I ran it again and this time it wasn't puffing black smoke but it's still spraying some fuel out of the carby inlet. I tried the mixture screw but it didn't make much difference. It might be lean as it backfired and stopped.
What you called a backfire sounds like what the trade calls a spit. Essentially it blows back through the inlet manifold which results in a blast of petrol-air mixture puffing out of the carburetor inlet. It is often on fire as it puffs out. This is the main reason it is unsafe to do carburetor tuning experiments with the air cleaner removed.
Have you checked the ignition timing? Does the engine have normal compression? Spits can be caused by extremely lean mixture but they can also be caused by grossly incorrect ignition or camshaft timing. If you have a sheared flywheel key or a camshaft drive failure, much of this could be explained.
