Trying to set the timing for a 1984 Chevy C10 with a straight 6 250 engine and it seems to be happy when it is set at TDC pointed towards the ground. What is it supposed to be at 4 degrees BTDC or -4 degrees ATDC or what? Does not have rpm gauge to figure rpms husband has gone through and fixed mixture screw several times. When he puts it into reverse it backfires uncontrollably. Please help we are lost. He is getting frustrated and starting to regret this truck

 

Timing should be set at 4 BTDC with the advance disconnected. Has the distributor been removed? Could it possibly be set in the wrong tooth? The starting point for the mixture screw should be 1 1/2 turns out.

 

The distributor has not been removed. How could I tell if it is in the wrong tooth? I will tell my husband and see what happens. What could be causing it to backfire?

 

Check plug wires, sounds like you have two swapped. Is this electronic spark control, electronic distributor? The pointer in the distributor should be at #1, with the timing set at 4 deg. btdc, follow the firing order around the cap, that will ensure the plug wires are correct. I don't know the firing order on a 6cyl, it will be on the intake or somewhere.
Also if it is electronic spark control, the gear that runs around the pickup coil should be at the pick up end, there are teeth that rotate with the distributor shaft, and the teeth are what "fire" the spark at the point (look where the wires go into the coil, you should have one of the teeth at that point when the distributor is at #1 cyly, and distributor pointer is at #1 in the cap.
If the distributor hasn't been removed this should already be as stated.