Johnson 9.9 HP modifications

[rjjmsj]

I was talking to a place that trades outboard engines about the possibility of swapping my older Johnson 9.9HP for a 15HP outboard. While they didn't have any to swap, he mentioned that there is a carburator swap that can be done that will give the same power as a 15 HP. That sounded very promising, but he was not able to provide any further details....can anyone reading this tell me more?<br /><br />Rob

 

[kev_79]

Re: Johnson 9.9 HP modifications

this happens a lot. When you hear people referring to a motor being detuned. I happens even with the big engines. Some powerheads, ex: merc 100, 115, 125 are exaclty the same, except for exhaust restrictions, and carbs, or, either. Some boats are only rated for certain hp, and some places won't let you in with a big motor. The manufactures get around making a jillion differnt block, by detuning certain ones. Also makes most of the part interchangable, saving costs. From what I have heard, most 9.9's are detuned 15's, so a little bit bigger carb will let it breath out more hp. Hope this helps you better understand, and good luck.

 

[JB]

Re: Johnson 9.9 HP modifications

The carbs will swap, RJ, but the rest is myth. Wait until there is a 15 available and save yourself some bux and grief.<br /><br />Red sky at night. . .<br />JB

 

[rjjmsj]

Re: Johnson 9.9 HP modifications

Well, it does help...sorta. At a minimum, it reinforces that the guy wasn't pulling my leg...but what I'd like to know, if anyone knows, is where to turn to find out how to actually either get the new carb, or get the work done. I am not a big boater...I've got a 12ft Olympian (glorified rowboat) that could benefit from some extra get-up-and-go, and this would be alot cheaper than either selling the 9.9 and buying a 15HP (max. rated size for this boat), or buying a 15HP outright. The engine runs well and if I can do this, it would be great, but I really don't know where to ask for help...I bought the engine second-hand, and don't have a manual or even know the year.<br /><br />Rob

 

[rjjmsj]

Re: Johnson 9.9 HP modifications

JB, is your comment about the rest being myth referring to the comments by Kev_79? What he had to say certainly makes sense. In the automotive world, better breathing certainly creates more HP (put an engine on a dyno, and the hp is a function of many things, but it is NOT limited just because of the displacement), so why would you say that better breating (ie. better breathing carb), could not produce more HP?

 

[clanton]

Re: Johnson 9.9 HP modifications

Along with the carb is the exhaust tuner and sometime the port timing, and timing curve. Have you priced these parts.

 

[JB]

Re: Johnson 9.9 HP modifications

My comments were not directed at Kev's post. He posted while I was writing. I was responding to the myth that by changing the carb RJ can get 15hp. He has to change around $1500 in parts to get 15hp, and he can swap for a used 15 for a darn sight less than that. <br /><br />Of course a bigger venturi will breathe better. Higher domed pistons will give higher compression ratio, raising the ports will allow it to go to higher rpm on the torque curve, bigger (and more) reeds will further improve the breathing, etc., etc.<br /><br />The return is not worth it. Changing one component in the chain is like putting one big link in a small chain. . . you've still got a small chain.<br /><br />Red sky at night. . .<br />JB