Concerning Bubbles & Vapor Lock

Posted by TxGoat On 2023/5/18 12:40:05

I noticed this morning when I started my car at about 78F, that the glass filter bowl was completely full of gasoline with no bubbling evident, even after the engine ran for several minutes. After driving about 10 miles with the temperature outside in the low 80s F, I stopped and looked at the filter bowl. It was about half full of gasoline. The car was running fine. I drove another 10 miles home and parked in the barn and opened the hood to see that the bowl was half full and the gasoline was bubbling.

After about five minutes, the carburetor begain to leak gas at the throttle shaft.

I expect that when I start the car again this evening, it will fire up and die, then act like it is out of gas.

I think what is happening is that the filter and gas line from the pump to the carburetor are heating up from the radiated exhaust manifold heat and pressuring up enough to push gas past the float needle flooding the motor.

When it's hot, the fuel pump seems to be pumping bubbles instead gasoline, which makes for hard starting. Once it starts, it runs just fine, but the fuel pump does not deliver gasoline as it should when its hot and I am cranking the engine with the starter.

Moving the filter bowl down to the fuel pump outlet might improve things by keeping it cooler. It's connected to the carburetor float bowl, and fully exposed to exhaust manifold heat.

I believe that adding a fuel pump heat shield and replacing the inoperative vacuum advance will improve the situation, but I think it will always be an issue with todays "gasoline" in hot weather.