Question for Joe

" Joe, I have a 91 model 25 hp

" Joe, I have a 91 model 25 hp johnson... My aluminum bass boat is rated for 35 hp... Is there anything relatively simple I can do to increase my horsepower?... I always heard that you could convert a 9.9 hp into a 15 hp by changing the carburetor... Is there anything similar that would work on my 25 hp?? Thanks for your advice.. "

"T.McCoy.... The 9.9hp to 15hp

"T.McCoy.... The 9.9hp to 15hp easy conversion has been an exception, at least to my knowledge. I haven't heard of any way to upgrade the 25hp though. That's not to say it can't be done, just that I'm not aware of it.

Joe (30+ Years With OMC)"

" Changing the horse power oft

" Changing the horse power often involves other things. On the mercury motors the old 7-1/5 hp had smaller transfer ports than the 9.8hp. ON OMC motors the difference between a 90 hp and a 115 /140 is Carburetor bore size , Diameter of intake ports and exhaust tuning.It is not often that changing the carburetor results in an instant gain. "

" So it's easy to change a

" So it's easy to change an OMC 9.9 to a 15? What do you have to do? Just cahnge the carburetor, or is there more to it? Also, would this shorten the motor's lifespan? I've heard that it would on many conversions/rebuilds because you're turning more rpms. Thanks. "

"JdKline..... Think about it.

"JdKline..... Think about it. Straight from the factory, the only difference between a 9.9hp and a 15hp is the carburetor. It doesn't make any difference who installs the carburetor. There is nothing else to change and no damage will ocurr.

Joe "

The exhaust stack is differen

The exhaust stack is different on the 15hp (later models ) it allowed motor to breathe better and build up the extra revs to put out more power.

"JdKline.... Yes, that notice

"JdKline.... Yes, that notice by Sparky got by me. I've been doing some work on three of the older 9.9hp and 15hp engine in the past week, and apparently my mind was wandering in those circles. My apologies for any misunderstanding, and a correction follows.

When the 9.9hp and 15hp came into being in 1974 and up to the date of 1980, the only difference between the two was the carburetor. Starting in 1981, there was a change made in the inner exhaust. I'll install a 1981 or later 15hp carb on a 1981 or later 9.9hp when I get the chance just to see what effect it has. If anyone has already done this, I'd be interested in the results.

Joe"

" Joe, You say you have parts

" Joe, You say you have parts for some of these OMC motors. I wonder if you have impellers for the 9.9/15. That's one I need at least every year. Do you have it, and what would you charge me for it? Thart conversion sounds like more than what I want to mess with. I'm pretty happy with what I get out of that 9.9. It's a '90, so what Sparky said would definitely apply. I've gotten it up to around 30 mph in my jonboat(with 1 person/light load) and cruise at 14 with 3 people, so I'm happy with it. One more question, the manufacturers' suggested rpm range on it is 5500-6500 @ WOT. What part of that range should I be hitting? Anywhere in there, or the high/low end? I've heard that the load the boat is under has something to do with that. Thanks for your time. "

Error Encountered Here. Post I

Error Encountered Here. Post Is Listed Below.

"Jdkline.... Sorry (I chec

"Jdkline.... Sorry (I checked) I am out of the impellers and water pump kits for those 9.9 & 15 hp models. The 5500/6500 WOT range you mentioned, what they're saying is that you should hit at least 5500 but do not exceed 6500c. If you were running with a tach, you would want the WOT reading to register about 6300/6400 on the tach for a perfect Boat/Engine/Prop setup.

Joe"

" I am doing a 9.9 to 15 conv.

" I am doing a 9.9 to 15 conv. I have read alot about this some say that the heads are dif. but I can not find that they are. My 15 carb. has a 58 main jet, but my local dealer only showed a 54 & 56 main jet listed & a 9.9 having a 38 main which is what is in my 9.9 carb. what is the correct jet for the 15? The motor is an '84 9.9 let you know how the later 9.9 does with 15 carb. "