Hey, I am looking for some ideas on what could be wrong with my 420. It back fires through the carb and will fire off intermittently. It started this randomly one day. It is relatively new and not at all abused. I have checked the timing, spark, new carb. Went through all the internal and no luck. I did not take the head off yet but it has compression so I do not think it is a valve issue. Has anyone experienced this?