These are some notes I wrote in response to a message on the Toro list. Various theories had been advanced regarding excessive ping (knock or pre-ignition), you will see from the text a partial response to them.
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First of all, back the timing off until it doesn't ping. THAT should be your starting point.
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Just my 2c worth. When a distributor is calibrated by the manufacturer, they have to chose a setting that "most" examples can live with. As the production tolerances ensure that there is variability between each distributor/engine combination made, the settings chosen are generally slightly conservative, i.e. the static advance chosen is one at which about 95% of cars are OK. This means that for most they will take increased advance over the factory start point, but for a few, they need less. Then factors like fuel, age of car etc. etc. come in. The only way to fine tune the static advance is by driving the car, the factory setting should be seen as a get you started point. However, if you are more than about 2 degrees retarded from that, I would start to wonder why (could be modern reduced octane gas). If you can take more than about +5 degrees, again I would start to wonder why.
In a modern car _light_ ping is an OK situation, it is deliberate and does no damage. Usually in rain it vanishes. Light ping will only be OK at partial (light) throttle settings, it will always go if you open the throttle. If it doesn't then either you have a busted vacuum advance or another cause of ping. I think this is a function of unleaded gas having different properties than the leaded kind (not just the octane...). If the ping develops into a deep rattle, serious and expensive damage will result, the piston crown will start to break up and spall off, but not straight away!
If the engine is overheating the plugs might show why. It is hard to describe an overheated plug in words, try to find a photo, but they look clean and wrong! They have a sort of white appearance and the electrode is a bit glazed. I am assuming that you have the correct temperature grade plugs fitted. If the plugs have overheated do not re-use them, fit new ones. I think the state of the plugs is a better guide to overheating caused by too weak a mixture or wrong plug grade than the temperature of the coolant. Of course if the radiator is not cooling properly then you get overheating and that will result in ping, it is all a matter of chicken and egg.
I'll be interested in other people's views, but if you get ping with virtually no advance, (and vacuum advance disconnected, but plugged up on the carb. side) then you have a carbon build up problem (better than 80% sure and assuming mixture about right). However, if you don't then carbon build up is probably not the cause. If the ping goes with a modest retard I would tend to look elsewhere. A heavy carbon build up starts to act like a glow-plug in the combustion chamber resulting in premature ignition. In really serious cases it significantly raises the static compression ratio too, but compression ratio is a very complex subject. Basically at light throttle settings and modest (below 3000 rpm) engine speeds you can forget carbon build up as being a cause of ping due to compression ratio changes, there is not enough fuel/air in the combustion chamber to make any difference to anything. Remember that it is only at highish revs and full throttle that the combustion chamber fills with its maximum amount of fuel/air, the rest of the time (like most of the time) there is less, far less, in.
Modern gas mixtures have detergents in them that make carbon build up much less likely than before and "de-coking" a car is a rare event. Before the war it came round say every 20,000 miles. This improvement is also a function of modern engine oils. Still, at 200k miles a de-coke might be useful. Modern (port fuel injected) engines need the valves kept clean of carbon on the induction side because the carbon acts like a sponge and causes an erratic/confused mixture response. The fuel should be sprayed at the hot inlet valve where it instantly vaporises and is then sucked in as a gas. In a carburettored motor, the ideal is for the fuel to be carried in a very fine droplet form in a fully mixed air stream. In practice the fuel runs down the outside of the induction port and is sucked into the piston as great gobs of liquid which then have to be vaporised before they can burn. This is one reason why fuel injected cars are more fuel efficient. However, it does mean that carbon build-up on a carburettored car happens less on the valve stem/port side of the valve and on-valve carbon build up matters less anyways.
As another test, use a different octane fuel. If reducing the octane makes no difference then there is no point is using octane enhancers. Once you have the right octane level for the engine, making it higher gains nothing except lost $$. Also try another brand of fuel, sometimes that can fix the problems. If using Texaco, try Shell or Chevron or Exxon. It will take a tank or two to work through though. I think I would start with running on lower octane for a bit and see what happens. If things immediately get very bad then shove in a booster and go back up. If a lower octane looks like being OK (just), and taking it over 100 makes very little difference then I would suggest that the problem is not compression ratio related.
It is _possible_ but unlikely that the valve timing is out.
PS, Pinking is an alternative term for dieseling, i.e. when a car continues to run after you switch the engine off. Some people call dieseling pinking. These notes have no relevance to dieseling.