So it seems from posts on here carbon can stop noises of a knackered engine, quite true, so why not recondition it then.
It is very true that all engines will have carbon in the oil as a waste product of combustion leaking past the rings, this is what blackens the oil.
Carbon on a piston will do nothing to promote a better seal, as the only seal is between rings bores, so any carbon is ground away by rings anyway!!
Some engines can indeed become noisier, engine flush is not always the correct stuff to use for noisy tappets, hydraulic valve lube is though, but again depends on how far gone they are.
I always do it at every change, with oil changes at 12,000 miles, my cossie has 163,000 miles without a bottom end rebuild, still going perfect and has regular dyno sessions for hours at a time(doing developement stuff), and I do use it very very hard.
I use wynns flush as it is not as harsh as others so disolves gently the crud/carbon, I leave it in for a minimum of 200 miles before oil change.
A cheaper way to do it is to put some DIESEL fuel in the oil, as this has the same effect.
tabetha