Does EVAP use the port that vacc advance does not?
That’s a complicated question because it depends on year/engine/tonnage and federal/high alt./CA emissions schemes, but in short I would say yes. You want to run your vac advance to a manifold source, and your canister control to a ported source. The canister has two jobs: to store vapors when the vehicle’s turned off or idling and to help facilitate the purging of those vapors when the throttle opens. If it was hooked to a manifold source, it would constantly purge when the vehicle was running, and I guess the eggheads at GM/emissions overlords c. the 1970s deemed purging at idle unnecessary.
In a perfect world, vacuum advance would have constant vacuum access, and that’s how it was until things turned smog complex. Then you had distributor delay valves and thermal vacuum switches (TVS) switching from ported to manifold respectively to exert greater control of emissions between cold start and operating temp.
Emissions got way too complicated on these trucks from about 1981-86. I’m in the camp that I think some of this equipment is good to have (e.g. PCV, canister, sometimes EGR), but a lot of it is junk (e.g. EFE, air injection, myriad of TVSs/valves, and an overall spagettified vacuum system), and it should be simplified as much as possible. Some people want to get rid of all of it, and some people live in places where you can’t so I guess I’m somewhere in the middle leaning towards practical deletion.
