Have a 94´ 535 Virago, I have replaced the spark plugs and gaped them to spec. Runs well at idle and when riding but, when I let out of the throttle it backfires out of the exhaust. Once in a while you will hear it backfire through the carb. Any help on what to do next to fix this would be greatly appreciated. I am DIY, cannot afford a high paid mechanic. Thanks.