Carb Jets, and Increasing Horsepower

[MBIEGANEK]

I have a 1981 Evinrude Outboard on a 18foot triplehulled fiberglass boat (sorry I don't know the model). I can achieve about 30mph at WOT and I am interested in increasing my HP so that I can change to a 21 pitch prop, (currently running a 18 pitch). (Although I have to admitt I have not yet put a tach on the engine to verify RPM's.) <br /><br />I have also purchased the rebuild manual for the engine, and have spent many hours reading / online searching out part schematics / diagrams searching for the differences between the 90hp, 115hp and 140hp models. After asking a mechanic, and reading many of the posts on this board I have recieved the standard, "They have different exhaust systems / Carburators ect", but in my research I have been unable to locate a physicall part difference between them. The only differences I have been able to find to date is that they are jetted differently. This is not true for all the models, but specifically the 1981 model. (It and 1985 appeared to be special years for similiarities amoung the 3 different hp ratings. Does anyone know if these differences actually exist between the 3 models of 1981, and if their are differences can a person just order in the different parts and replace them as I am certain that the Powerhead Block / Drivetrain is identical between the 3 HP ratings.

 

[Dhadley]

Re: Carb Jets, and Increasing Horsepower

The port timing is different on the 90. The 115 has the same port timing as the 140 but the 140 has a "tuned" exhaust manifold. <br /><br />All three carbs will interchange. There are differences in throat sizes. The 140 heads are slightly tighter. <br /><br />Without a tach you are lost. Once you have a tach you can establish a baseline. Once you know where youre at you will know where youre going.<br /><br />Good luck!

 

[MBIEGANEK]

Re: Carb Jets, and Increasing Horsepower

Thank you for the info,,,as luck would have it I found an excellent link for the "dealer" exploded views of engines at www.boaterssupply.com a few hours after putting up this post. In comparing the different motors, I found that the 90 and 115 were almost Identicall with the 140 having several major differences, including the ones that you have mentioned above. (The most prohibative to me being different heads for the 140hp). The one difference between the 90hp and 115hp that I am unable to resolve is a spacer inside the Carb Body that is only found on the 115hp and 140hp. Should I replace the other parts of the Carb to bring it up to 115hp spec, what effect will having / not having that spacer have. (Under the Listing I found, the Spacer is not sold independantly). <br />Thanks<br />Michael

 

[Dhadley]

Re: Carb Jets, and Increasing Horsepower

The carb throat size is different. Putting on a larger venturi carb with the 90 hp port timing will do nothing except hurt the holeshot and probably burn more fuel.<br /><br />I cant place what spacer you are talking about in the carb body. Even though the carbs are different they use the same kits. <br /><br />If you are talking about the filler blocks in the intake, dont worry about them. In theory they take up dead air space but they dont come into play until after a lot higher rpm than you are (most likly) turning now.<br /><br />Until you do the port work you will not gain anything as far as horsepower. However, once you get the tach installed we can work on your set up. <br /><br />Keep us informed!