A 3/32" fuel line will more than handle that engine and much larger engines. Line size is not your problem. How old is that 20? Maybe it is time to install a rebuild kit. At minimum I'd disassemble the carb and inspect the diaphragms to make sure they are soft and pliable. More than likely that is your problem. Another area to check while the carb is open is the small strainer screen under the pump diaphragm. It can be removed, cleaned and reinstalled but replacement is better. They are notorious for getting a sort of slime, clear coating on that screen that dries and restricts fuel flow. Once hardened, that coating is very hard to detect and remove. A screen is included in every rebuild kit and kits are only about $10. Valley View RC among other places is a good source for the kit.