Bajaj pulsar 180 dtsi backfiring while starting? why? and missing after starting.

Check first if all cylinders spark plugs are firing, seem like you are missing on one cylinder. this will cause as well backfiring.

if this is not the case then:

backfiring can be caused by too rich fuel mix or by wrongly set advance. there is unburnt fuel present in exorst and this cause backfire. these two are most "popular" causes of the backfiring.

hope this will help you solve the problem, do not forget to vote...