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15 hp Johnson with a 9.9 carb, any advise?


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I bought a 2000 15 hp Johnson short shaft, rope start outboard new from the dealer in 2000.

I just was having issues with it running (7/12) and in the process, my OMC certified mechanic (Not same place, but about 55 mile away) told me that it had a 9.9 carb on it. The only difference between the 9.9 & 15 is the carb and the reeds in the intake manifold. The exhaust and everything else is the same. The reeds were not changed or swapped from the factory as the paint was virgin and so were the gasket area, not touched/pulled apart.

He suspects that it was swapped out by an employee at the dealer before I bought it. He had a hard time believing it was from the factory.

I have always been the slowest 15 hp of the group, but never would I have thought to look at the carb part #.

Do you think I have any recourse with the dealer, being it has been 12 years? I think I am SOL! HELP WITH ADVISE, who would I talk to? Service Manager, Store Manager?

Any advise on how to approach them? I was thinking of going in person, with the original purchase receipt and motor.

The new carb would cost me about 3 bills to buy new, dealer cost is $175.48. If they won't help me out, I am planning on asking if they can sell it to me at cost. (Of course I won't start asking for cost).

I also was thinking, maybe, just maybe, they caught some employee in the past doing this exact thing or stealing and they would just take care of me, stating, "Yeah, sorry about that, we had a few like that come through, well swap it out for you".


There are more issues with it still and my mechanic is baffled, but I will address this later, unless someone here has some ideas? It idles down great, trolls great, but WOT is like 1/2 throttle or less.

We took off the carb, intake manifold, head, lower unit, tested the coil pack, power pack even removed the wires from the coil ,reversed them along with the plug wires and got same result.

Seems like the top cylinder is running heavy and the lower is not. If i reverse the plugs the lower cylinder cleans the one that was just heavy and darkens the one on the top. The pistons look good, and the skirts are intact. Wrist pins did not loose a snap ring or anything.

It only has one carb! How can this be? Intake manifold is clear. Gaskets are good. Looks like I will address this now, sorry. Hope this all makes sense, I could use the help from any backyard mechanics, or professionals.

Thanks for your time and advise, in advance,


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Some motors the difference between the 9.9 and 15 is only the carb.I have read some changed the reeds in later years.BUT the throat of the carbs are noticably different!! after all those years I'd say your out of luck pressing the seller.Thats to bad.I look often for the 15HP carbs on a common selling place.There HARD to get, they go quick and buying from OMC or even the lower prices parts suppliers is costly!

Do you know if that later model the lower hole pulses the fuel pump?? I am not up on late models,but the earlier ones had lower hole problems cause they operated the pump by pressure from the lower hole.Which made it foul out often and the top hole was just fine.

Personally I would have done alot more trouble shooting before I broke the whole motor down.

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Had to come back and say>>That gasket between the exhaust chamber and motor chamber,If its leaking will rob from the motors preformance.Making it run about 1/2 throttle when the throttle is wide open.Now you exposed it, it cant be tested so replace it with some good gasket sealer 3M Marine sealer.

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Sorry but after that length of time, I wouldn't expect much out of the selling dealer. I'd check around on the evilbay and the used outboard engine place in the cities for a replacement carb.

I'd replace the coil pack with a known good one, seen and heard of this type of problem many times - ignition components test good, but still runs bad fouls plugs, new coil often solves the issue.

Good luck!

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I plan on bringing it up, not expecting too much after all of this time. The mechanic that found the carb, knows the dealer, and will vouch that it is original and no one has tampered with it.


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Ask the certified mech what the specific changes were made in that year from 9.9 hp to 15 hp.

I say that because it depends on the year.

That ranges from using the same carb but without the spacer between the carb and intake to reduce air. So while it uses the same carb body without reducer the main jet is larger and or the venturi is larger.

A tuned exhaust tube or same exhaust tube but different carb.

The condition you mention along with results from the swap is a strange one. Is there any chance the plug wire is being grounded out when throttle is applied from movement of the mechanical advance?

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