A timing light is the best, but a old rule of thumb for on the road fixes, is loosen the clamp on the distributor and rotate it in the direction of increased idle. That's advancing. Rotate it for the fastest idle without faultering. Then retard the timing 100 RPM's and you will be close. If it sputters, try adjusting the carb mixture screw to smooth the idle out to the best. If that doesn't work, you may need to do a valve adjustment. One or more may be tight, or not enough clearance. John