DAMMIT RAY ! (old indian saying)

I'm old school with cars and motorcycles . Most of my bikes and all of my cars are mid 60's to early 70's two strokes and a couple 4 strokes. They all have carbs . Then I got this non running 1988 BMW K 100 . I had to learn all about EFI. This bike is the same as automotive. It's basically a 4 cylinder car motor laid on it's side and stuffed into a bike. What I learned working on this fuel injected liquid cooled 4 cylinder BMW motorcycle is that the slightest thing can cause problems like you are describing and the first place to look is at all the electrical connections.The whole system is run from the fuel injector computer and the mass air flow sensor and the temp sensor in the cooling system.There are so many things giving that computer input and all it takes is one loose wire,or corroded connection and you get a backfire or it runs rough ,or it won't run at all, Check all the plug in connectors and use dielectric grease on them as a starting point .Then if it still has issues look for air leaks. My money is on an electrical problem causing the initial problem of the backfire and now an air leak as a result of the backfire.