1978 Johnson 9.9 vs 15 carb jets

[aaronmt]

I just recently picked up a very original and very clean 1978 Johnson 10r78m and id like to convert it to a 15hp.A friend believes the early motors like this one were simply just jetted different from the 15hp.I went through ever part including the tuner and every piece of the carb. on boats.net diagrams and all parts are exactly the same.The carb rebuild kit is the same as well and that confuses me.Certainly the main has to be different at least.Does anyone have a idea what it would be on the 15.Would carb kits be come with 2 mains and you match yours up or in my case switch? Thanks Aaron

[Fastjack]

Most carb kits do not include jets . Also look at the Spark advance specs. Perhaps factory OMC part #s by serial # will tell the tail

[jswain3]

Check out www.leeroysramblings.com

[kev29p]

The 9.9 carb is different. Much smaller throat.

[dwhitford]

The 'throats' (more correctly named the 'throttle bores') are the same, but the 15 carb's 'VENTURI' (the narrowest restriction of the carb's air flow) is a quarter-inch (40%) bigger, which is substantial.

[rmorris]

If you get a 15hp carb put it on an your motor will be a 15

[jeff55v]

Don't mess with the carburetor jetting. The carburetor jets alter the fuel to air ratio. This ratio is critical for lubrication of two cycle engines.
Try changing out the entire carburetor, as previously suggested.