9.9hp to 15hp conversion done

Well after must research and scouring the internet for a carburetor I finally was able to up the HP on my little 9.9 johnson a little. The last piece of the puzzle for me to make it electric start also came in today, the electric start bypass cover is very elusive, almost as much as a 15hp carb. I will post pics of that conversion next weekend.

"new" old dirty15hp carb off ebay


"new"new looking carb off ebay after a complete tear down and rebuild.


original 9.9hp carb still in place, pic to reference recoil starter for reassembly


15hp carb installed and putting everything back together


Electric start bypass cover, very hard to find


electric start flywheel and bypass cover, just need new starter which i am ordering brand new online this week.


pic showing where a new bracket will need to fabricated for the fuel pump since it is where the starter is going to be now, you can still by the bracket but they want $110 for it and for that i will just make something.

PS, i did start it and it seems to run and idle better than before, started on 2nd pull, taking it out in the morning to do some fishing, will fine tune and get performance improvement data and post when i get back, between this and the jack plate, i hope i gain some MPH

You will be pleasantly surprised how much more speed that 5.1 extra horse is going to make... I know i was

Looking forward to your results. Wouldnt mind squeezin a few more hp out if my 1990 9.9

verdict is in, i was able to get some running and fishing in before the sky let loose. WOT rpm increased 750 to 5450rpm and just a tad over 21mph, very noticeable increase in acceleration but only picked up 3mph from my fastest day with old carb, not too shabby I think, kinda was hoping for a little more. Still seems like a lot of the boat is in the water, but that may just be me, I am used to boats with much bigger engines. Next time out might try moving the pin in a notch and see what happens. Not sure how much the jack plate provided as i didn't run it with the old carb. Motor actually seemed to start easier and idle better than with the old carb, cant wait to put on the electric start parts this week. Overall I am pleased with the conversion, few more adjustments and it will be there.

Glad my day didn't start like this

JaxJonBoat said:

... but only picked up 3mph from my fastest day with old carb, not too shabby I think, kinda was hoping for a little more.

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Yeah, that doesn't seem right to me, but i'm no expert...

My first motor was an Evinrude 9.5... I clocked it with my GPS at 16MPH, with 242lb me, my two 90 lb kids and all our fishing gear...

With my mercury and mariner 15's, i'm getting a HUGE difference in speed (i have not tried checking with the GPS yet... Next time i'm out i will.) But my ButtDyno says we are going much much faster now... Definitely more than 3 mph

A few years back I done the 9.9/15hp carb swap and I got the exact same results as you. A big boost in take off power and a 3 mph increase in top end.
Even though there isn't a big increase in top end speed the extra take off power and quicker throttle response makes the carb upgrade well worth it.

Do you have the parts numbers for each carb? Or is there a way to tell them apart?

are you running the same prop?

Bugpac said:

are you running the same prop?

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big question......get the right prop. no magic answer since different props are suited to different boats, loads etc..... to keep things simple, start with what's on a 15hp.

You mentioned there is still alot of boat in the water. If you move the trim pin in you will have even more in the water! You have to raise the engine and take speed and RPM measurements at that height and move the trim pin in and out at each height point. Only then will you know what the boat/engine likes. This is time consuming and everyone likes instant results......up to you!
When your RPM goes up and the speed stays the same you are done (height wise). Beyond that you are just increasing slippage. There are stainless props you can explore as well.

From what i have seen, 9.9's and 15's come with the exact same prop, usually a 9.5x10 and that’s what mine has. After doing some math it seems that i am probably running the speed i am supposed to be going using some of the formula's i ran across on other post. I found this on the internet:

Factory specification rating for RPM of the 9.9hp is at 5000 & the 15hp at 5500. With operating range for the 9.9hp at 4500 to 5500 RPM, & the 15hp at 5500 to 6500 RPM.
The outside visual dimensions of both carburetors appear the same, unless you look inside the throat. The 9.9hp carburetor’s internal throat is .625 dia., 15hp internal throat is .875 dia. The outlet throat dia. is the same on both & match the manifold port dia. The idle jet venturi system is also slightly different between the two. The part number for the 9.9 hp Carburetor Assembly is #388936, while the 15 hp Carburetor Assembly is #338273 up to about mid-1987.

When i held to two up side by side you could really see the bigger throat of the 15hp carb stand out. Looks like i am at the lower end of the rpm range, need to work on that. Anyway i will test some more this coming weekend, hopefully with the electric start upgrade installed.

PS i meant to say move the pin out, like you said that would just push the bow down even more, i am on the second pin now and my jack plate is nonadjustable, was typing too fast.

I have a 1974 mercury 9.8. Is this possible this motor?
Please pm me so I am not hi-jacking this thread.