9.9 to 15 hp conversion trouble

I replaced my 1983 9.9 Johnson carb with a 15. It was supposed to be the only difference between the two models. Ran fine on the water hose so I took it to the lake. Initially it ran fine giving the expected hp boost. After about 10 minutes it died. I found I had forgot to open the vent cap on the gas tank cap. But even after that it would not stay running for any length of time, 1 to 2 minutes. If it helps, I would sometimes have to use the choke to restart it. I checked for gas line kinks (both hoses are new). I don't know much about how the motor works. Fuel is fresh and mixture was acurate. I really don't know where to start. Are there any additional modifications needed with this change that I wasn't told about such as timing, plugs, etc?
Thanks for any suggestions.

Re: 9.9 to 15 hp conversion trouble

Welcome to iboats.

Did you thoroughally clean the carb, including all orifices and passages before installing? Probably a dumb question but you never know.

Along with a larger carb, (you DID change the whole carb right, not just the main jet, because that will NOT work, that will only give it too much fuel, and not enough air to run properly at top end, causing it to flood out), there is also a shim, part #325038 to go between the leaf valve & the stop for the 15 hp(cheap, about 3 bucks). This allows the 15hp to breath with more ease. The final difference is the round tuned exhaust housing, which again allowed it to breath with more ease.

Your new carb assembly should be part #338273.

Re: 9.9 to 15 hp conversion trouble

You may have damaged the fuel pump by having the tank vent closed.

See if you can keep the engine running by the application of light steady pressure on the primer bulb. If so, you may have to rebuild or replace the fuel pump.

Re: 9.9 to 15 hp conversion trouble

Chris, thanks for the advice. I dont have a tank to run it in yet, only a water muff, but I will try that as soon as I can.