[rbohm]

first you have a lean condition when the engine is cold, thats why you are getting a backfire. you need toset the choke on it to where it is closed when the engine is cold. as stated you have a vacuum secondary carb, and the secondaries will not open untill you have enough of a load on the engine to where they are needed. you can tune the opening point by changing the spring in the vacuum actuator. as for the timing light, the craftsman one is good. you only need a tach to set the idle speed, not the timing.