Try the original flasher. The new bulb amperage might be enough for it to work.
The flasher is a mechanical device. The lamp current goes through a resistive wire wrapped around a bimetallic strip that bends when it gets hot. The current through the resistor wire heats this strip, it bends, and the bending opens a switch contact that shuts the lights off. No light = no current = no heat, so the strip cools down and bends back to where the switch closes again and the cycle repeats.
The Pep Boys etc stores have mechanical flashers and also "electronic" flashers - these as a rule have a very simple electronic timing circuit and a relay that switches the lights.
There are two terminal and three terminal flashers. Two terminal mechanical ones are sensitive to the lamp current, more current makes the flash rate faster and less makes them slower. At some low lamp amperage the flasher stops flashing, the bimetallic strip doesn't get hot enough top bend the switch contact open.
Two terminal electronic ones are relatively insensitive to lamp amperage but still need some lamp current. They all have one characteristic that I think is dangerous. Because the internal electronic timing circuit has to be powered through the two terminals, it has no power when the turn signal switch is off. When you turn the signals on, the lights must start "off" to allow the flasher's power supply to charge up (with the switching relay contact closed, there's no voltage across the two pins to operate the electronics) then it turns the lights "on for the first flash. This means you have a 1/2 second or more delay from when you flick the signal switch until the signal lights come on.
There are two kinds of three terminal flashers: ones that will work in your bike and ones that won't. The ones that won't use the third terminal for a dash light or alternating flasher output but are just like two terminal flashers in operation. The kind that works have a ground terminal and these power the electronics from the input terminal to ground: because they don't need the voltage between the input and output pins to power themselves they turn the signal lights "on" as soon as you flick the switch, flashing it to "off" in a little while to start the flashing. This type will work with any lamp current from zero up to the rated maximum number of lamps.
Just about any electronic flasher has a lamp failure sensing function. A flasher rated for four normal lamps will flash at a different rate when less that four normal lamps are connected. This can be faster or slower than the normal rate but faster is more common. Personally I like the fast flash rate with LED signals, it gets attention. If you really want the "normal" rate you can add load resistors to draw more current and "fool" the flasher, or - if you're electronically savvy - open the case and disable the lamp fail function. You just have to figure out how it works and then make it not work. This is usually not too hard, there will be a shunt wire and a voltage sensing circuit across it. You usually just have to break the circuit trace connecting the shunt to the sensing transistor or chip.
