My Yamaha F4 would stall every once in a while. Every time I removed the carb drain screw, bits of varnish would fall out and then the motor would start right up. Then somebody from the MG club told me they all use the most expensive Shell gasoline. It is the only gas with 0% ethanol. Ethanol is able to absorb moisture from the air which creates jelly (varnish) in your carb. I switch in June and never had a problem since. My Yamaha F20 used to take 3 or 4 pulls to start. With the new gas, it cranks up first pull. It's definitely worth an extra 10%.
That being said, I make sure I run my motors at least once a month to keep fresh fuel and lubrication in the system. I fixed a number of guy's motors and the problem was the same every time and always on the small engines.
