G'day If your engine keeps running with the carby completely closed, then it is getting air from somewhere and you almost certainly have a leak. One problem I have seen is carby barrels which are too small for the carby housing (badly machined). One temporary fix to prove the point is to put some grease on the carby barrel and give it a run to see if this improves things. Silicon grease will not wash away so easily and is a semi permanent fix for some. I suspect you are going to be buying another carby eventually. All the best. Michael from Oz