I've tried Seafoam and recently B-12 Chemtool. The results of the B-12 were more pronounced and noticed than the Seafoam. The B-12 really helps the injectors.

Not knowing the tune-up status of the van as you had stated, it would be ideal to start there. New ignition wires, new cap-n-rotor (brass/copper contacts only), new air filter, new PVC valve, new air breather bell and a new set of Autolite Single Platinum plugs.

As for oil, if the highmileage 5W30 for this time of year has low sulfur content, then it's a deal, else 5W30 Castrol or Quaker State would be ideal. Avoid the really low cost oils like SuperTech (Walmart brand) and the ACCEL brand.

 

Well read up on how a battery and alternator work they only kick in at 1000/1200 rpm not at an idle speed. Read about Cold Cranking Amps (CCA) for start up, this detemines your battery size, which may not be correct. You use more power under load than in Neutral. It is as simple as going into a shop and asking them to load test the battery. and if you are saying everything else is OK then this is why I believe the battery.

A new battery certainly helped my Van get up and running.

As for Gas if you have a Marathon nearby or a Route 66 they have the best quaility, never go to a loaf and jug. Have you tried running higher Octane?

A premium Oil Filter has also made a huge difference to engine performance they only cost an extra $5 over standard Filters

 

Well, I say the things are okay which I've checked myself -- I replaced the fuel filter, checked the air filter, changed the oil (forgot to mention that), checked the compression, and checked out the plugs and plug cables myself.

I didn't do a premium oil, but I did try higher octane gas -- and there was absolutely no difference I could observe.

The local mechanics around here all claim that Shell has the best gas you can get in this area -- there aren't many 66 stations and NO Marathons (66 is one of my favourites, personally) -- so I've been going with their recommendations.

I'm still rather dubious about the battery being the problem (I've had a lot of battery problems in other vehicles in the past; this doesn't have the same feel at all), but I'll get it tested we can eliminate that variable.

I'll also give B-12 a try, since -- I forgot to mention -- the van did apparently sit a few months before I bought it, and I'm certain that there's probably residue left over from that period still gumming up the engine. Keep in mind, though, that it didn't have the problems I'm describing when I bought it; I have a good ear for motors, and it sounded almost perfect then, and only started getting weaker about 1,500 miles later.

Actually, I forgot one other quirk about its running -- it has a small irregularity to its idle; it seems to backfire very slightly every so often, and since the compression was good, the best guess I was able to come up with was that the valves are probably sticking or have a weak spring somewhere.

 

The occasional backfire at idle makes me think spark plug wires. I have seen pristine looking wires light up like a light show at night, and it is a fact that wires break down faster in vans than in any other vehicle due to the heat and limited ventilation under the doghouse.

YOU CANNOT say wires are good just by looking at them.

I don't know much about the method of distributor advance on an '83 but this is also high on my list for your symptoms.

Also remove and clean the battery to engine ground, battery to firewall and add a engine to frame ground if one does not exist.

These Vans are notorious for bad grounds causing seemingly unrelated issues and they should always be eliminated as culprits in the beginning.

 

I just changed my iginition coil today maybe that could help, the one I pulled out (1996) look pretty well used so I assume your one will be wanting some refreshing.