Its normal, moisture from the air is turned to steam in the cylinders and condenses in the exhaust pipes and muffler and is then forming water droplets at the exit of your tail pipes and carbon collects there street mufflers have small weep holes in them to drain this condensation! Dont worry till you get clouds of heavy white steam that's usually damage!! Larger carbs and cams increase the moisture drawn in with more air and you see more at the tail pipe! Mine blows smoke rings with the lumpy cam my grand son thinks its cool LOL But when pipes all get hot you wont see the steam but as long as the pipes are cool the water will condensate! The longer the pipes the longer it takes to get hot!!