If your motorcycle is like my car, the 12V bus voltage varies from 12.0VDC to about 14.7VDC, with occasional spikes that might go higher, and alternator ripple which might be 250mVAC riding on the DC level. During cranking, the voltage can sag as low as 8V.
The 555s are ratiometric devices, meaning that their internal trip points are pegged to 1/3 and 2/3 of the voltage at the the 555's VCC pin. Bypassing the 555's Vcc to Vss pin with 10uF to 100uF will help hold that voltage steady. I would use a 100nF ceramic in parallel with a 10uF to 100uF electrolytic.