I have a 1963 gas Farmall 706, in good condition. No smoke, fluids kept changed, recent plugs and points, new float and needle valve in carb. My problem is that the tractor will start great, running on all cylinders and then after about three/ four minutes it starts to miss, occasional backfire, then eventually stall. Let it sit a minute and restart, same thing, becomes rough and stalls. I have restarted this tractor several times until it warms up and then it runs great the rest of the day. Gas flows well, I recently started to burn 93 octane, which I think might be the problem, but can't figure that that is causing the rough running until it gets warm.. Any help would be apprciated