Johnson 15Hp won't stay running

Pardon the ignorance, as I am new to boating and boat motors. I have recently acquired, what I am guessing, is an 80's vintage Johnson 15HP on a 14' boat. I believe the motor has very low hours, as you can virtually eat off the motor when the cover is removed. I went to winterize the thing last fall. The boat had not seen water for about two years. I flushed the fuel system and replaced it with fresh pre-mix (100:1, as advertised on the side of the motor). After launching and pressurizing the fuel system, the motor fired and ran great for about five minutes, then started sputtering and then died. I can get it running, but I gotta continually horse the choke and throttle. There appears to be no air filter to change, and new plugs made no improvement. I have heard through friends that there is a valve on the top of the fuel tank that must be opened after pressurizing. Is this true, or do I need to take this puppy in for a check up?

Re: Johnson 15Hp won't stay running

1st and most important, remove the sticker that say 100:1 fuel mix. there was a service bulleting years age, to use 50:1 fuel mix. too many motors blowing up, due to lack of oil. add the rest of the oil to your tank. if you have a metal johnson tank, it has a built in vent in the cap, no worry. plastic tank has a knob in the middle of the cap that, has to be open to vent the tank. there is an arrow on the fuel line, has to point to motor.

i believe you need to clean your carb. http://forums.iboats.com/showthread.php?t=244013

Re: Johnson 15Hp won't stay running

The tank is an older steel one, although I can't confirm it is a Johnson tank. I removed and blew the fuel lines out up to the carb. Didn't pull the carb off, though. I've rebuilt 2-stroke motorcycle carbs, so this should be a cinch. Thanks for the response and link to the carb rebuild. Yeah, I thought 100:1 was a little lean, but it was a factory sticker, so...

Re: Johnson 15Hp won't stay running

get the sticker off.