Richard ... The parts that you are discussing are expensive. Be sure that you or your mechanic knows a lot about these units or has spent the time with the manual fully understand the function of each component. First be absolutely sure that the linkages are adjusted correctly at idle. Then try the tests described above. Since you believe that the engine is lean when warm, if the cold start valve is not leaking, the thermostat is working (test in hot water on the stove) and is the correct thermostat for this pump (fits with the correct seal and clearance), try making the fuel mixture a little richer by lengthen the pump control rod by a twist or two for testing. If this corrects the problem, there is an adjustment in the pump to increase the fuel flow. The linkages should be reset according to the manual. This internal pump adjustment is not "normal" and should be your last resort if everything else checks out.
Try continuing this on the "Vintage" board.
190SL, 230SL 5-speed, 95 E320 Wagon, 01 E320 Wagon, MGB, Boxster 'S', 190SL "Barn Find"