I read your other post and almost jumped the gun with an answer. Then I reread it and saw that you had checked the timing. My first jump the gun answer was you were changing the timing with your two dwells. BUT, it was the wrong direction. The lower dwell would result in higher timing (more advanced), which should make it run better.
When I realized that I didn't reply. The M115 Stromberg carbed 4 cyl is very picky about timing. Be sure you are at least at factory setting or above. (I would recommend 5 degrees over factory settings). By factory settings I also mean proper operation of both vacuum advance/retard and centrifical advances.
Based upon the back fire issue I would say you are either retarded (or deficient in secondary ignition capabilities/coil/rotor/etc.) or lean. On the Stromberg the choke was a constant problem and the oil resevoir needs to be full of oil. The main diaphram (easily removable by removing the top (three screws if I remeber correctly)should be checked before adjustments are made.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 05-14-2000).]