Stihl BR550 back pack blower 4 mix .. whats been done so far new fuel lines - primer bulb - fuel filter - carb - coil .. adjusted valves to 4 thou new spark plug gaped at 20 thou .. It starts runs .. but has a miss when running ??? it throttles up i adjusted carb .. but it idles with a miss ?? this is the first 4 mix so not sure of the ins and out of this type of unit .. need assistance with what is the miss all about ?? Thanks !