I have a1988 s10 i am trying to get timed but when you try to start it it back fires thru the throttle body

This is only a suggestion but I had a similar problem. I have a 1987 Camaro Iroc z fuel injected. I had a similar problem with the car back firing through the throttle body. It turns out it was the TPI sensor mounted on the front of the throttle body. The sensor is mounted on the front of the throttle body to the left on my car, the opposite side of the linkage. Its black in color and has a electrical connection to the back side of it. Its triangular in shape and mounted with to hex head screws if this is of any help to you locating it. Try adjusting it first that may correct your backfiring problem like it did for me. If this was any help to you leave me some feed back.