I've recently pulled and replacedthe heads on my 1990, 350 tbi, and now it won't start. I have spark and fuel to the throttle body. I have also set the rocker arms to one turn past zero lash. I initially thought the timing might be out 180 deg., but have tried the timing both from tdc and 180deg. When I try to start the truck, it coughs and backfires through the throttle body, and has even thrown flames. The truck has sat for almost 2 years, and is old fuel, but it seems to burn all right in the intake This one has me stumped. Any suggestions?

 

Gotta be in your timing. You do anything with the cam or timing chain? Could you have either a tooth missing on the timing gear or the chain jumped.
Double check your distribuor orienation starting at TDC.
Also, check your valve springs, rocker arms and push rods. Make sure the valves are really opening and closing.