I believe the OEM valve can be disassembled for cleaning. You might want to try lapping the plunger where it seats against the body of the valve to make sure it can seal well when idle vacuum is applied. Chances are there's a constant leak that shouldn't be there, which is causing the idle speed problems. A weaker spring might help- - - -nothing says you can't do some tinkering to try to improve things. Also, check the throttke plate in the carburetor. Some of them have an air bleed hole in the flap valve that can be plugged with a pop rivet if the idle speed is too fast.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!