Re: 9.9 Evinrude / Carb upgrade
from the site:When the 9.9hp and 15hp came into being in 1974 and up thru 1980, the only difference between the two was the carburetor. In 1981 there was a change made in the inner exhaust housing for the 15 hp OMCs,. This was a sort of tuned exhaust, allowing the engine to breathe better, helping to up the RPM, which upped the HP, in addition to the different carburetor. The 9.9 inner exhaust housing stayed the same as it was.<br /><br /><br />The carburetor is different in that the inside diameter of the throat is larger on the 15 hp. This larger throat allows more air to enter the engine. The main jet is also larger allowing more fuel to be sucked in. The combination of these 2 factors allow the motor to gain up to another 1000 RPM, increasing the horse power. Do not think that you can up the horsepower by simply modifying a 9.9 carburetor by replacing the main jet with a 15hp main jet. All you accomplish is that the engine is being fed to much fuel & not enough air to properly mix with it & the engine will then blubber (floods out) at the top end. It can run OK on the lower end & idle OK, but it will have less top end power than with the original 9.9 main-jet. Believe me, as I have been there/done that. Matter of fact that is what got me into writing this article, as I could not readily find any information on the subject.<br /><br /> <br /><br />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<br /><br /> <br /><br />The outside visual dimensions of both carburetors appear the same, unless you look inside the throat. The 9.9hp carburetors internal throat is .625 dia., 15hp internal throat is .875 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 1987.
