block MASS if added indiscriminately can make matters worse, unless it is added to the mount the engine is attached to.
rotating mass is good too, the old rule of thumb was that a 12" crank handle an average man could wind up up to a ton of rotating mass without too many problems.
I once saw and started a 1.5 hp 700 rpm lister d that was coupled to what must have been 3/4 of a ton of flywheel mass (an old press brake flywheel) which was used to store up energy for a big (AFAICR IRO 200 ton) press brake, that D would make a quality electric motor feel rough, after the brake was tripped it took a good minute or so of full throttle to wind the flywheels back up and the governor tripped the carb back to tickover
as geno has discovered, extra flywheel mass is always a good thing, and for stationary engines running at constant rpm it is quite hard to get too much of a good thing, I suspect around 3,000 lbs on a 6/1 would be getting near the point where windage and bearing losses became significant