answer:You are correct on both counts. When an engine is not yet at operating temperature, it requires a richer fuel mixture to run. This is due to the greater friction of the more viscous oil – more power is needed at a given rpm to avoid stalling. If you’ve ever driven a car with carburettor induction, that is the function of the choke. Late carbie cars had an automatic choke, and modern cars are fuel injected, so the car takes care of this for you. As you increase the revs, the engine is receiving more than enough fuel/air to run despite the increased friction (there is no risk of stalling, like at lower revs), so it reverts to a more standard fuel/air mixture. You’ll also notice that the engine will idle slightly faster when it is cold. Also, when the engine is cold, the components are not at their operating size. When metals heat up, they expand. Therefore a cold engine is running with slightly larger gaps between moving parts, which decreases efficiency, and allows more unburnt fuel through into the exhaust manifold. This will increase your hydrocarbon reading. It also isn’t the best idea to be revving an engine too much when it is cold, as the greater oil viscosity means more wear on the moving parts. I personally change gear at 2,500 – 3,000 rpm in my car when cold, but can rev out to 7,000 rpm when warm.