Continental O-470 after cylinder overhaul

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Continental O-470 after cylinder overhaul

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?

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O-470's don't have the greatest induction so you may be having unbalanced mixtures to each cylinder. Is your balance tube at the front of the engine plugged or leaking?

Cylinder compression limits are usually good in the 60's too. The orifice in your differential pressure tester indicates what the lowest value allowed with that tester is (when you open the orifice valve with the outlet hose capped off).

Thanks! The crossover tube and all other joints are sealed well - but I will check the crossover tube for blockage.

John W. Sibole, Jr. A&P/IA

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