Another cam idea: The present camshaft could be set a few degrees retarded, by means of an offset key in the crank or cam gear, at little cost. This change results in later closing of the intake valve, thereby reducing compression pressure and reducing the bad effect of low octane fuel, especially at slower engine speeds. This would give about the same result that a longer duration camshaft gives as a side effect at low revolutions, but should not upset carb jetting or needle type.

A thick head gasket may reduce beneficial turbulence in the combustion chamber and make the octane problem worse, even while it reduces compression ratio and pressures at all engine speeds.

If the engine is a B18A, does it not have a vacuum advance on the distributor to supplement the centrifugal advance? A vacuum advance gives extra spark advance at part throttle when the engine can use it. The basic centrifugal timing position or advance rate can be reset to what the engine will accept at full throttle at various engine speeds, and then the vacuum advance can be adjusted --with shims to shorten or lengthen the spring in it, or whatever works-- to regain what the engine wants at part load. A vacuum gauge from the intake manifold really helps, to keep track of what happens at what degree of part load.

Lots of cheap fun in making these experiments, if done with care, one change at a time.

I would also check that oil deposits in the combustion chambers is not part of the problem. Fuel additives that claim to remove valve deposits, like Chevron's Techron-based "Clean-Out" , seem to have cured oil consumption problems in a 1966 Ford 289 of mine that has never been overhauled. It used to foul a few spark plugs and make a little blue smoke. Not any more. When I changed its worn cam timing chain set and advanced the cam another 4 degrees with an offset key, I had to take out all of the spark advance I had added over the years, including the extra vacuum advance. It's a very happy engine now.

Charles Greenlaw
Sacramento, California