Could be a bad switch or the engine may not be grounded to the frame. Try connecting the ground wire directly to the engine. If that doesn't work and you have an ohm meter then measure continuity across the switch. It should be open (infinite resistance) normally and go to a short (zero resistance) when the button is pushed.
If you remove the cap from the top of the carb where the cable enters you'll find the needle. There is a metal disk on it that you can move up and down to adjust it. I think moving the disk down will increase the fuel flow.
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