Billbo,

Since octane rating is merely the anti-knock index of a motor fuel, using more expensive gas won't help. In fact, since 93 burns at a lower temperature than 87 does, using 93 may create MORE carbon deposits. This is because of the cooler--and therefore less complete-- combustion.

It is, however, possible that the carbon fouling is due to low QUALITY, not low OCTANE gas. More expensive gas probably has more detergents. Plus, cheap gas might have impurities that don't burn cleanly. You might want to try a bottle of fuel system cleaner.

In short, all other things being equal, higher octane fuel ALONE will probably worsen your deposits and decrease overall performance.