I have an 86 Shorty with a 400 sb and a Q-jet. When I let it set for 3-4 days without starting it, it will miss for about a minute and then it will smooth out and run fine. It will also puff out a little white smoke every now and then. The motor was given to me and was it was supposed to have been rebuilt before I got it. Ya'll have any suggestion what to look at first?