as a 15 and had to send it back. The carb are otherwise very similar, so it is hard to tell them apart. You have to look straight down the throat of it.
On my previous video, before modifications, mine would also go 22.5 mph with minimal weight in the boat. The 9.9's really slow down once you load the boat to the maximum capacity. In my case, it was 13.4 mph. With the 15 hp, I ran 21.1 mph with maximum weight.
I love these motors
Is your reverse broken? heading into that dock fairly quickly, reverse would help you there.
Yes. When I have a few people in the boat, or am pulling a tube, it really makes a difference. I got 21.1 mph (15 hp) vs. 13.4 mph (9.9 hp) at maximum load.
Chad Wassink I got 15mph on a 6hp evinrude rebuild with no porting so 25 is pretty good I would love to get it ported but it’s a good motor I have a 9.5 I’m working on
What size and pitch prop did you change to?
Did you ever do the tuner swap?
Which prop did you end up putting on?
All I could think of is the cottagers looking out going who in hell is that nut lol
The carburetor is the only thing that made a significant improvement for me. Seems to account for 99% of the improvement, but I would have to start back at the original setup to really know for sure. The exhaust makes it louder, but adds nothing to the top end. There was no top end difference with the shims either. Perhaps acceleration may have changed with either modification, but I was not testing for that so I don't know. I changed the prop and got the top speed to 26.9 mph (light load) now, but acceleration is a little slower.
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 have a 76 evinrude that I went thru and it starts 1 st pull 90% of the time. I would love to have a 15hp carb but have NO idea where to find one, please help. Thanks Glen
Keep your eyes peeled on ebay
Ebay
yeah, did you find it was worth the time and money for the power you got?
Where did you get the carb?
Nice comparison. It would also be interesting to see what difference a higher pitch prop would make. I believe a 10" is standard for the 9.9, you could probably go to 11" with the new carb.
Probably not. I just covered that in the thread above.
A bigger prop will give you less top speed smaller gives you more top end and a lower hole shot
Great test-Wondering about a higher pitch prop for better top end is why you didn't see nearly as dramatic a top end speed gain(with minimum test weight) after 15 carb installed,as you did with maximum weight-- props can make a huge difference
No, you don't change prop pitch, because you're getting the power from higher RPM. You have to let the 'new' 15hp rev to get the ponies. If you overprop it you'll bog it down. You want that motor running 1000 RPM higher than before.
Hess lake?
Reed spacers and tuned pipe
Half moon lake?
25 that's pretty fast
Appears convincing enough when comparing the 9.9 carb video to the 15 carb video, but I would also like to see the 9.9 carb perform with just you, the motor, gas can, and oars, as is shown in this video... (and not shown in your 9.9 carb video). A positive result would probably have me looking at putting a 9.9 carb on my 1986 6hp Mercury.
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.
What size boat
Man i havr that same boat but a 14 ft & same motor
How big of a motor can you put on that boat and what size is the boat
its a 1236 rated for 15hp (I have the same boat)
Depending on the year of that engine, you need to add reed valve spacers for it to gain a full 15 hp at the prop. exhaust tune pipe and carb is not enough.....Depending on the year, some 9.9 already had these spacers, and some years don't......It's important to have them if this one has none !!!
Thats what get with non 83 johnson 9.9 no conversion on a v hull 12 footer
Hey what size carb would be best for a 2007 mercury 2.5 hp??? I wanna go faster...lol
So buy a bigger motor😂 you not going to get any kind of speed out a 2.5hp🤣
Sylvan Lake
Loaded at max weight of 540 pounds? so you, 2 teen girls and a dog weigh in at 540 pounds? Im confused... did you fill a live well too or something?
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.
how did you mod the exhaust?
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.
My 9.9 stock will go 22.5 mph
Hi ...you do not need a carb, all you need is a main jet and reed valve spacers........only on some models....Note depends on year, a lot more mods may be required....nice job
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.
jou wil go faster with a difrent piched prop
Theres no way with a 15hp anythonh
Anything
If you don’t have a boat license and are using that conversion that is 15 hp and the most you can have without a boat license is 9.9 hp. Your technically not allowed to use it without a boat license.
please prove me wrong. I want to make mine like that.
"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.
Plenty quick enough
Good video. Check out my brand new 2017 15hp Suzuki on my channel. I just reviewed it. It's fuel injected.
You should have had one of those kids 🧒 drive, that would be less weight..................Use your head mate. 😭😳😘😒😩
Yeah, that would have helped.
Sylvan Lake