Two float bowl screw holes in an expensive (and not available as a separate part) carb main body I have were already helicoiled, but the person who did the repairs did a crappy job and now the inserts themselves have pulled out. I tried replacing the inserts, but one of the two new ones simply pulled out again, because there are simply not enough threads left over the length of the insert. So... anybody every run oversize float bowl screws in this situation?

It looks like my next size-up options are 6mm x 1, 1/4 x 20 or 1/4 x 28. I'd prefer to go w/ 6mm (smallest change) but the existing holes in the main body are already about .020" larger than the pre-tapping drill bit size and I'm concerned the result would be the tapped threads for the larger inserts might not have enough "meat" to engage properly with.

The concern w/ either of the the 1/4" options is that float bowl would need to be opened up "that much more" for proper clearance for the oversize bolts, and the metering block might also require clearancing. It would be nice to fix the issue w/o having to on more parts than necessary.