Vapor lock is caused by boiling fuel which can be due to temperature only, or can be caused by a restriction in the fuel line. Any filtering that is in the fuel line should be after the fuel pump meaning you want to push fuel through the restriction, you don't want to be sucking on it. The lower pressure caused by the obstruction with the fuel pump sucking on the line lowers the atmospheric pressure and thus lowers the boiling point of your already volatile fuel. Obviously summer time aggravates this condition. If push comes to shove, I have gone to an auto parts store and purchased an adjustable electric fuel pump and set the pressure around 2 psig for my 115 HP engine, mounting it right at the tank. Just want enough pressure to feed fuel barely faster than the engine can use it.
If you have an inline water separating fuel filter between your tank and your engine that could be clogged.
Kinks in your fuel line?
Long distance to your tank?
Long vertical distance from tank to engine?
Fuel line to engine, if removable, connectors could be a culprit since there is a restriction there....my boats were straight plumbed if I had a built in tank....no removable connections at the engine.
Your fuel line could be degrading and particles are partially blocking the line. Happened to me at 10 years of engine age.
Your squeeze bulb check valves could be defective, or clogged buy chips from above comment.
Your engine's fuel pump's check valves or diaphragm could be defective.
