Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Have you ran a cylinder test? Loosen the spark plug wires and with engine idling, remove one wire at a time, engine should continue to run. If engine dies when one wire is removed, the opposite cylinder is not firing. With only one cylinder firing, the exhaust seems rich. It may not be running on one cylinder, but it is a common problem with twin engines.
removed wire one at a time stayed running on both. Idles good starts right off everything is ok until about 1/4 to 3/8 throttle then starts to miss real bad and blows out black smoke. Pulled plugs they are very black sooty looking. Could something have been done wrong on carb rebuild to cause this?
Choke flap in carburetor loose? May not be staying full open.
see question still open, did you get to check the choke?