What kind of gas should be used for a Honda vtx1800?

My VTX1800 backfires quite a bit on Regular 87 I bumped it up to 89 and it cut out most of the backfiring....but runs best on Premium

With a 9:1 compression ratio this engine should run fine on reg 87 octane fuel. An engine that backfires is doing so, not because you are using too low an octane of fuel, it is backfiring because the timeing is not correct or the plug wires are not connected correctly. They can also be a few other things wrong.