Happened to me too! My problem was crap in the carb.

 

I had the sampe kind of problem with my z400. a had a bit of backfire, splutering engine and and engine cut out once and a while.
It was all due to a lack of fuel in the tank... try that and see what happens. if you havnt already

 

I had the same problem, checked plug checked float bowl. It turned out, not enough gas in tank. Remember the Eiger main fuel pickup tube sits high in the tank. Because mine would only sputter and backfire, and stall with the petcock in the ON position, when in RES or PRI it would run fine

 

Thanks for all the info guys. I had fuel in the tank, that was not a problem. The carb had come unseated from one of the couplings it rides in. This allowed for a lean mix accounting for the sputtering and backfire. Evidently the last backfire caused enough of a gab that it was definitaly to lean to run. This would have been missed entirely had the plan not been to take the carb apart to begin with. I now have the machine back and it runs like the day I brought it home. At least for now anyway. Again I appreciate all the information I recieved from you all.