New motor backfiring through carb

If the balancer and timing cover are accurate ,'or at least have been marked to be accurate when timing...with a pen' you should be able to just line that mark up to 0*... and you're either going to be right on or 180 out ...so if it cranks and still coughs out the carburetor and even back fires once then it's 180 out, flip it and fire it up.