Help me please, to make a long story short I purchased a 90 zx-11 from a friend that had sat for probably 10 years. I took it to a bike shop and they replaced fuel filter, fuel lines, cleaned carbs, new plugs, new valve cover gasket, new throttle cable, new oil and filter, new coolant and hoses, and cleaned and sealed gas tank. Got bike back running decent, but shop owner said I might need new fuel pump and relay because my two fuel lights continuously flash even when gas tank is over halfway full. Bike would start up and run fine for the last two days. However, today I put new premium unleaded gas in tank and some techron fuel cleaner. I let bike idle for 5 or more minutes and every so often it wants to backfire now. It is hard to start and im afraid of running it because of loud backfires now. Any help would be greatly appreciated.