Fuel line fitting
The solution is to work the end of the fuel line such that it is stretched open to fit the connector.
Most fuel lines are made out of urethane which is very flexible in addition to being oil/gas resistant. In order to increase the opening on the end of the tube, use a pair of needle-nose pliers inside the tube, putting the entire end of the pliers in the tube. Then push the tube out by opening the pliers. Work the pliers around to soften the tube. Do not use extreme force that may rip the tube.
Once the tube is softened use a phillips screw driver to stick in the end of the tube to make it even wider. Then after the tube has conformed to the screw driver, pull it out and insert an allen wrench that approximates the size of the connector.
Let the tube sit on the allen wrench for 1/2 hour or more. The tube will maintain the opening size of the allen wrench but will eventually shrink again. So once you remove it off the allen wrench, immediately place the fuel line end on the connector.