TR2/3/3A '59 TR3A SU carburetor

For the first 10 miles, i.e., when cool, the engine runs beautifully, then it starts to chug a little, then more until it is difficult to maintain its idle at traffic lights and constantly chugs during aceleration and down shifting. I've topped off the oil in the carburetor and walked thru the manual's procedure for adjustment and tuning. Everything checks out. Very perplexing. Would appreciate any advice.

What are you doing with the choke while all this is going on?

The car starts right up, and runs smoothly, so I don't use the choke for very long. When the car starts to chug, especially at stop lights, I'll choke it so it doesn't stall.

It sounds like your mixture is very rich. You should need a lot of choke to start, and reduced slowly over 5 minutes or so. If the car starts with little or no choke, then you have a rich mixture, which then becomes relatively richer as the car warms up. You are helping the idle with the choke by bumping the idle speed with the choke cam.

I would check to make sure the choke returns fully to the stop on both carbs. Then check the fuel levels where the needles enter the jets...it should be visible at the jets, but not flowing over the top of the jet. Finally try leaning the idle mixtures. If you are able to fully lean the nuts with little or no change in the car’s running, then the needle(s) are likely worn from improper installation.

Just to add, pull your plugs and inspect -I wouldn’t make any mixture adjustment until you confirm the plugs look rich too (black and sooty)