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I and many others have been through those problems. A search on carb will tell you a lot. Wish I could tell you there is a quick fix. First start with fuel, pull the line loose at the carb and check for good heavy flow/stream for 2 or 3 minutes. You could have rust in tank plugging the line. Also check any screen at the carb. Gas flow problems show up where it will hold an idle or move in low gear but doesn't get enough gas to get that instant spurt at high loads. It could also run till it dies(the carb empties), then it sits a little while and the carb fills back up, and then starts again. Check your sediment bulb setup for flow and replace or clean if necessary. Get rid of any type of inline filter used with lawn mower engines. Gas running out the carb indicates float sticking open, float hung up on gasket, or not adjusted to close right, or needle sticking. I hope the guy soaked the carb in a carb cleaner and blew out the passages with an air compressor. Those viton needle tips will stick and I've had to replace a new one of those with another new one. My float had a depression where the needle rode in that when properly adjusted still didn't allow the needle to seat. The pin holding the float could be loose. A sticking needle or float can sometimes be knocked loose by a light rapping on the carb with a wrench. I had my carb apart a dozen times before getting it right. Even with all that and getting the carb right, like with mine there could be a multitude of other problems that just enhance the whole situation. Gas soaked plugs that never dry that need replaced. Points with a bad spot or out of adjustment. A weak condenser. Rotor & dist cap. Plug wires starting to go bad. I replaced all that stuff but when combined with a new coil, it now runs good. My old coil was just weak. If those things haven't been replaced in a few years then it is time anyway. I'd start with points, condenser, dist cap, rotor, plugs if wet or crusted, the coil, and lastly the wires. You can check the wires by running at night to see if there is any spark around the dist cap, etc. Also pull the coil wire and spark plugs wires and look for a good sharp blue spark. The backfiring is indicative of electrical problems with the spark such as a cracked dist cap or coil, etc.. Too early to suggest a worn distributor. I worried that mine might have a governor problem cause mine wouldn't run in second or power up a slight hill but replacing the electronics and particularly the coil fixed it even though the coil wasn't getting hot and acting the way some of the posts here describe a bad coil. Fixing the carb and electronics also took care of my need to have full choke on to start it. Needing full choke is indicative the fuel mixture too lean or an air leak around the intake manifold. I had very good suction at the carb intake and spraying carb cleaner around or going around the intake manifold with an unlit propane torch didn't produce any increase in rpms which would be indicative of an intake manifold leak.
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