I'm getting there!
 

I have a 1976 450SL and thanks to this forum, I have solved some of its engine problems. However, the engine still sputters and backfires through the throttle plate when first started. After I let it warm up for about 5 minutes, it smooths out considerably. I have recently cleaned the screen on the warm up regulator and this helped, but the above problem still persists. Any ideas on this one? Thanks for your help.

Have you checked your timing?

DO/Check the following

1.eliminate leaks in air intake system:
2. Check Auxiliary Air Valve for opening
3. Cold Start Valve leaking:
4. Cold control pressure not correct:
5. Injector Leaking:
6. Injected fuel Quantities low:
7. Basic Idle incorrect:
8. Fuel Leaks:
9. Fuel injector Clogged:
10. Control Plunger in the fuel distributor binding:
and as Alabbasi noted
11. Idle speed, Ignition Timing, and mixture out of tolerance: