There are different part numbers that do not cross between the cobra regulator in an impco system on the af82c (pre CARB) and the af82d model cobra, (CARB tier 1, a 2003 manufactured truck of which the s15g system is, unless the truck has the s15g system that was pre-CARB for speed controlled [Home Depot]) trucks.
I can confirm that putting the wrong regulator on the truck causes bad running.
If you have confirmed positions of all vac lines as correct, you may also want to confirm no side play on the distributor shaft (up and down play is ok , but.000 free play, left to right, is a must).