[Mart]

Do you think it may be running over rich?

If so, could be a few things.

Damaged power valve.
Float setting too high.
Float punctured and semi submerged.
Poor quality needle valve.
Too high fuel pressure.

All simple things to check.

I also had a faulty distributor cap that would make the car run worse the longer it ran. Maybe because as it warms up, compression improves and the load on the HT side of the ignition goes up and the spark tries to find an easier path to earth than firing at the plugs.

Mart.