Idle issue

[boyce.watson]

I have a 1984 c-10 with a 305 4 barrell carb that is stock to the bone.  It has automatic transmission with air , power brakes and power steering.  It is a scottsdale with a 3 speed automatic, not aod.   It has 102k miles on it and my dad has owned it since 7k miles.  The truck ran crappy when I got it and turns out the air pump system was defective, so I removed it and plugged the manifold holes and vacuum feeds.  The truck ran fine again.  It started smoking when you first crank it up so I replaced the valve stem seals.  They had fallen apart.  I put new spark plugs, new fuel filter and new air cleaner in it and cranked it up.  As long as the choke keeps the idle high, it runs fine but rich.  When the choke releases, the idle is very slow and the engine seems to quiver, smoothly but quivering.  It doesn't die but just quivers more when I put it in gear because the idle drops even lower.  I have checked the timing, removed the valve covers and readjusted the valves, twice.  I have replaced several cracked and leaking vacuum hoses, removed and capped the egr valve and plugged the vacuum feed.  After all this, the engine hasn't changed one bit, still slow quiver.
I connected a vacuum gauge to the vacuum port on the rear of the manifold and it shows low vacuum and "late ign timing".  The distributor will not adjust to fix it.  Help.....  All help is appreciated.  I am new to this site and hope to get my problem resolved.
Thanks

[wired1141]

what rpm do you have the idle set at?

[boyce.watson]

I haven't touched the idle.  It ran fine before I replaced the valve stem seals.

[emod20]

Air pumps pump air into exhaust ports, never seen air pump cause a running problem, having said that if problem only started after replacing valve seals going only on what you describe i would suspect a valve to tight possibly an intake not closing all the way. i would do a cylinder balance test if it was me. with out machines i used to do that by pulling plug wires off one at a time and listen and watch for RPM drop. for years now i been using a test light. i hook test light ground clip to a ground and use the poing of test light slip it under boot where wire comes out of cap. this cuts each cylinder out. (i have done this for years many engines) it does the same thing as a fouled spark plug. anyway a weak cylinder shows up pretty easy. some times you need idle up a little high. you can raise idle some to see if that helps. if engine is warm and choke is open, with engine off open throttle and close choke by hand and release throttle. this will keep choke high idle cam on high and choke plate should flop open then start engine and it will idle high untill you tap throttle one time. nayway the cylinder balance test well point out any cylinders that are not up to par. just my thoughts

[boyce.watson]

Thanks for your input, problem solved.  It was a bad vacuum advance chamber that mounts on the distributor. Apparently it affects idle unlike some that have no affect at idle.  I checked it on the vehicle and I could not draw a vacuum on it with a vacuum pump and the rod would not pull the advance mechanism inside the distributor.  I removed the distributor and replaced the chamber with a new one and "bam" problem solved.  It starts easy and idles smoothly both in and out of gear.  Wow! I am glad to get this problem solved.  I had been advised that I had the valves too tight by more than one hot rodder, thats the reason I re adjusted the valves 3 times.
This 84 has some kind of computer mounted behind the glove box, with wires that come through the firewall of which one of them goes to the distributor.  I guess it controls many thing including ignition timing at idle , don't know for sure but what I do know is that it won't run without that wire connected and it won't idle properly without the vacuum canister working properly.