Eventually, it will not run strong and OK.
While carbon deposits build up, at least three bad things happen:

1) Heat transfer from the combustion chamber to the fins in the head is reduced.
2) Carbon crests and points stay incandescent (like the glow plug), and initiate the combustion at the wrong place and/or time. That messes up the time and place at which the expansion wave hits the top of the piston.
3) The volume of the chamber is reduced, which increases the compression rate, leading to detonation.