I just ran into something like this on my racing kart. The pilot jet worked reverse of normal meaning the mixture screw is actually metering the amount of air. When you turn it all the way in, it is giving it the most amount of fuel which is what it is looking for. If that carb is in fact set up like that, this means the pilot jet is too small (turn screw all the way in and get the most amount of fuel which it likes and which explains why it runs so much better with it turned all the way in vs what you'd expect is it dying all the way in).
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