I've just had all the cylinders overhauled and chromed on my O-470-A. After the run-up prescribed by the overhaul shop, there was a lot of soot on the spark plugs in the #5 cylinder. There also seemed to be excessive vibration while running (new Lord dampers and the prop statically balanced). Warm compression on #5 was 67; others were in the 70's. I tried a subseqent run-up with the primer capped off--same result. Later, a cold compression indicated 75. I swapped plugs with another cylinder, and after another run-up, the #5 plugs were covered with soot again. When installing the cylinders, I cleaned and inspected all the lifters, performed leak-down checks, and checked the dry tappet valve lash. I tested the resistance of all the spark plugs and megged all the spark plug leads.
It seems that #5 is either running excessively rich or for some reason has incomplete combustion. Do O-470's just have poor mixture distribution at low power? Any ideas?