Charley,

Some on-line checking indicates your Holley "List 660" carb appears to be original equipment for your "old fire truck". By definition, this is listed as a Holley Model AA-1 (see screenshot attached taken from the below web site listing - all credit to "The Carburetor Shop" for posting this information for all to use).

https://www.thecarburetorshop.com/TruckkitsG.htm#TkitGMC

I myself don't have any experience with this model carb, but others will hopefully chime in with some help on what might be causing your predicament.

Will note that many have suffered the results of ethanol in today's fuels and its bad effects for older equipment not designed for it's use.

Dan
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)