[Bob Phillips]

Thanks Malcolm and David for your thoughts.
I think maybe you have it- aircraft plugs are too difficult to service in the field? seems to make sense. Much of what I have read suggests radials have a bad habit of fouling plugs and need frequent changes of plugs. So this could be due to a too rich mixture at low speeds or too much oil working its way up to the plugs? I have pulled apart engines where the top rings are all seized- probably due to oil leakage toward the combustion chamber. Too much oil will certainly foul the plugs, an ongoing problem in radial type engines.