Backfire, when starting my model a

[dmancha]

Trying to time my model a. Do not have or cannot find dimple w/timing pin. So i just felt no. 1 piston for tdc. Adjusted points to where light bulb (attatched to ground and to left side of coil)comes on and then turn cam to where light just turns off. Results-back fire. What else could cause bf? Or do i need a better method of timing?
Would appreciate any advice. Thank you all.
Note: I would consider myself a novice.