86 4Runner runs very rough on cold starts after sitting a couple days.

I'm at a loss here. My 86 4Runner with a 22RE runs extremely rough and backfires out the exhaust if I try to rev it. It seems fine if I drive it every day, but if it sits for a couple days it gets bad, a week or two and it's really bad. It takes about a mile of driving before it smooths out and runs fine the rest of the day. No codes, I've let it sit and bore scoped the cylinders and looked at the plugs, no signs of oil or coolant burning. Any thoughts? The bottom end has about 20k on a rebuild but I don't think anything was done with the head when it was done by a previous owner. It's just confusing me that it seems to be dependent on how long the truck sits.