[abus319]

Acerna
Hopefully someone with more skill than I have will chime in soon, but Backfire can be several things. Usually its related to timing, but can also be a lean fuel condition as you suspect. I don't know which carb you have but putting an ounce of fuel into the air horn instead of dumping it down the venturi is a much safer approach and will give you a longer run time. That should be enough time get your fuel pump going.
These new fuel blends have a VERY limited shelf life so if it sat very long before you tried to start it 3 months ago you will probably have more work to do.
I have to pick up a grinder in San Antonio so If you need help or just want to chat I will pm you my number.
Oh and a fire extinguisher is a great tool to have nearby