Timing setting question

[Hair]

1990 350 1500 2wd auto

Put a new distributor in my truck - old one died.

setting the timing - unplug the wire on the firewall and set the timing and the truck runs fine. I actually set it on 6 deg seems to run best there. Turn truck off, plug wire back up, start truck and it runs like crap backfiring and timing is off. Unplug wire and it runs fine again. Of course I have no timing advance but it runs fine otherwise.

What's the deal?

[booger69]

you have to unplug the wire set at 0 un hook the battrey plug wire in then hook the battery up the timming will set its self
thats how i had to do my 94 should be the same

[ChevyTech]

On these trucks, if you have to set the timing outside the spec of zero degrees there is usually a spark control system failure.

You should check the trouble codes.

Setting the timing using the correct method will set a trouble code. This is why booger69 said to unhook the battery after you set the timing, which clears the trouble codes.

After driving the truck, see if it has set any trouble codes. This can be done without a scan tool. There are instruction in the sticky at the top of this board.

Here are some things that can cause backfiring:
Insufficient fuel (Lean mixture)(Low pressure)
Incorrect ignition timing
Crossed plug wires
Bad distributor cap
Flat exhaust lobe on camshaft
Head gasket failing between two cylinders