Boy, that a good one. the only way fuel can get to the booster is either through the jet or past the power valve. You said the gas was coming out of the sight glass. That has to mean th bowl is over filling so were are back to the fuel inlet. I know you said the float looks good but sometimes they will fool you. You say it is plastic. I asume you mean molded dense nitrophyl(sp) and not hollow. I have never seen a hollow plastic float but then I've never seen a lot of things.
If it is nitrophyl they do sometime get saturated and heavier than fuel and will force it out the vent tube and out the booster. If you are in doubt let the float air dry out for a while and try it again.
John