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I've taken a small sharp chisel to the head and using that turned the screw enough to loosen it up. You can also get a little more spray in that way and a pair of vice grips might be able to get it then. The cutting of a slot like you say might do it to but it would help to free it up a little first.

Vice grips are a common tool that most hardware stores should have. I've successfully used them several times to break loose carb boot screws like shown. If you have to take a hammer and screwdriver and spin the boot out of the way to you can get a better angle at the screw. You should be changing those boots regardless do don't worry about possibly damaging them.

Yup Ive used vise grips too . Its my go to tool for screws like that if you can get them in there . Almost every carb i touch have to vise grip at least one screw . Try and get the real ones made by vise grip if you can . Most aftermarket ones are junk .

Don't forget heat.

Good Ridin'
slmjim & Z1BEBE

Vise grips did it. Finally!!

Thanks for everyone's advice and help on this one!

Special F**K off to the rusty ass screw who gave me a headache all week.