JOHNSON 9.9 TO 15 W/CARB SWAP

[BRIANLODGE]

I was told I can upgrade my 9.9 johnson to the 15 hp Johnson by switching the carb. Is this ethical? What's all in involved? I basically see 2 nuts on the carb studs, throttle cable and air intake. Looks simple enough. To gain 50% on HP for a couple hundred seems reasonable. Any advice or assistance would be greatly appreciated. FYI- the bore and stroke of the 9.9 and 15 and all other specs are exactly the same. Just different cabs.

 

[wilde1j]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

I'll bet there's more. Are all of the exhaust components the same, the exhasut/intake port timing the same, are the crankcases for the different HP's the same P/N? What you may get is just more fuel consumption with no appreciable power increase.

 

[Tacklewasher]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

Depends on the year of the motor. Later model 15's had a different exhaust as well as the carb change. Doing the carb would get you to around 15hp at the flywheel, while the exhaust change got you 15 at the prop (from what I understand).<br /><br />I've done the switch myself and have gone back to the 9.9 carb because the 15 wouldn't idle right. It needs a rebuild and it was easier to put my old carb back on for now. But at WOT it made a big difference. With my father-in-law in the boat, it was the difference between planing and not planing. <br /><br />As to the ethics, I guess you should register your boat if you have a 15hp on it. At least here, anything 10 and over needs registration.<br /><br />The hardest part of the swap is removing the starboard side carb nut. I haven't figured out how to do it without removing the pull starter and I can never get it back on right. I have an unsprung spring at home right now from my last attempt.<br /><br />I really should get my 9.9 working as a kicker for my bigger boat though. I've been using my electric but I hate having to cart the batteries around for it.

 

[BRIANLODGE]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

The boat is registered. I meant is it ethical to run 9.9 decals and upgrade it to 15 hp. What if I go on a 10hp limit lake?? Just keep the throttle back I guess or not???

 

[umblecumbuz]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

Hi ALWAYS...,<br /><br />If you're on a restricted hp because of lake regulations, of course it's not ethical. It's illegal. If there's no restrictions where you boat, then there's no legal problem.<br /><br />Check your insurance cover though. <br /><br />There's more to getting 15 hp out of a 9.9 than swapping the carb. Probably need to fit new exhaust tube, maybe reeds (some 15's had different reeds with a spacer). Maybe even need to play with carb jetting to get it to run smooth. The extra power came from increased revs, so the whole exercise rests on increasing the upper rev limit. A prop swap might also be needed to get the best out of the new setup.<br /><br />Easier route might be to sell yours and buy a 15.

 

[BRIANLODGE]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

Still considering a yamaha high thrust motor. That's if I get a fair trade from the marina. I paid about $2100 and the Yamaha is about $2500. I cant get them to put a price in writting. The mechanic said to me on the side don't be surprised if they only offer $1000. on trade. So a trade is probably out and trying to sell it will be a pain. I'm just trying to get a little more out of it. Other identical boats pass me on the water???? I mean exactly the same.size, style, hp and bought from the same marina???????

 

[OBJ]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

You can get more out sometimes just by how the engine is set up on the transom. Tank a look at the anti-cav plate (that fin right over the prop) and see where it is in reference to the keel of the boat. If it's below the keel, raise the engine so the plate is 1" above the keel. You should also bolt the engine on if you do this. This and maybe a 9.25X8 prop will get you what you want. It will make a difference in performance.

 

[CATransplant]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

This is the 9.9 on the 20' pontoon, right? You'd get a little more performance from a 15, but your lake limits you to under 10 hp.<br /><br />Don't waste your money trying to convert your 9.9. Yours is a newish outboard and the conversion won't work.<br /><br />Concentrate on getting a prop that won't lug that engine down. You were on the right track with that 7-pitch prop. It will probably let you get closer to the ideal revs on that little tiddler.<br /><br />Moving the outboard up or down won't make much of a difference on a pontoon boat, since it's running in unobstructed water anyhow. What little drag it will have from being a little deeper in the water won't make enough difference to measure.<br /><br />Trim angle, on the other hand, might make some difference.<br /><br />Still, with just 10 hp or so, you're going to be moving around slowly with a 20' pontoon boat, so the trick is to get that little engine running at its best RPM to take some of the load off it.<br /><br />Re-propping will help. But, when you post a question here, you should included all the relevant facts. The fact that you're using this on a 20' pontoon boat is the main fact people need to know. <br /><br />Go to the place that sold you the motor and get a 7"-pitch prop for your 9.9. Ask them if you can try it to see how it works out and ask them if they'll give you a break on the price if you give them your almost-new 9-pitch prop. They can sell that to someone with a little aluminum boat.<br /><br />Your RPMs will go up, the little engine will purr along happily, but you'll still be pushing slowly along. There's nothing much you can to about that on a 20' 'toon with a 10 hp motor limit.<br /><br />Best of luck!

 

[OBJ]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

Being on a toon does make a difference as CA said. There is a four blade "high thrust" prop for your 9.9 availabe....8* pitch. But do get the engine up as far as you can without losing water flow to the pump.

 

[BRIANLODGE]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

Where can I find the 4 blade high thrust prop. I've found several 4 blade to fit but they are like 11-13"?

 

[CATransplant]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

That prop is available from your dealer. But try to get it to try out before committing to the purchase.

 

[umblecumbuz]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

from CA:<br />Concentrate on getting a prop that won't lug that engine down. You were on the right track with that 7-pitch prop. It will probably let you get closer to the ideal revs on that little tiddler.<br /><br />Moving the outboard up or down won't make much of a difference on a pontoon boat, since it's running in unobstructed water anyhow. What little drag it will have from being a little deeper in the water won't make enough difference to measure.

Sometimes when giving advice we forget the obvious. What CA says is right on the nail. Thank goodness I didn't suggest trim tabs!

 

[locojoe]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

hi...I have a 76 johnson 9.9 (model 10r76h) and id like to get some more jame out of it doing the 9.9 - 15 carb swap. a couple of questions if anyone can help.. Im told i need to change the reeds to accomplish the swap(by the dealer) but do the cylinders have to come off to get at the reeds and is this swap possible with my model engine.. Im after screwing up my carb by makeing the main jet bigger now it idles great and trolls but thats about it ( i kinda knew I was gonna mess it up but couldnt help myself..I also welded my recoil spring to its housing so it will never esape again!!


Any help on this would be great I gotta get back on the water!!!

Now i think i will fill the jet with solder and bore the jet back to its original size .040 any idea what size drill bit that might me or if I shuold go alittle smaller and try to lean it out?

 

[ezeke]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

If you want more information, the best on-line source is at the following link, just scroll to the bottom of the page and you will find a storehouse regarding the 9.9 and 15 horsepower motors.

http://www.wisnersinc.com/Fishing%20Information/Fishinginfo_main.html

If you buy the carburetor with the larger barrel, you can increase the horsepower. Some 15hp models had a totally different flywheel and exhaust.

Some had different reeds, but I don't think that would make much difference.

Edit: In the above referenced link, go to the normal maintenance section and then do a search for "conversion". The point of reference is that he says :

"The larger carburetor will gain the most in increased RPM, (about 1000 RPM more, according to OMC) which in this case also relates to more horsepower. If you insist on squeezing that last ounce of energy, then maybe the aftermarket reed valves, polishing & deburring the intake manifold & replacing the exhaust housing will be of some benefit. But, if you are trying to push it this far, at wide open throttle for an extended period of time, my thoughts are that then the rest of the engine may not stand up for long, so go to a larger motor."

That is consistent with the information in my 1980 OMC Service Manual, which covers both 9.9 and 15 HP, and lists the full throttle operating range of the 9.9HP at 4,500-5,500 compared to the 15HP at 5,500-6,500. [see the "Specifications" page 2-2]

 

[reload]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

Also I think the manifold that the carb bolts to has a larger throat.

 

[mikesea]

Re: JOHNSON 9.9 TO 15 W/CARB SWAP

simply,there IS more than just the carb.Ive looked into it,after doing the carb only swap with no noticable differance on a 12 ft.alum boat,not worth the trouble