The carb is worth disassembling and cleaning and blowing out passages. But it would be worth watching the timing while the engine is doing that (one guy in the car on the brake, the other with the timing light).

It could be as simple as the plugs loading up too. You might want to keep a set of hot plugs on hand for trouble shooting the engine in the pits/garage. Cold plugs don't like too much idle time, especially when the mixture is rich.

Try leaning just the idle with the idle feed screws.