It may not be what you want to hear but I'd run a compression test. It's a bit of an annoyance on a van but it seems like the next likely step.
Sounds like a vacuum leak.
Check for vacuum leaks. It sounds like the carb may need to be adjusted.
Because your sitting in it.
I have one but cap and vaccuum advance are missing.
The vacuum diagram is located on the catalyst sticker on the fan shroud. If that is missing, locate a GM repair manual as most DIY manuals will not cover this material.
how do you hook up vacuum lines in a 1988 Chevy nova
Sounds like you lost the vacuum supply to the controls. Check under the hood near the master cylinder area for cracked or disconnected vacuum lines.
There should be a white sticker, on the radiator top cover. It is a required sticker, for smog purposes. If yours is missing, you can go to your local Chevy dealer, and get one. I actually remember a factory campagne having to replace some of those.
can I get a diagram to replace vacuum hoses ona 986 chevy astro van
I need a lay out of all the vacuum lines for a 1991 Chevy van.
Check for a vacuum leak -sounds like it is starving for vacuum and reverts to the default position
Test it with a vacuum gauge.