As you have already stated a carb of questionable or unknown condition could benefit from a simple rebuild. Pay particular attention to cleanliness and getting the float height correct. I agree that leaky gaskets are bad.
Interestingly, a carb that is actually lean in the idle circuit can display symptoms of richness at idle. This results from a carb that stubbornly wishes to run without the benefit of sufficient fuel from the idle circuit. How does it do it? It tricks its owner into opening the throttle blades too much to keep it running exposing too much transfer slot or worse yet inducing the dreaded nozzle drip (fuel being pulled from the main boosters at idle). Insufficient idle bypass air can also induce this condition. I guess my point if I have one, is to make sure the throttle blade angle is set in the correct relationship to the tslot and then if it won't idle properly find out why.
Stick your nose down the carb will idling (carefully) and see if you see fuel dribbling out of the mains. If no drip try putting a kit in it and see if that helps.
Correct ignition timing is an important part of the equation.
There will be other more knowledgeable members along shortly to help. This site likes a good Qjet mystery.
Last edited by TommyK; 09-10-2011 at 04:25 AM.