As above but would say grey smoke is rather more air than fuel (also lean mixture). Black smoke is overrich mixture. A wideband lambda gauge is a must to prevent engine failures (unfortunately the fewest have this kind of gauge installed):

• lean mixture = high heat load, a hole in a piston can be the result
• overrich mixture = cylinder wall and components will wear faster cause the fuel flushes the oil away

Would check the points above before driving it hard.