How to Recognize Stale Fuel. Stale fuel is not only inefficient in the motor that's running it but can also cause serious damage to parts and mechanisms. Being able to recognize stale fuel is important, since the fuel should be drained and replaced immediately. Follow these steps to recognize stale fuel.
Open the gas tank and either bend over the tank's opening to smell the fuel or use your hand to waft the scent towards you. If the smell is sour or is similar to varnish or nail polish, then you're most likely dealing with stale fuel.
Place a small, clean stick such as a Popsicle stick into the tank and lift it out. Check if the fuel is sticky and if it clings to the stick to recognize the fuel as stale.
Let the motor cool for a few hours if it's recently been used. Dip your finger into the open gas tank to see if the fuel is cool or warm. If it's noticeably warm, then it's most likely stale.
Run the motor through a normal use cycle. Monitor the performance of the motor and listen for sputtering or any other unusual behavior. Keep a rough estimate of how much time the motor was able to run to see if its efficiency is diminished, which is a telling sign of stale fuel.