Small Engines (Lawn Mowers, etc.): Gas blowing out of Gas Cap, gas caps, vapor lock

Question
I have an Ariens 911 walk-behind with Tecumseh 5HP engine. It's 27 years old. I have gas blowing 2-5 feet out the top of the gas cap. I've never seen this question asked online and can't figure it out. Engine runs fine. Doesn't smoke. Any ideas would be helpful.

Answer
Richard
There is too much pressure inside the tank Is the cap metal or plastic?  You may need to Get one at one of the big box stores for less than $3.00 if i remember right.I would supect the gasket is missing from the cap? The pin hole allows air to flow into the tank and prevents vapor lock in the fuel system. All lawn mower gas caps have this air pin hole, so buy a new gas cap that has the pin hole if the current one doesn't.