From Wikipedia I quote them on fuel air ratio by weight. “A stoichiometric mixture is the working point that modern engine management systems employing fuel injection attempt to achieve in light load cruise situations. For gasoline fuel, the stoichiometric air–fuel mixture is approximately 13:1”.
So for a 4 cycle engine to run the weight of air needs to be 13 to 14 times the weight of fuel to be burned.
If that fuel air ratio gets out of whack she don’t run so good.
Depending on MFGr and model if there is a gas tank attached to the carb as there are on many 1-7 hp engines there is a fuel pickup tube with a fine wire mesh screen at the bottom which sucks gas off the bottom of the tank. If debris of any kind (crud) clogs the screen, it will need extra vacuum which the choke provides in order to get gas thru the plugged screen and up the tube into the carburetor. Also as said here by someone else in this thread, the main and idle jets can get gunked up. Often there are tiny openings in the ventury region of the carb which need to be cleaned out.
I use a very fine diabetic insulin needle to clean out these tiny openings. Your tip cleaning tool for oxyacetylene torch has a variety of slightly larger reamers for bigger jets. Remember that you only want to remove aluminum oxidation and varnish deposits, NOT metal. If you enlarge a jet even a little it will not meter fuel or air as designed and therefore the engine will not run well either even though everything looks clean!!!! In other words if you ream a jet oversize it will let to much fuel thru and the engine will always run to rich. Another thing to look for is whether the mating surface between carb and engine block is leaking air. If so that alone will cause a lean condition to exist and in turn require more fuel to sucked into the engine so it will run. Get to big a air leak and the engine will starve for fuel even with the choke on. Another place air can leak in is around the throttle shaft. Thats easy to check just by trying to wiggle the throttle shaft. If it wiggles around in the casting you need to consider placing a leather or felt washer on top of it or getting the carb body rebushed.
Many newer small engines have one or more orings which are intended to provide an air tight seal between various components manifolds and the block. If an oring is out of position or damaged it doesn't do it's job. On some carbs there are primer bulbs which are made out of a material which is affected by various gasoline additives. You will find that on higher end equipment the synthetic components will last for years even with the additives. A lot of cheaper engines use easily dissolved tubing and other rubber parts which don't last. If you get a hole in the primer bulb which leaks either fuel or air, there is another problem. Seems like maybe the engine manufacturers are in a p***ing contest with the fuel suppliers over who is to blame. I take the position that the engine is the end user and should be designed for the environment and FUEL which it will be subjected to. Go figure. I have a 20 year old Husqvarna 44 chainsaw with the original fuel lines still in it. It runs great. Gee how could that be? Good plastic tubing obviously. I have repaired 2 year old throw away saws that needed new fuel lines already.