I used to be a service engineer of pneumatic systems, and when l realised, my EN500 petcock is spitting light fuel vapor on the top of the carburetor, l knew what happens: The diaphragm is not sealing any more, the fuel can pass it, and - because of the vacuum pulsation in the suction line - my petcock acts as a small membrane pump. The vacuum valve follows the vacuum pulsation at low rpm, and this eats up the diaphragm. The solution is easy: You have to install a pneumatic averager between the vacuum connector and the petcock. The pneumatic averager is a very simple thing: 20 cm of vacuum tube (6mm internal diameter) and a throttle in it, at the vacuum end. My throttle is a 10mm long 6mm bolt piece. If l push it into the vacuum tube, it plugs it totally, so l filed one side of it, to make a little gap. Now l have a throttle and a chamber after it. The pulsation cannot shake the diaphragm in the petcock any more. I installed a petcock repair kit and made this cheap mod. I am sure, this will result in a long-long petcock life. p.s. I take the vacuum from the right side carburetor vacuum connector, just because it is further from the petcock. Of course l plugged the unused left side carburetor vacuum connector. Now my petcock opens 2 seconds after engine startup, and closes 2 seconds after engine stop. Excllent!