I cleaned carb and put a rebuild kit in. I set float and nedle valve per instructions in the service manual. Before rebuild it would not idle at all. Now it will idle about 1200 rpms for about 20 seconds after throtle up,then it will fall off to about 500 rpms for about 10 seconds then die. It seems to run up ok (good responce), but blows black smoke and leavesplug black after just a couple of minutes. Any suggestions ?