I have a Diablo flip chip and Powershot2000 (propane) system.

If I get on it hard with the chip set to performance setting and pane turned off I make a lot of black smoke. Repeat the above with pane turned on I make less smoke.

Propane workes by improving the efficiency of the burn. Since black smoke is mostly incompletely burned fuel it stands to reason that burning more of the fuel will decrease the amount of smoke.