Cold, lean engines backfire. You probably didn't use enough choke. I like to use full choke and crank an engine until it fires without touching the gas pedal, then release the choke about halfway to keep it from flooding and stalling. Using a little choke for a couple of minutes during warmup is usually needed, depending on the outside air temperature.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!