RE: G20 ei won't start anymore
Check the spark at night with lights turned out... a reliable spark will be bluish in color. A yellowish spark will fire the engine occasionally when the mixture is perfectly suited but not consistently enough to run.
Also, the spark needs to hit the ground bar, if the spark jumps from the electrode and grounds somewhere other than near the tip of the ground bar, the plug has a problem and the spark is not interfacing the fuel mixture in an optimum position.
Either of these two issues could produce the occasional firing but not starting.
Generally a carb problem can be diagnosed by first pulling the plug and insuring it is dry. Then priming the plug hole with a small amount of fuel, reinsert the plug and after several flips if there is no burst of running, the problem is likely ignition. If there is a burst of running then the problem is likely a carb issue.