If your engine is healthy and properly tuned it should idle just as smooth as today's cars. Rough idle can be caused by a number of things. Usually it's dirt in the carburetor. Do you have a new fuel tank or the original. A new carburetor doesn't do any good if your tank is loaded with rust and scale. It will get clogged in a matter of minutes. Even with a fuel filter. You may also have a vacuum leak. I would disconnect all the vacuum hoses from the intake manifold and plug the connectors and see if it helps. You may also have a leaking vacuum advance in the distributor. That will also cause rough idle.

John

The idle is smooth as silk in neutral, it's once I drop it in drive, it is an original gas tank, so I'm sure it's full of gunk.

I meant park, and neutral.

Engine idle rough?

Hi Mike, I see you are up in Dave Dare's ~ simplyconnected, neck of the woods! Detroit! If that is the original tank, you might want to get a new one, rather than taking it off and trying to clean it out. I did that and it ended up leaking again, so I bought a new one. It would be a good investment, and Spectra, the company that makes them, is across the border in Canada. They make all the gas tanks these days for these old Tbirds of ours. You can find them on eBay by googling for Spectra gas tanks for your year Tbird. Or you can get them from Rock Auto, and use the 5% discount code they give us.

I would change the least expensive & easiest to begin with, & that is the Fuel filter & see if it makes a difference.

Chris......From OZ.

Check vacuum hoses first, then I'm going to stick my neck out here. How sloppy is your timing chain? When they stretch, much of your low-end torque decreases. At idle with no drag, not much torque is necessary so it should run smooth. When in gear, the story changes quickly. Another sign of a stretched chain is that hesitation when you give it gas at a light. A new chain delivers instant response because valve timing is not lagging behind the crank.

How many miles old is your timing chain? If you don't know, take the distributor cap off, put a long wrench on your crankshaft bolt and rock it back and forth. How many total degrees does it move before the distributor responds?

An old engine is easy to move. I normally put one hand on the rotor while the other hand rocks the crank. Start at about 5 degrees BTDC so you can see the hash marks in both directions as you feel the rotor. - Dave

This could also be your torque converter full of crud. When was the last time your fluid was changed, including draining the converter?