Rebuilt 273 - starting for the first time! Help!

I had to crank mine a long time (too long) to get it started. I had put some gas down the bowl vent in the carb but not enough. If You're belching fire out of the top of the carb, the timing isn't right. Double check the timing by static timing the engine. That will get you very close. Have someone crank the engine and you can run the throttle and spray some carb spray into the carb. Have a extinguisher and a damp towel handy just in case.