Carbon build up in the throttle body

[Sedan]

Hi guys,

I drive a manual 08 sedan and started experiencing an engine knock at cold start. After few minutes of driving the knock disappears and does not occur till the next cold start.

I took the vehicle to a mechanic and found a thin layer of carbon build up inside the throttle body. Did a clean up afterwards and the car drove fine, but after few weeks the knock started to develop once again. I removed the throttle body and saw there was again build up of carbon. Cleaned it up, car drove fine and after another few weeks, knocking and carbon build up again.

If it is relevant, the car is very slow to respond say when you pick up after stopping uphill. It takes that much more time to respond to the pressing of the accelerator.

Could you explain a reason for this scenario please.

Thanks in advance.
Cheers!