here’s some background:
the truck was purchased last year off craigslist. the PO did not tell us about several problems, including failed coolant temp sensor, and the fact that the distributor had been recently replaced but had been set at the super-advanced timing above, in order to appear to run well.
the truck was driven for quite a while by people who weren’t (ahem) paying much attention to the sounds it was making. at some point the exhaust manifold cracked, causing massive loss of power etc.
at this point i starting trying to troubleshoot. have a parts truck – a 1996 mighty max that has most parts same. swapped the exhaust manifold and first part of exhaust/cat converter, which solved most of the problems, but still had a low power issue.
then redid the timing belt, which was off a tooth (common issue on this truck apparently). this improved it to where it is now, but in redoing the timing on the distributor discovered that the base timing had been set way off. with the base timing correct per timing light (and also in a more sensible place near the mark on the housing), the above situation w/ backfires etc.
most recently, i swapped the distributor from the parts truck to see if it was the crank position sensor. no dice.
this distributor apparently has neither mechanical nor vacuum advance – just the crank position sensor. one symptom is that when the ECU is jumpered out of the timing equation for setting base timing, the timing light is perfectly steady. but when the computer is setting the timing, it jumps around randomly on every rotation by up to 6 or 7 degrees. same with both distributors.
thanks for your thoughts and help!