Disclaimer - coming from some CIS experience, no direct MB knowledge.
Sounds more like an extra air valve problem than WUR. Even if WUR went very rich or very lean, revs would not go very high. For peace of mind, put a gauge in between distributor head and WUR, check cold and hot pressures. Hot should be ~50-60% of system pressure, cold around 20psi or less.
Check that you have 12V to the WUR, extra air valve, thermotime switch when hot. If the extra air valve opened while hot you would have a high idle all the time. Sure the throttle is not sticking open?

HTH
jp