I guess I like to think of the simple things first, if it had not been run all winter and there was no Stabil or other fuel treatment put into it, then the carb could have gotten gummed up causing it to run lean. That would cause the back firing and limited fuel would keep it from revving up to full RPM. Does it hit the wall with no load or only when driving? The next thing I would look at would be the fuel delivery system, how are the fuel pump and lines? Next step after checking those would be plugs and then wires.

Good luck and ket us know how it works out

Pat