Is there a Carb rebuild kit? I have a leaky right carb...
Your fix is likely much easier than a full carb rebuild. If your engine is running well, a full carb rebuild will be wasted effort.
Remove the float bowl, paying close attention to the gasket integrity and installation - one very likely replacement item. This could very well be the only problem. A bit of silicone grease (commonly available packaged as "dialectic grease" in autoparts stores) can make gasket installation much easier.
Failing such an easy fix, find some container to catch gas, hold it under the carb and turn on the petcock. Push up on the floats until the flow of gas stops - the flow should stop when the floats are roughly parallel to the carb body. If it doesn't stop at all, you may need to replace the rubber float needle valve. If it does stop but requires pushing the floats higher than parallel to the carb, you need to adjust (bend) the little clip in the middle of the floats that attaches to the needle valve.
If after all this the carbs still leak, it may be that your floats are shot and no longer floating - although if this is the case you would have running problems as well.
All parts referenced here (as well as all other carb parts) are available piecemeal from any BMW motorcycle dealer. I prefer mailorder from Max BMW for my parts needs.