Hello all. First time posting. Long time viewer.

I work for crown NJ and I have an issue with a doosan gas truck. I was here a few weeks ago for a no start, but it started normally and drove fine, I pulled the codes and there was a code for o2 sensor lean. I ordered new plugs, coil harness and a o2 sensor. Returned to the location and the truck will not start. I replaced the plus, wire harness and o2 and began looking into the issue. I tested and cleaned the parts of the fuel system down the line. Filter by the tank, electric fuel cut off before the regulator, the regulator and then the carb. And tried starting the truck after each piece. I removed the top of the carb and the diaphragm was covered in residue and so was the carb. I cleaned the diaphragm and carb and see that in the center of the diaphragm is a small cone shaped piece that sits inside the center of the carb and moves when with the diaphragm. The seal on that is rotted out. I cleaned the carb and reinstalled all the parts and the truck started right up. I let it idle for a few mins, turned it off and it started right up again. I went to test drive the truck and it ran fine for a minute or two and then started to back fire. Loud back fire. I reinspected everything I did, verified everything was back in place. I believe the issue is the seal on the diaphragm and I'm currently waiting to see if my parts department can get the diaphragm. Could it be a regulator issue tho? This truck has a history of no start calls due to the fuel system

Thanks in advance.
Steve