The LED bulbs are much brighter, but they have a different internal resistance, which the standard flasher module very much counts on having. So there are 2 ways around this...

1) Install an electronically controlled flasher unit. In this way the flash rate is controlled by an internal circuit board and NOT the physical bulb itself. (These will work with either type or even mixed bulbs.) The basic resistance type flasher unit is cheap, around $6. The circuit board controlled unit is usually around $25.

2) Add a "resistor pack" to the circuit. This adds the missing resistance back to the flasher circuit, so that the LED bulbs have the total resistance that once again match the older incandescent bulbs. This sounds easier, but the ones I've seen are rather large (about 4" long) and fitting under the tank or inside the HL shell looks to be an issue.