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The carb on my 55 Patrician is giving me fits! After having it properly adjusted and running great for the last week, it started acting up. Rough idle, tendancy to stall when in gear and the brake on.(Runs great going down the road). Ive tried to adjust the idle screws on the front with no luck. Ive disconnected all vacuum and plugged them, but no luck.
Now for the real kicker. After it sits a few minutes, its back to normal. New cap, rotor, wires, bug, coil and resistor have been installed already. Ive also sprayed the usual places looking for leaks. Is there a modern carb that can replace this?

There are several threads from a few years ago recommending the 500 cfm Edelbrock 1403 as a more or less drop in.

https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=39307#forumpost39307

https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=35352#forumpost35352

You can read thru the threads and find out what extra might be needed to make them fit the air cleaner or if there is a clearance problem somewhere. There have been a few that have made the change and whether it solved all the problems is unknown. There are many using the stock carbs without issue.

The 4GC is an excellent carburetor and properly set up should give fine performance, no need to think in terms of replacing it. Just how did you set the idle mixture? With the help of a vacuum gauge you will get a very precise adjustment.

Can we assume the carburetor is in good condition?

There are many issues with the 4GC some of which are design issues known by the manufacturers back then which have stop gap remidies at best. Some of these issues are amplified by the 352 intake design . The duals used may be less affected due to the carbs not sitting directly over the exhaust cross over. I have a bulletin on heat related issues still being battled by stop gap remidies nine year after it use on the 352. I also feel raw fuel and fuel vapors may be causing oil dilution on the engines and washdown and may excelerate wear and lifter noise. Be careful with the floats and watch for flooding.
I replaced one on a 352. The car ran flawlessly for years - oil clean- no lifter tap. No rich smell- no fuel smell- no need for elec fuel pump. The car could sit for 4-5 or more weeks and start right up with 2 pumps.
Of course i also had a hotter spark with the pertronics and had disabled the crossover in the intake as it was of no use with our planned usage. If you are in a hot weather climate and or are not using as a daily driver in cold ambient temps I would recc the same.

The carb was bought from a guy that had it rebuilt, but never used it. It ran pretty good until this morning. Can you walk me through setting with a vacuum gauge?

Check the secondary throttle linkage. Many idle problems are really the secondaries not reseating properly after they've opened. Wiggle side to side, tap with a screwdriver and see if that changes the idle.

jack vines

Yes. The weak secondary return spring and poss misalignment of the secondary throttle blades (if they were removed at rebuild)will contribute to the blades not seating at idle. If you have no experience adj the base idle and mixture with a vacuum gage further discussions may be beyond the scope of DIY repairs. There is a detailed adj procedure for all linkages in the svc manual which must be performed during the rebuild before any on car adj. are you running an electric fuel pump or just the mechanical? Do you have the proper 4gc for this application? If so it should have the removable plugs in the bowls. Are you sure the floats are set properly and not sticking? Are the needle and seats updated to disc and seat or original?

Maybe not as helpful as others... but I'm planning on Fitech EFI. Look them up and see what you think. You will need to convert to negative ground though.

Is there a source for the secondary spring?

To set idle mixtures with a vacuum gauge:

Connect the vacuum gauge to read vacuum in the intake manifold, you can take it from the power brake fitting (but remove the check valve) or from the vacuum take-off for the wipers but before any junctions, but do not take it from the vacuum take-off for the distributor vacuum advance. Start the engine and allow it warm up and idle at a reasonably correct idle speed (450 rpm +/-). Select either idle mixture screw and begin to screw it in (clockwise, leaner mixture); the vacuum should begin to drop quickly and significantly. Then begin to richen the idle mixture by screwing it out slowly - the vacuum will begin to increase; stop when the vacuum ceases to increase and repeat the procedure with the other idle mixture and then go back and recheck the first one.

The optimum point you want to be it is with the leanest mixture (screws further in) consistent with the maximum vacuum. An engine in good internal condition and in good tune should give a very steady (no significant fluctuation of more than 1") vacuum reading of 18 to 20 inches of vacuum.

PS - you might want to check and make sure the ignition timing is correct before you do this.