New owner in a 1910 house with upgrades. Kitchen sink has quite a distance to the main outlet (septic system) It is 1.5' out of sink and then in the basement it goes to 2". The sink is vented to the outside, 1.5" and it has a 90 on the top. All lines including the vent have been taken apart and are clear. I can run the water in the sink at wide open and it goes down the drain but when I let 1/2 to 3/4 of a sink full of water out it doesn't want to go down the drain, and apparently is backing up in the vent. I took the vent apart and stuck a garden hose down it and at 1/2 flow it drains but if I go to full water flow it comes back out the vent. I think that it must be a venting problem.

You don't have a venting problem, you have a partial drain blockage. Snake drain line.

Agreed, its your drain line. Do you have a clean out anywhere so you can open the main?