How old is your MDS 40?? The OLD black head engines that were imported by estes model rocket company were set up for low or NO nitro(high compression) most would simply not run on 15% or more nitro. If you have one of these engines try some 5% nitro... that and a good plug like the OS 8 you already have SHOULD yeild a good running sport class engine... some the engines also had air leak issues at the carb base and needles... once you can get it running take a close look at your fuel line for air bubbes.

John