HC shows a moderate increase and CO shows a large increase; what is this likely indicative of?

A moderate increase in hydrocarbons (HC) alongside a significant increase in carbon monoxide (CO) is indicative of a rich fuel mixture in the engine. When the air-fuel mixture supplied to the engine is too rich, meaning there is an excess of fuel relative to the amount of air, it can lead to the incomplete combustion of fuel.

In this scenario, the combustion process does not have enough oxygen to efficiently burn all the fuel, resulting in higher emissions of hydrocarbons, which are unburned fuel particles. The large increase in carbon monoxide, a product of incomplete combustion, further supports this conclusion. CO is produced when combustion occurs in a fuel-rich environment, where there isn't enough oxygen available to fully oxidize the carbon in the fuel to carbon dioxide (CO2).

Additionally, while conditions like an ignition misfire can cause increases in HC levels, they typically do not correlate with a significant rise in CO. Similarly, lean mixtures and advanced timing would not lead to this specific pattern of emission increases, as those conditions usually result in different combustion characteristics. Hence, the data provided on HC and CO levels strongly suggests that the engine is operating with a rich mixture.