1977 2.7 back firing

[patrick300]

I am working on a 77 911 with a 2.7 engine. When the car was sent to me the ingnition switch was taken apart. Well i fixed that problem and i decided to start the car it started right up then it ran for like 20 seconds then backfired and shut down. I noticed that the cdsi unit was replaced with a prema tune electronic ignition unit, but the distributor still has points. I was looking around and noticed that the fuel mixture unit has a sensor on the back of it and the plug has 3 wires coming off it but one is broken right at the plug and i cant find another wire so i dont know if it went to ground or wasnt used at all. i am hoping someone has had this problem before, i know that the 2.7 is known for back firing and messing up the airbox . i just put the popoff valve in it. When i started it up i didnt have the filter and top on the airbox so i dont know if that makes a differnts, any help with this would be great i am new at these cars

[T77911S]

after about 20 sec, the thermo time valve (TTV) opens and the vacuum to the WUR makes the control pressure (CP) rise about 1 bar which makes the car lean out. if the CP's are wrong, it could have gone very lean and backfired.
you did not say if it would start back up. you can remove and plug the vac line on top of the WUR to get it started and keep it running, but you will not be able to drive it like this. sounds like you need to check your CP's. check the poppoff valve and make sure it is seated/closed properly

fuel mixture unit? i assume you are talking about the air flow meter (AFM) and fuel distributer (FD). there is a plug on the back of the AFM for the fuel pump and the cold start valve is back there.
if the fuel pump does not run with the key to on and motor not running, then ithe AFM switch is working.

[Paulporsche]

Have you checked the points, control pressures and mixture?