My truck wont Start?

Parked my truck, a few hours later tired to start,
wouldn't turn.  So far i have replaced spark plugs
and cables, spark tested and they all are getting
power. I replaced the crank sensor. The pump
comes on and fuel is sprayed in from the line. I
have tried starter fluid 4 times, it didn't catch.  The
4th time i had a friend spray into the tube and it
blew fire. There is always a back fire.  And white
smoke comes out the tail pipe. It wants to turn.  I
have air, spark and gas. I am so lost. Please help
me.  This truck has always been maintained and
only ever had spark and fuel problems.  I feel i have
covered all that.  But im still stuck.