You are obviously running lean when cold, which is causing a backfire on acceleration when the engine needs a richened mixture and not getting it.

I would guess that the shop looked at the basics like choke adjustment. Gmask has explained the importance of carb synchronization very well.

A couple of additional things to check or have your shop check.

- Top plate warping. Remove the top plate and rub it across fine emery paper taped to a piece of glass. If the sanding marks are smooth, then you are OK. If not, you can continue sanding until you get a flat surface. You may be able to compensate for some warping by using two top gaskets. The MB gaskets are a bit thicker and may do a better job than the ones that come in the rebuild kits. Several posts on this.

- Check that the accelerator pumps are working well. If you look down the carb and open the throttle quickly, you should see a good squirt of gas from the little tube.

- Make sure that the secondary throttle valves are opening correctly. They are vacuum controlled and can get stuck in the closed or partially-open position. Either of these conditions could cause backfiring.

- Do you have the tube from the exhaust to the air cleaner in place? Is the flap in the air cleaner working correctly?

If you like tinkering or can find a good carb guy and want to keep the car original, then mess around with the Zeniths. Otherwise, look into the Weber conversion from JAM Engineering.

I would disagree with MPNYE though, the Zeniths are pretty basic and simple carbs. The key to keeping them working well is to get the heat risers to either open and close correctly or make sure they are jammed shut. Lots of posts on this as well.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe