I've melted about 300 pounds of wheelweights so far, and have not experienced anything like you describe. I don't think 900 is high enough to separate out the antimony, but you shouldn't go over about 650-700 degrees.
Sort your wheelweights! Zinc weights, at least the ones I've run across, have "ZN" on them somewhere. I've gotten both stick on and clip on types. You do not want Zinc in your bullet alloy. Keeping your pot temperature in the recommended range will keep the zinc weights from melting...they'll just float on top like steel. Steel weights usually have "FE" on them, and I've gotten both stick ons and clip ons. If you get some lead stick on weights, they're nearly pure lead, so you might want to set them to the side.
Smaller weights give smaller overall yield. Large weights will give better yields. From what I've weighed so far, 80% yield is about right.
cast boolits website
ETA: Be
absolutely certain that anything you put into molten lead is 100% free from moisture, otherwise you may have an "explosion" and end up with melted lead flying everywhere. Also, you might want to smelt the wheelweights in a different pot than the one you intend to use for casting. The less gunk you put in your casting pot the better.