Given the 900HC, you are either dumping a bunch of fuel, have an intake leak after the meter, you have no/little combustion in a cyl., or your cat us shot.

I would check the valves, the spark plugs(see if you are running rich), and for a vacuum or exhaust leak.

You may also want to have the O2 sensors checked/replaced depending on age. The non OBDII sensors are only good for about 80k, and tend to get lazy w/age, your ecu could be getting a false lean condition and is trying to remedy the situation by dumping fuel. Any loss in fuel economy.?

I don't think any type of additive to the fuel will reduce those numbers.