I don’t know what is the type (natural vent / induced draft) and efficiency of your furnace but since it is a new appliance utilizing a B vent (category I appliance) it is probably somewhere around 80% +. The flue gas temperature is typically above 275°F, closer to 400°F / 450°F (higher efficiency – lower exhaust gasses temperature).
The further you touch the vent pipe the cooler it should become but close to the furnace collar it will definitely burn your skin within a few seconds. It is also the hottest at a startup and once the chimney draft has been established the pipe should cool down a little since the exhaust gasses velocity increases.
There may be some obstruction in your chimney as well which would increase the temperature further and could result in Carbon Monoxide poisoning (make sure you have CO detectors installed
in your home) – get it checked just to make sure that it is clear.
I wouldn’t make any adjustments at the gas regulator unless you’re certified to perform them.