That is the problem. The bike uses an old heater element style flasher . When the bulbs come on, they heat up the element. When it's hot enough, it flicks over to the side which breaks the circuit (turns the bulbs off) and allows it to cool enough to flick back over to it's original position. Then the cycle repeats again. With lower wattage bulbs, you don't create enough heat to heat the element properly which can cause a slow flash or no flash at all. If the wattage is highe, it will cause the flasher to flash faster than the desired rate.
You've got two options.. replace the bulbs with bulbs of the same wattage (assuming the LP signals can take the add'tl wattage witouth melting or (better option IMO) replace the flasher with an electronic flasher (should be available at any auto parts store as a direct replacement for around $10). Additionally, an electronic flasher will allow you to use LEDs instead of bulbs in your turn signals since they rely on eletronic timing (instead of heating) to flash the signals.
BTW ..the flasher is located on the left side of the bike, mounted to the frame with a rubber mount, behind the side plastic near the rear seat. It has two wires going to it.