HI.
Backfire could be caused by incorrect ignition timing
Start with some of the basics first
Fuel in tank.
Is coil giving out good spark when cranking.
Are you getting a good spark at all plugs.
Can you hear fuel pump running for a few seconds when ignition turned on.
All pipes and wires connected around engine.
Check inside both ends of plug leads for corrosion.
Check inside and outside of distributor cap and rotor arm.
Has distributor come loose and moved position.
Has timing belt jumped a tooth. Will throw out ignition/camshaft timing but may still start up.
Dave
February 2011