Slant dies when put in gear.

I have a '66 with a 225 /6, and it's a 904 trans. The thing recently started sputtering and dying when in gear- runs/revs fine when in neutral or park. I brought the idle set screw up (more gas) and that does not help, but if I take it down it just dies. I have a 1920 carb, and MSD 6AL ignition. Is there a chance it's maybe my timing, or does it sound more like a fuel/carb issue? I know it's getting plenty of gas through the lines and like I said, it idles smooth and revs fine (until I put it in gear!).