Before you bend the tab on a plastic float, be aware they can and will break... If you use a heating device, such as a screwdriver(chisel, nail, ???) stuck into a hot flame, you may have better luck modifying the float level without breaking the tab. The heat should be applied to make the plastic pliable, and then you can move it, hold it in its new place, let it cool, and it will likely stay where you put it.
Be aware that most times those floats are so precise they do not need any adjustment, and doing an adjustment is not a 'factory approved' solution. Generally, if a plastic float is 'off', something else is worn, such as the needle, seat, float pivot pin, or the 'hinge' section of the float itself. If you had water in the gas, and it was left in the float bowl over winter, and froze, I suppose it could 'modify' the float setting, but more liklely it would have cracked the plastic and you'd have a 'sink' instead of 'float'.
When you put the bowl on the carb, it may be that the float is getting pushed upward by the float bowl, or a ridge/bump in the float bowl, and closing the needle valve. Generally, you should not have to touch the float to make it work. You may have gotten the wrong float for your carb. Is it the same as the original you are replacing? Is it possible that the needle retaining 'spring'(springy metal) is out of position?
When you hold the carb inverted so the float weight is closing the needle to the seat, blocking airflow, does the float body(roundy tubular part) appear to be roughly parallel to the gasket surface where the float bowl would seal? The float should be roughly horizontal when the carb is inverted. If it is too high, as in the float is riding at an angle, high on the end away from the pivot, I would inspect closely the needle fitting, the retainer, hinge, etc, and also compare those bits to the old one. There should be very little variance, almost to the point of being so minor that it cannot be measured, between old and new when installed in place.
Be careful if you do some melty work, as for sure you are making the parts non-returnable if you modify them.
Some have used a Bic type butane lighter to heat up a metal implement to soften the float tab material, FWIW.
tom