I flipped till my arm was tired, came back and did about the same in 20 minutes.  Was getting plenty of gas I knew the ignition worked because I had tested it the night before.  That's when I remembered taking the battery out and charging it because the bench was too full to get the plane on it.  Plugged the dang thing in and 3 flips later the engine was running.

Wish is was that simple for you as you're getting pops, however, make sure the plug is tight (mine was also loose but ran) the boot is on firm and then check the timing. If that doesn't get it T.O.M. will be buy with the correct answer.