Briggs opposed twin carb runs rich at idle.

I have a Briggs carb # 692814 on a model 461777 0137E1 code 9802035A ( opposed twin 2 20.5 hp engine engine) that runs rich at idle. I have rebuilt the carb ( ultrasonic cleaner ) and used a new kit and the included with the kit jets. It improved the idle somewhat but not 100% as it is still slightly rich at idle. Is there a way to add a mixture idle adjuster to this carb?? Or am I overlooking something. This is the first fixed mixture carb I have run across that is rich rather than lean. The carb does not appear to be flooding over and runs well at all speeds except for a slightly fat idle that is driving me up the wall. No gas in the oil --air filter clean--same problem with air filter off the engine. Thanks in advance for any help or thoughts on this. Rick