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You have to be super careful about not gouging the cylinder head, but I would probably try separating the carb holder rubber from the cylinder head so that the penetrant can seep in. Heat from a heat gun or hair dryer might help. maybe cut the carb holder a fraction of an inch or so away from the cylinder head, then pry the rubber to get some separation. The carb holders are junk anyway, so you can destroy them if need be. just don't gouge the cylinder head where the carb holders mate. And you could cut away the rubber around the screw heads to use the wood block. or just use a good fitting screwdriver or screwdriver bit instead of the wood block. The fasteners get oxidized to the cylinder head & you want to break that bond, but don't go overboard. just tap tap tap using a mallet. spray the stuff. tap tap tap. give the stuff time to work. repeat.



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Using your Macgyver skills, a small cotton wad may be held in position against each screw head, and then keep the wads soaked with your favorite brand of penetrant. Which should help provide a more continuous uninterrupted time of penetration. Continue the wad-soaking process for a few days or longer. And hopefully, the penetrant will eventually seep down past the head and into the threads.

A pretty good diy penetrant is ATF mixed with acetone.

Good Fortune!