Re: Johnson 9.9 to 15 hp conversion??
Hello. I have a 1975 Johnson 9.9hp. Many years ago when this series came out in 1974, I was at my local Johnson dealer who had 40 rental boats with these very engines on them, and a 15 on the tug they used to bring large boats on the moorings into the dock when customers wanted to use them. They told me they liked the 15 for the tug because it had more pulling power. They had a new 9.9 and 15 in the sales office, and they looked identical, even with the cowls off. I went to the mechanic who was OMC master certified, and had just gotten back from OMC school on these new engines. I asked what the differances were, and he told me it was only the carb. By 1977, the 9.9/15 got solid state ignition, and as mentioned the 15 had a different exhaust tube, oh, yes, the reed spacers too. In 1982, I bought a used 1975 9.9 and purchased a new 15 hp carb for it. The power increase was well worth the $78.00 carb price, and it soon broke the upper motor mounts, so I had to replace them! Over the years, I would go scrounge parts from the local bone yard at the marina, getting used 9.9 and 15 engines from them that had busted lower units from people hitting rocks. Anyway, I tried the different exhaust housings and tubes, and added the reed spacers, but it affected the low speed trolling and idling so I went back to everything original but the kept the 15 carb, it runs best this way, as designed.
Others here get very confused about this because for any given year, with the exception of the first three years (1974-76) there are more differances, but even so, as I found out, the carb makes the real change, the other stuff is not noticable by itself and really not worth the hassle. I was lucky to have free aceess to these parts and try them, but went back to stock. Much of the controversy arises from people who have not actually done the conversion, as they say, seeing is beleiving.
As pointed out above, it is quite interesting that after OMC made all these different changes to the 15 exhaust, at the end they went back to using the square exhaust tube on both models, just like they were originally, what's that tell you?
Study the engine specifications and you will see both engines are capable of reving much higher than the RPM specified for their rated horsepower, so they are more powerful than the BIA certified HP rating.
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