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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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With unleaded fuel, you have to run the engine for about an hour to get a good plug reading and then you want to run it at the speed you want to take the reading at.
The screw on the side is just to adjust the idle speed - the fuel screw is the one on the bottom parallel to the float bowl - if the carb has never been worked on before, the fuel screw may be capped preventing you from seeing it and adjusting it.
The slide needle adjusts the fuel mixture between 1/8 and 3/4 throttle openings - it has nothing to do with idle - if it runs clean at partial throttle openings raised one notch, you can leave it there.
The only thing that affects the fuel mixture at idle is the pilot jet and the fuel screw - the fuel screw should be adjusted when the engine is at full operating temp - run for 15 minutes or more.
The only thing that enriches the mixture for cold starts is the choke - if the choke butterfly is closing, then only the float level, quality of fuel (age, octane and blend), amount of compression pressure and spark affect starting. When then choke butterfly is closed an abnormally high vacuum pulls on the fuel in the float bowl and sucks more than normal through all the jets - the rich mixture ignites more easily, but produces a cold burn - the engine starts, but is low on power until it gets hot enough to run without choke.