Carbon build up

Just pulled my iron SCJ heads off what causes carbon to build up on the pistons,valves and combustion cambers it's comes right off the pistons with a razor blade I'm have SS valves and harden exhaust seats install and would like to know if this is normal if not how to keep it from happening once I get it back running thanks for and help

cold engine operation, rich fuel mixture, Short trips, cold operations, never having the engine up at temp.

Sound like cold operation in my case for years I would just start it and let it warm up and shut it off that won't be the case when I get it back running I plan on driving it like I stoled it thanks for to info

Oil consumption also.