My 71 doesn't have the bowl vent valves either. Basically all the throttle control valve does is give it a little more gas under load at idle (like engaging the transmission). You could turn the idle up a bit and it should run fine. I would start by looking at your chokes, mine was doing the same type of things and I unplugged the electric element on each choke so they open according to the engine temp and it seems to have fixed my warm running problem.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section