There may be a time when your engine runs rich and the carburetor cannot be adjusted to the lean side. Too rich is too rich and needs to be fixed.

Leaky floats:
Check the floats first. Remove float cover, lift out float, and shake it. It should be light and dry. If it sloshes with fuel inside it has a crack somewhere. Any fuel inside the float is bad, making it sink and fuel level goes too high (making it run rich, or fuel overflows).

If there is fuel in the float, put it in a pan of hot water, weigh it down to stay submerged, heat gently (not boiling the water) until it bubbles out where you will see location of the crack. Continue with gentle heat until all of the fuel inside boils off as vapor and it stops bubbling. Remove float from water BEFORE cooling. Allow to cool, then solder up the crack. A leaking waist joint can easily be soldered. For a crack in an open surface, solder on a thin brass patch. A crack near or on a corner can be difficult to repair. Otherwise replace the float.

Float level:
When floats are sealed and don't slosh, check float level. Set at 7/16 inch per the book. When you get it right fuel level should be slightly below top of main jet where you can see it, but definitely not overflowing the bridge in the carb throat.

Correct needles and height setting for the needles:
If you still can't adjust it to run a bit lean, then check the needle. Needle has a shoulder that should be flush with bottom of the air piston. If too high, loosen the set screw and pull the needle down until shoulder is flush. Also check the number on the shank of the needle to be sure you have the right needle for your application.

Worn needles and/or jets:
If correct needle, and the needle shoulder is flush and it's still running rich, you can try setting the needle shoulder 1/32 inch below flush (slightly exposed). That's equal to about 2/3 turn on the adjusting nut. This can compensate for a worn needle or worn jet. If it still runs rich, then you may need to replace the needle or jet (or both). They are not supposed to wear, but if it has been assembled with the main jet off center it can rub and wear.

Worn needle and work jet will both cause larger aperture for fuel flow, making the carbs run rich. If they are worn too much to properly adjust the fuel mixture, the needles and jets will need to be replaced. Do be careful to get the jets properly centered to avoid metal contact and wear on these parts. See video Centering the Jet.

Incorrect seal washers for the jet bearings:
There have been some reported cases of jet bearing seal washers being too thick. The upper and lower jet bearing seal washers should be about 0.020-inch thick copper material (like a thin shim). Some of these have been reported to be more than 1/32-inch thick (more like 0.035"). Thicker upper washer pushes the whole jet and bearing assembly downward. Thicker lower washer pushes the gland sealing nut downward. All of this lowers the upper travel limit for adjustment of the fuel jet, which my result in too-rich mixture at idle (and no way to adjust it lean enough). Solution here is to measure thickness of the jet bearing seal washers and be sure the correct thin parts are installed.

High altitude running:
I drive in mountains often enough to notice this effect fairly often. When driving up Pikes Peak in Colorado (USA), the entrance gate is close to 9000 feet, and the peak is slightly above 14,000 feet altitude. At 10,000 feet you have 30% less air than at sea level, and at 14,000 feet about half of the air that you would have at sea level. The most noticeable affect is like having an engine only half the size with very low compression ratio, so power output will be dramatically lower. The engine can still run okay, and it will still get you there, but you may need to catch a lower gear and stand on the throttle with a little more patience.

Driving the car to high altitude can cause the carburetors to run richer. As air pressure drops the air will be less dense, and it will require wider throttle opening to get the power you need to climb a hill. For a given mass air flow, the flow velocity will be higher with less dense air, and this will draw slightly excess fuel flow from the fuel jets, making the fuel mixture richer. To compensate for this effect you can turn both carburetor mixture nuts as much as two turns leaner when running to these altitudes, and reset the mixture leaner when returning to lower altitude.

This may be an acceptable adjustment if you regularly run at high altitude, but re-adjusting fuel mixture for frequent changes in altitude can be a pain. The car will usually run okay with richer mixture while you are on throttle, but will likely not want to idle in thin air. My easy fix for this is just to reset idle speed at higher altitude, and then set the idle speed back to normal once I get back down to lower altitude again. If you will commonly be running at high altitude, and not going down to sea level very often, it may be appropriate to install leaner (thicker) needles in your SU carburetors.