Lack of power - do I need to change my jets

I fitted a Simonini pipe to my PX125E a few weeks ago and found a nice improvement in power. I checked my spark plug after a few days and all was well. A few days ago I noticed a lack of power when accelerating from a stand still.

I checked my plug today and it was heavily fouled with carbon. I’ve cleaned my air-filter again as a precation, but I only did it a few weeks ago so I don’t think the problem lies here.

Do I need to change my carb jets???