Re: Looking for 15hp Johnson carb for my 9.9
Something that escapes most people who have trouble with the 9.9 being able to produce 15 horse power is revealed by very close scrutiny of the manufactures specifications.
It is impossible to design an engine to deliver exactly 9.9 hp, so what they do is rev the engine until it produces 9.9hp and take that RPM and publish it as the rated output. Actually the engine can rev above this point and produce significantly more than the 9.9 rated hp.
If you look at the rated HP RPM of the 9.9, it is far lower than the maximum full throttle RPM, and if you extrapolate that you will find that at it's full throttle RPM rating, the 9.9 is putting out about 12 HP. The 15 puts out about 18 HP at it's full throttle rating. This will vary a little because all engines are not exactly equall due to manufactuirng tolerances.
My 1975 9.9, converted over with a 15 carby revs out to 7460 RPM on a 9-1/4 x 8 prop. That is almost 500 rpm over the rated maximum, and it loves it.
The above information I want to thank John Gill of Holloways Marina for, as he explained all of this to me back in 1978 when I first began inquiring about these engines for my 12 foot fishing boat. At that time, John had recently completed his OMC Master Certification on this then new series of engines and had all the inside scoop on them from the OMC school he attended back east. Seeing as Holloways had a fleet of over 30 rental boats with these engines, he was a specialist and expert on them, and still is for that matter. Most, if not all of my knowledge on the 9.9/15 series comes from an intimate association with John, as every year I visit him on the weekends at the local marina and see all these engines taken apart, or being serviced. He loves to show me all the weird problems he has found and discuss any issues I have with my 9.9. I have seen virtually every model of the 9.9 made in pieces at his shop over the last 36 years, and this has been a very unique and special oportunity for me, plus the bone yard out back is a treasure trove if you have a 9.9/15.
Incidentally, there are a couple different conbustion chamber variations, one has a weird wedge shape near the plug to help stop fouling at trolling speeds, intersting huh? Synching the plugs also helps, try it: Face the end of the negative electrode of the plug towards the exhaust port. Use BR7-HS plugs, they work great.
BTW, Chris's outline above of the differances is accurate to a T and I have seen each of these variations first hand. Most perplexing is that eventually OMC went back to the old square exhaust tube on both the 9.9 and 15, as obviously it was not that critical.
More revealing is what the APBA guys have found when using these engines (15ss) for hydros and tunnels in Class J (junior) racing. All they do is add a restrictor plate between the carb and manifold to keep these engines under 10 HP, which tells you they put out more than 10 HP in stock form.
Most air cooled 2-stroke cycle engines of 200cc displacement (the 9.9/15 are 216cc) easily put out 25 HP, and liquid cooled 125s can easily approach 35 HP, so it becomes apparent that the 9.9/15 is quite restricted and capable of allot more power. For goodness sake, my old 1970 McCulloch 101 (125cc) puts out 18 HP on gasoline, and it's a single cylinder air cooled engine. Thus there is no question about the power potential, it's there. The reason the ratings are so low is for longevity, a restricted carb allows full throttle operation all day, without over stressing the engine. All outboards are always down on hp compared to auto and car engines of the same dissplacment for longevity reasons.
.
