ser1esii

I just bought a series 2 rx-7, it worked fine when I took it for test drives but when we tried to drive it home we got a little way down the street and it stalled every time we wanted to stop unless we kept it revving to something like 3-4k it wouldnt idle, and it was very hard to get started again. the guy I bought it from said to just put new spark plugs in, which worked when I did that the next day after towing it home, but for only about 20 minutes and after that the bottom two plugs were very black... top ones were fine. he said the other problem is it's bridge ported but has the stock carby which is limiting it, please help I'm new to rotary engines what do you think the problem is, what should I do?

Oh it's the stock 12A engine too btw