1978 MGB leaks gas from 1 of the 2 SU carbs, runs rough and will not hold idle. car has not be started for 1 month.
You probably have the float level set too high or the carb float has a hole in it
Get a set of Grose jets to replace the old style needle and seat. Replace the floats while you are at it. http://www.northwestimportparts.com/MGB-Carburetor-Parts- i58.aspx
Moss Motors -- http://www.mossmotors.com/Shop/SearchResults.aspx? SearchText=grose&WebCatalogID=0
Leaving an older car sitting for a month without starting it will cause problems with the carbs getting gummed up. Run a good fuel system cleaner like Seafoam and drive the car more often. My car with 3 SU's does not like sitting but a drive of a few miles will usually clear up a rough idle. I installed Grose jets 20 years ago and have never had a problem with fuel overflow! If your car sits a lot be sure and use Sta-bil in your gas as well. Good luck!
The leak is probably the small cork gasket on the lower jet housing. If it has not been driven in awhile, they dry out and can leak. Before you spend a lot of money, remove the lower jet and check the gasket.
Those carbs should be easy to get apart ... my guess is if it sat for a while the float may be stuck or the needle is not closing the feed
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