Most likely you're working with a pilot jet instead the older idle mixture. They are meant to screwed all the way in, lightly seated. These do clogged and must be carefully cleaned as they are a jet.

Now to the back fire and I know many used the term incorrectly. Is it out of the exhaust or the intake? Either one can a sheared flywheel. Out of the exhaust can happen excess unburned fuel is present as these engines uses the wasted spark system. IE: spark plug fires during the exhaust stroke.
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