[PanhandleShantyman]

Progress report: after getting the manifolds back on she fired up no problem, with no more dieseling or backfiring after I reset the initial timing to spec at +4. Once I adjusted the idle mix screw for max vacuum, I was super happy to see the vacuum gauge reading steady at 18 inches Hg on manifold vacuum, way better Than the 5 I'd seen before. Then I put in new spark plugs and wires and that really smoothed out the idle even more.

She sounded great idling in the driveway, so I decided to take her out on the road for basically the first time since bringing the truck home. Then I hit a new issue in that the truck was really stumbling under any significant amount of throttle. I limped to my destination, had a beer and a sandwich and thought about it, then went home and decided there must be a failure of the timing advance at speed, so I put the timing light back on and found that that the mechanical advance was at least doing something and could get me up to about an additional 20 degrees advance at 2500 RPM, with ported vacuum not hooked up to the vac advance. Hooking the vacuum advance back up however gave me no additional advance at steady 2500 RPM, so I figured either that linkage is just frozen up or the diaphragm is shot, and as a result my total timing must still be too retarded at speed. I was running out of daylight so I didn't dig into the distributor at that time. I just decided to advance the timing to +13 to make up for it and see how she ran, and that worked great. Truck idles a little higher, at 650-750, but the stumbling on acceleration is gone, and the truck fires up first try and turns off with no dieseling or backfiring.

Next steps I guess are to figure out why the vacuum advance is not doing anything and fix that, and also make sure my mechanical advance is reasonably close to factory curve. Then maybe I could get her back to running smooth at recommended initial timing. Although, I've read expert opinions on this forum about how the low +0 to +4 recommended timing advance on these trucks was primarily just for emissions control measure designed to make the exhaust run hot for better smog combustion with the A.I.R. injection system, which my truck doesn't have. (I see a bracket for holding the A.I.R. pump, but there's nothing on it, I guess maybe the previous owner just got rid of it?). Should I just forget about factory recommended initial timing in that case and give my engine the extra advance it seems to want? Any downsides to that approach longterm if I just keep the initial advance a little high like that?

Anyway I am super happy to finally have this truck performing well enough to drive around my local area and run an errand or two! Maybe tomorrow I'll drive it down to the Christmas tree lot put it to use hauling a Christmas tree back home.