I have a Holley 4 barrel 4160 600 cfm carb on my GMC 302. I worked fine for years, but recently it develpoed a major problem. When I press down on the accelerator peddle and then release it the carb stays open. To my hand, it feels like liquid (gasoline) is preventing the carb from closing. It will close by hand but only at a slow rate -- kind of like a shock absorber. I plan to rebuild the carb but thought someone might know if I need to replace a specific part rather than buy a complete rebuild. Any ideas?