Welcome! Need support, you got it. Or share you ideas and experiences.
Anyone have info for this, aside from the carburetor change? This is for a 1990 model. It was 11 years ago, but when I tried to do the "common" conversion via carb swap only, it didn't work. Results were: raw fuel out the exhaust, no significant power increase. After contemplating this for several years, I have come to the conclusions: The exhaust tube for the 15hp is different than the 10hp, the exhaust ports in the 15(exhaust housing casting) may be different than the 10, the timing advance cam may be different. There is absolutely no info regarding this in the service manual or parts list except: I saw a different exhaust tube in a photo of something unrelated in the service manual. Johnson went to the trouble of being sure the necessary info was not available in their documents. I have not found a dealer who was helpful in any way. Does anyone have a genuine 1990 15hp for sale so I can get the necessary parts, or any info for this to satisfy my curiosity?
In modifying the 9.9 OMC so far...The carb change/Boysen reeds/the exhaust tube. It would be cheaper and easier to find a 15 horse and slap your hood and engine number plate. If you're cheating with your buddies...fine. If your fooling the Ranger...that's a different story! I am looking for a EKT or JKT 11 engine. That was the Evinrude and Johnson "A" class racer with the "quickie"-style lower unit. These engines were modified by Second Effort, OMC's speed partner.
Well your right on more than one count! The Johnson/Evinrude 10/15hp are the same except for carburetor, slight port/timing changes and the majority of the boost to 15hp coming from the tuned exhaust system on the last 10% of the throttle twist. Johnson built--up to a few years ago--a mini hydro ‘racing’ version of the 15hp motor. Aside from the radical lower unit and prop it seemed to be mostly a highly tuned exhaust system that gave this motor it’s 'zing'.
Sorry, don’t have any info. on converting the 10hp to the 15hp though…
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