Continental O-470 after cylinder overhaul

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johnsibole's picture
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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Scott.N's picture

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).

johnsibole's picture

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

grahamgoad's picture

I see this post is almost a year old and maybe the issue is resolved.  As the other comment said the O-470 intake system is very inefficient, here is a tip that I got from Steve Knopp of P.Ponk aviation when I was assembling my O-470R for my 180.  Install all the intake elbows without any hoses and without the cross over tube, then sight down the intake on each side and check the alignment, if any are misaligned swap them around from cylinder to cylinder until you get the best alignment then number them.  On my engine I had to take a couple sets and keep swapping around until it was near perfect, (I have heard of some taking the elbow to the belt sander to change the angle on the surface where it mates to the cylinder, not sure how the FAA would feel about that).  I have an engine monitor and all my EGTs are within 50 degrees and the CHTs are within 20.  At cruise settings the data looks like it could be from a fuel injected motor with balanced injector nozzles.

Graham Goad

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