I have a 71 250C also. I replaced my Zeniths with Webbers (eventually) but have rebuilt them (Zeniths) in the past. "Assuming" the car did not exhibit these characteristics before you rebuilt the carbs, and nothing else was changed, then you can pretty much figure the problem is in the carbs/manifold mounts. Did you use new insulating blocks when you put the carbs back on? If not, did you clean the old gasket material from the blocks and make some new ones? Two other areas to check. All the rebuild kits I have seen for these carbs have more than one gasket for each mating surface. They look pretty much identical BUT THEY ARE NOT. If you don't use the proper one you may block an internal passage and/or open an area that needs closed. The top (cover) plate warps due to people over-tightening the aircleaner screws. This causes air leaks. You can surface them by working them over some 300 to 400 wet/dry sandpaper on a FLAT surface (bench). When the surface of the plate is all clean and shiney, they are surfaced. It is also possible that the metering jets and/or air correctors are reversed. Note. the air corrector stamped "4S" is for the PRIMARY side NOT the secondary side ("S" is not for secondary).
One last item. These carbs need to be "flow balanced" to work properly. The amount of air through each carb should be equal at idle. I have had strange things happen when the were far our of sync. Best way to do this is with a flow meter but you can get close by using a short piece of heater hose from your ear to the PRIMARY on the carb(s). Try to get the sounds balanced. If you have one carb rich and one lean nasty things happen.
Hope this helps a little.
BTW. Go buy a cheap 13mm open/box combination wrench and cut it in half. Makes carb removal/replacement much easier for the inboard studs.
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T
[This message has been edited by DANTRCAV (edited 02-17-2000).]