They are NOT leaking at the existing o-ring on the older black style solenoids..
The problem seems to be that on the older solenoids (5110) they do not have an o-ring groove out at the very nose which is where they leak. They have that little thin hard plastic rib that squishes into the bore where the solenoid installs to seal them up. It is made out of the same hard plastic the rest of the solenoid body is made from and that seems to be the problem. If it had an o-ring there it would probably work fine and COULD be used more than once too.
When they do leak, they are not leaking gas into the carb but rather pushing it back around the nose and down the side of the solenoid back past that little rib into the groove where the diaphragm supplies the gas to the solenoid where it then gets "injected" into the carb. To me this helps explains the sputtering and dying issues as the motor tries to come up off of and return to idle.
I looked at an old one thinking I could maybe add an o-ring groove on one to try it out, but the area it just to small and thin to be able to machine one in. That is why I tried the gasket sealer. Also probably why Stihl came out with the newer versions? I imagine but have not measured to see if both O-rings on the newer white ones (5104) are the same size.
