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My auto has been setting for a while and now the carburetor is not working normally. Seems to be running very lean and does not want to continue running etc.. There is the standard 531 carb on it. I do have a spare 643S carb which fits the '48-60 senior Packards. I was wondering since I am going to have to take the carb apart and perhaps get it to function better, if I should instead clean etc the 643S carb. It appears like it would be completely interchangeable. Can anyone give me any advice of whether it would be wise to install the 634 on it instead of the 531? Would there be any benefit or drawback to changing?
Running it seems, very lean. What to look for especially, when disassembly and cleaning? Helps to pump the accelerator, to get started.

auto.nut

That 643 looks like a close match to the 531.

I've been looking over the specs and the two carbs appear to have the same size primary venturi at 1 7/32. The 531 calls for about ? turns more on the idle mix screws.

The question "What to look for" has a rather lengthy answer, but since you asked.

Leaking or late closing anti-percolation valves can kill the low speed to high speed transition (20 to 25 mph). If the AP valves don't close you'll never make the transition and drive home at about 20 mph.

Misadjusted metering rods can also may the low to high transition difficult. That's the adjustment that uses tool T109-113.

If the engine doesn't run at low speed then I would check the fuel delivery, float level, and idle mix adjustment . . . and then address the first few items.

All of the Carter materials stress 'check everything else first' . . . like tune-up before the carburetor is messed with. Have you double checked the items in the Carter 'punch list'?

If it only runs on fuel delivered by the accelerator pump I would suspect a compromised idle circuit (dirt/rust blocking the air or fuel passages).

If the mix screws are wildly mis-adjusted in either direction the engine will not run well, if at all, on the low speed circuit.

dp