The engine is the heaviest component on a complete racing kart

If you consider all the components that make up a racing kart individually, the engine is the heaviest. Around 17 - 18 kg on a KZ, a little less on an OK. In percentage terms, an engine accounts for more than 20% of the total weight of a kart, certainly more than the bare frame, which is around 18% of the total.
However, the highest percentage of total weight belongs to metal components and nuts and bolts: added, together, they account for 30% of the total mass. Clearly, only if we consider them as a whole: taken individually, no component has the same weight as the engine.
If you wanted to create a "ranking" of how much the various components of a kart affect its weight, the ranking could be summarised roughly as:
metal components and nuts and bolts – 30%
engine – 21%
chassis – 18%
fairings - 14%
wheels – 11%
plastic components – 6%

The ideal pressure varies depending on the model of the tyre, not the circuit. To achieve the best performance, the value that matters is that of the ideal pressure at running temperature: each tyre has its own, which stays the same on any track. Generally, tyre manufacturers always state reference pressures for each model in their range: the minimum value when cold is the one below which there is a risk of bead dislodgement. The recommended cold value is a rough indication (because it varies depending on the track conditions and temperature) which should allow the optimal running pressure to be reached. Lastly, the value of the optimal running pressure is the one that guarantees the best performance, albeit without resulting in premature tyre deterioration.