9.9 Johnson to 15 hp with Carb jets?

[kalloway]

Someone mentioned that I could increase the hp in my 1978 Johnson from 9.9 to 15 hp by just changing the carburetor jets. Is this correct? Does anyone have any experience doing this?

 

[Fitch]

Re: 9.9 Johnson to 15 hp with Carb jets?

In most cases you have to put a larger carb on and sometimes a different exhaust tube.

 

[charliesused@cs.com]

Re: 9.9 Johnson to 15 hp with Carb jets?

It takes a change of the middle jet, the only jet to <br />the larger and the reed intakes behind the carb. As <br />far as the exhaust ive heard many mechanics say dont even bother. im talking of the tube connected to the powerhead. If you are that desperate to change it wont matter. The jet in the top carb and the reeds behind the carb.

 

[the machinist]

Re: 9.9 Johnson to 15 hp with Carb jets?

On that motor you need to change the carb. If you just put a larger mainjet in you get more gas but not enough air for proper combustion & the engine will blubber as if you are actually choking it. It will run OK at an idle, but you will have less power at high speed than before you changed the jet.<br /><br />Go to the article I wrote for Puget Sound Anglers for a detailed expanation of how to work on that series of motors.<br /><br /> http://www.pugetsoundanglers.org/Fishing Info/Johnson 9.9_15.htm

 

[Boatist]

Re: 9.9 Johnson to 15 hp with Carb jets?

Kalloway<br />Machinist is 100 percect right. Putting in bigger jets will get more fuel in the cylinder and the engine will flood at hich speed and burn more fuel at all speeds. You have to get the air fuel mixture right to get more power. That means a bigger carb throut for more air.<br /><br />If you just want more fuel in cylinder then just pull out the choke will do same thing as changing to bigger jets.

 

[charliesused@cs.com]

Re: 9.9 Johnson to 15 hp with Carb jets?

Gentlemen, what about the different reeds behind the carb. Look at the parts layout.

 

[Tacklewasher]

Re: 9.9 Johnson to 15 hp with Carb jets?

Having just swapped the carbs, but not testing it yet, I will add that the throat of the carbs are very different in size. The 15 is quite a it bigger in diameter and new jets on a 9.9 carb would not be enough.<br /><br />Now to see how it runs.