This is an interesting situation to me, I've battled with ignition knock many times over the years with Brit sports cars etc and have never really been sure that I've
cured the problem with the right approach-the ideal is to run the engine on the lowest (cheapest) octane possible, right?
The "octane" of gasoline can be viewed as a measure of the relative speed of combustion of that fuel with another gasoline of a different octane level, the higher the octane #, the slower the burning process or flame-front during combustion of the fuel in the cylinder.
Too fast of a flame-front (lower octane fuel) can result in a secondary flame-front developing and the result is more than one combustion event in the firing cylinder and therefore the characteristic knock or pinging noise of the uncontrolled, secondary combustion process in the same cylinder.
Some of the contributing factors that I've learned of to this secondary combustion in carb. engines can be any one or more of the following conditions: quality and octane consitency of the fuel used, incorrect initial timing setting, incorrect operation of either the vacuum and mechanical advance of the distributor ,altitude (timing should be referenced to the altitude range in which the engine is operated), incorrect type of spark plug i.e. heat range and exposure of the central electrode of the plug, exhaust valves not sufficiently seating and therefore cooling correctly thus causing pre-ignition from overheated valve "land " areas (check them to the specs shown in the shop manual), the actual compression ratio of the engine, temperature of the engine's induction air at the carb.
-Have you had the total ignition advance of your distributor scoped at all ?
-What's the characteristics of your engine? i.e.- 1.3 lt with what stage pistons, stock or high compression?
- What readings are you getting for warm cylinder pressures with a compression gauge (throttle fully open)?
-Did you have proper a valve grind when you rebuilt the head ?
-Have you tried different heat ranges for the plugs if your running higher than stock compression?
-Do you have the correct thermostat in the cooling system- this will determine the correct operating temperature of your induction manifold..
Keep us posted, I'm curious to know if you've founs a solution yet.
Best o' luck