Yep, as usual W8YE has nailed it. Here is a good tuning document, including a picture of remote venting of the carb as mentioned.

With careful tuning and a free breathing muffler you might get away with the carb you have. If it runs good and doesn't stumble when you open the throttle rapidly, you're good to go....

I am betting running a remote vent to the regulator cover will cure the in flight richening problem.

AV8TOR