9.9 Johnson

[dontblinkmax]

Hello, I have a recently purchased 1987 9.9 Johnson. I am curious as to whether what I was told is correct. The difference between the 9.9 and 15 horse is strictly the rpm's. How true is this? If so, is there a way to raise the rpm's a bit to have more horsepower? Thanks!

 

[JB]

Re: 9.9 Johnson

Not true, Max.

 

[edzzed]

Re: 9.9 Johnson

i have heard things like getting a different throttle linkage will give more open throttle. truth is i would say the carb has to flow more air to produce more power. kinda like taking an old 327 vette engine and swapping a two barrel carb for a four barrel carb. more air = more fuel = more power. on my 9.9 yamaha they tell me a new carb as well as change the exhaust system too. my diesel dually pick up had a stock exhaust pipe at 2 3/4 inch and i swapped it for a 4 inch. wow, what a feel it in the seat of your pants thing. guess opening up the exhaust allows for more air to flow. Ed

 

[JB]

Re: 9.9 Johnson

Think about it, Max. You are talking about a 50% increase in peak HP. It's gonna take a lot of expensive changes to do that.

 

[CATransplant]

Re: 9.9 Johnson

If you really want a 15hp Johnson, put up an ad on craigslist offering to trade your 9.9 for an equivalent 15. With a lot of small lakes in many states having an under 10 hp limit, lots of folks will make that trade, even up.

It's worth thinking about. Of course, if what you had in mind is having a 15hp with a 9.9 cowling, that's another story. It won't work, since the Fish and Game folks are quite familiar with that ruse, and know how fast a 9.9 will push a small boat. Go faster, and they'll come have a conversation with you. The fine could be enough to buy a fixer-upper outboard.

A guy I know recently got caught doing just that. He's not a happy camper.

 

[Robj]

Re: 9.9 Johnson

I have a 1994 Merc 30 hp. After studying the service manual which was for a 30/40 hp Merc, I figured out that all I had to do to make my 30 into a 40 was change the carb for a bigger one and remove the reed stops, that prevented them from fully opening. When I was calling dealers looking for a larger carb, they laughed at me and said that it could not be done. I found a carb and made the changes, it works great. Even water skied behind it, and I weigh 200 lbs. Top speed on gps at 3000 feet went from 38 to 45 km/hr. So to think that a 9.9 can be made into a 15 does not sound that unreasonable. I just think that you need to do more research, such as checking part numbers to determine what the differences are and if it can be done.

The rpm thing is simply untrue. Max rpm's for an outboard are in the 4500 to 5500 rpm range, regardless of the hp. Max rpm for any motor is a function of hp and the right prop so at full load it is within the desired range. If it falls under the range, then you need a smaller prop, if rpm are too high, then you need a larger prop. If you can change your 9.9 into a 15, then you will need a larger prop to take advantage of the increased power.

Good Luck

Rob.