The ST had crappy MPG, I can only imagine that a R100ST would be worse.
And I also agree with what others have suggested.
Before you toss in the towel, make sure everything else is as good as it's going to get.
Also look for airleaks around the carbs. Make sure the boots and other intake bits are all sealing up how they should be
And don't overlook the ignition system, check your plugs, HT leads, coils, trigger (Bean can) and all the connections between them.
On vacation for a spell