Motor won’t stay running

[1] Do not use silicon that will be exposed to gasoline or gasoline fumes. The sil turns to jelly.
[2] The pressed bead in the bathtub gasket should, by itself, seal the intake/heads. If you want to use an extra 'gasket' Permatex #3 or #4 [ non-hardening & hardening ] would be good. A very old product but still works well.
[3] It makes no sense [ to me ] that this is a vacuum leak. Why would it run for a few seconds? Or run at all? I suspect carb/fuel related, post #31.