Actually, I only posted one photo of the left side throttle body - the photo of the exterior. The interior photos, showing the left throttle plate from front and rear views, those weren't posted because they were pretty much the same before and after conditions as the right TB. On both left and right TBs, the throttle plate is much more carbonized on the front side of the throttle plate, the side that faces the cylinder. Not really surprising since that's where the gas gets ignited. The side facing the rear, towards the air box has much less buildup.
Throttle plate discolouration might have more to do with heat. The valves are closed when fuel gets ignited.
Now that you have soaked them, take compressed air and clean out the passages.
You guys cleaning throttle bodies - Be SURE to visually inspect the all porting etc for the big brass screws (BBS) so that they are whistle clean before reassembly. I used a Q tip with carb cleaner to clean the BBS area, and a soft wire to verify the porting was clean. It is nice to have the inside and outside clean per above, but the BBS area is critical for final setup, balancing, and freedom from surging.
My hunch is that it is the different tradeoffs between the throttle stops and the BBS settings that causes surging, since the net airflow to the engine is optimized via the TPS on only one side of the engine. The other side has to be close to the TPS-monitored side for the mixture to be within an ignitable range on that other cylinder.
Throttle shaft leakage could be an issue too.