I don’t see an issue with using base gaskets that have the larger than carburetor bore sizes as long as they physically fit the outside dimensions of both the outside of the carb base and the intake or carb spacer.  There is a problem in using the incorrect carb base to main body gaskets though as the various internal holes do need to match up.  The carburetor kits I use for Holley carbs can come with as many as four different base to body gaskets which makes it prudent in selecting the correct one.  Here’s a picture of some of the different carb base to main body gaskets.
 

Lorena, Texas (South of Waco)

Thanks Ted. I have been working on this Carb for the last couple of Days. Re-checked the Gaskets I replaced on the Plates and float Bowls on both Ends. Checked all the Screws to make sure they were tight. Opened the Secondary Plate a little with the set Screw in the Base.Tried it with front plates completely closed with only the Secondaries open a little. Nothing has made any difference,.It will run with about Throttle half way Open. It does "surge" even then. The problem is in the Carb. I put mine back on and it runs fine. I haven't separated the Base from the Body to check that Gsasket for seal and flatness yet. I will do that before I write this Carb off as being Defective which is maybe why it was sold. Any other Suggestions on what to check?.  

  

Sounds like something is defective Mark, a bleed hole or something, its not the needle is it? Im sure you would have seen that info Steve posted on how to setup the secondary slots etc ? If everything is right on it and set up properly it will run straight up without adjustment. Second hand carbs are such a risk.. dang!



"Came too close to dying to stop living now!"

Everything is setup properly as far as Throttle Plates adjusted, Float Level, etc . When I installed my 390 which was New it ran right out of the Box. I did have to upsize the Main Jets and setup the Throttle Plates as per Greenbirds Instructions but after that it ran great. This Carb I bought looks like it was never run.Clean inside and out but maybe it was run and couldn't be setup properly and was put on the Shelf. Somethings not right. 

  

Does it have manual choke?If yes maybe the small ball in the vacuum channel is missig,or the gasket that holds the secondaries vacuum housing isn't sealing right!That wuold explain the the major vacuum leak,or it's just the gasket between main body and base plate.

Electric Choke. 

  

While this may or may not be obvious, you might double check that the bowl vent tubes are clear.  On most Holley 4V carbs, those vents are located on the main body and not on the bowls.  The most common cause for the vent tube(s) to be plugged would be a mud dauber plugging them.

Lorena, Texas (South of Waco)

Thanks Ted. I actually thought of that too. Bought a can of spray Carb Cleaner and removed Idle Screws. Sprayed cleaner down the Vent Holes using the included "Straw" and a steady Stream of Cleaner came out where the Screws go. Still working on it. 

  

Finally figured out whats wrong with this Carb. I contacted a Rebuilder and sent some Pictures. He knew right away what the Problem is. This 1848 Carb has a 600 CFM Base Plate on it. Either someone messed with it or it was assembled wrong from the start. My Opinion is it was assembled wrong at Holley. It does not look like it was tampered with. Have a look at the Pictures and look at one on Ebay. The 1848 has no PCV Port and only One vacuum Source for Distributor. The Base on mine has PCV and 2 Vacuum Ports. It will never work with mismatched Base and Body.  Frankenstein Carb. Anyone need a like new 1848-1 Carb for Parts or put the proper Base Plate on a like new Carb?

  

Looks like someone butcher together this one, I doubt Holley assembled it with a few slot head screws..