Lean/Rich mixture

Hello everyone. My motor is a 1988 Johnson 15 HP 2 cycle, rope pull starter. I am trying to bring it out of winter dormancy. I have fresh alcohol free gas, new fuel pump and tank hose, plugs, wires and coils. Hooked up with muffs the motor starts, runs for a few minutes, then starts to die. I push in the choke and it surges back up to a strong running condition, then starts to die again after a minute or 2. If I turn the slow speed idle adjustment knob counterclockwise all the way to the stop, it needs less choke to keep it running. The choke, and the adjustment knob are leading me to believe I need to adjust the slow speed needle valve screw out further. The manual says 3/4 of a turn, which is where I have it set. I live near Denver CO, at about 5800 feet above sea level. I fish on lakes at 8500 feet and higher. Could be I just need a richer mixture at this altitude. Any thoughts on where I should set that needle valve screw? I am thinking 1, or 1 and a quarter turns. Are there any other things I should look at? I was having this problem at the end of last season, but had to give up and put the motor away. Thanks in advance. The Waco Kid.

Sounds like the idle jet is plugged up. Take the carb apart and give it a thorough cleaning. Would be nice if you had access to a sonic cleaner.

As above. the symptoms you describe suggest that the carb needs cleaning

ITYWF that at high altitudes, because there is less oxygen, you need to weaken the mixture not enrich it

You may need a different main jet ........ especially at over 8000 ft

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(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
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