Converting a 9.9 to a 15?

tonyscj5 · May 8, 2005

I was told it could be don simply by switching carbs? Is this so? Anything else involved? Does the prop then also need to be changed? thanks

WillyBWright · May 8, 2005

Re: Converting a 9.9 to a 15?

You were lied to. It takes a lot more than that. That was somewhat true of other models in decades past. It's usually not cost effective unless you find a cheap parts motor and can do the work yourself. Some of it is quite involved. And you have to do all of it or risk trashing the motor from doing it half-way and throwing the balance off. Considering outboards are often run at redline for long stretches, it doesn't take much to blow one.

dajohnson53 · May 9, 2005

Re: Converting a 9.9 to a 15?

Like he said, there's probably more to it than a simple carb swap. Do a search on this forum and you'll find many, many discussions on this same topic which detail the possible differences between the two and why the conversion is generally not considered cost effective. The confusion is because many manufacturer's build engines of different HP from the same basic block and lower unit but many other parts and/or specs are different, resulting in a complicated and expensive conversion. Good luck.

phatmanmike · May 9, 2005

Re: Converting a 9.9 to a 15?

put a 15hp hood on it, honestly, youll never know

CATransplant · May 9, 2005

Re: Converting a 9.9 to a 15?

"put a 15hp hood on it, honestly, youll never know" ------------------------ Yabbut, that spoils the fun of fooling the lake patrol, doncha know? Lotsa lakes hereabouts that ban motors 10hp or more. There are some guys who put a 9.9 cowling on their 15. The lake patrol guys are pretty good, though, in most cases. They see this 16' Lund V-bottom planing with that 9.9 on there....gotcha!

malibu4u · May 9, 2005

Re: Converting a 9.9 to a 15?

I had a 9.9 hp on my 12' aluminum boat and it got stolen. Insurance company replaced it with a 15 hp(cause that's all there was here at that time) and it was same year and everything(Johnson). I actually felt that the 9.9 was a better and also faster motor than the 15.

chris in va · May 9, 2005

Re: Converting a 9.9 to a 15?

I just did the carb swap on my 77 9.9. As I understand it, there was NO difference between the 9.9 and 15 from 77-81, just the carb throat diameter. Indeed I have both carbs and the 15 is much bigger. Now I don't know what a normal 9.9 is supposed to run at with a 14' jonboat, but mine hits 25mph with me and a little gear and a 10p prop.

dajohnson53 · May 9, 2005

Re: Converting a 9.9 to a 15?

chris in va: did you run the engine with the 9.9 carb? I'm curious about the change after you swapped. Thsnks.

herters · May 9, 2005

Re: Converting a 9.9 to a 15?

On 74-76 9.9-15 I have done the carb swap more than once. The larger throat 15 carb will give you more power/top end speed,BUT it will use a lot more fuel. The best way to spend your money is on a NEW prop. Hub slip and a worn down prop will rob you of a LOT.

chris in va · May 9, 2005

Re: Converting a 9.9 to a 15?

A lot more fuel? I went 6 hours (about 2 hours run time at WOT) with my 15hp carb and only used about 1.5 gallons. D, the 9.9 carb that came with the motor was somewhat in need of a rebuild, but I put the kit on the 15hp instead. Maybe one of these days I'll slap it on and see what happens. The guy at the boat rental place gave me an interesting look...

wayne h · May 10, 2005

Re: Converting a 9.9 to a 15?

the only differance is the carb do a search on the board this has been talkin about many times. i have pulled down a 9.9 and a 15 of the same year that was the only diffrance look up rings for both engines they are the same size so are the reeds

dolluper · May 10, 2005

Re: Converting a 9.9 to a 15?

Here's a good read on converting and other maintenance on 9.9 half way through it speaks on carbs[the choice will end up yours] http://www.pugetsoundanglers.org/Fishing Info/Johnson 9.9_15.htm

Chinewalker · May 10, 2005

Re: Converting a 9.9 to a 15?

On later (black block era) 9.9s and 15s, there are more differences than just the carb. They tweaked the exhaust tuner on the 15, among other things... The powerheads themselves remained pretty much the same, and the conversion can still be done, but it's more involved... - Scott

Converting a 9.9 to a 15?

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