Under a load the 1949 to 1953 flatheads vacuum drops off which causes the vacuum advance to back off the timing. Because the throttle is opened further under a load than normal driving the cylinders get more air into them. This increases the cylinders pressure which causes the pinging. The vacuum advance is designed to counteract the high pressures under load by retarding the timing. They could have created all of the advance using a mechanical advance, like the hipo Fords in the 1960s did, but this requires high octane fuel because of the extra advance during a load going up a grade or accelerating. You want that extra advance for optimum power if you are racing using high octane fuel. But the average customer wants an engine that will run on cheaper octane fuel. The vacuum advances name is a little misleading because it also retards the timing. It allows full advance under light throttle driving for better fuel mileage and performance. It was engineered so that if it did fail the timing would retard at idle rather than advance. A bad vacuum advance diaphragm would not make an engine ping. The only way would be if the mechanism that rotates the points plate were to get stuck in the advance position. Make sure all of the mechanical and Vaccuum advance mechanisms are free to return back to their fully retarded position. You can also check the vacuum advance diaphragm by pushing the arm in and then putting your finger over the vacuum port. If the diaphragm is holding vacuum the arm will stay in. If not, the spring pressure will push it out. That was fastest way to check them when I was doing tune ups. You also need to check that you are getting the proper vacuum to it.
Early flatheads with a "vacuum brake" use manifold vacuum. That means the source for the vacuum is below the carb's throttle plate. Vacuum will be there until the throttle plate opens up far enough that the air is all flowing through the carbs venturi. There is no more manifold vacuum at that point The late flatheads 1949 tp 1953 use a vacuum advance instead of a vacuum break. That uses venturi vacuum. That means the vacuum port is located above the throttle plate. There is no vacuum there at idle when the throttle plate is closed. As the plate opens and air passes by the upper port creates a vacuum. You measure the mechanical advance using an adjustable timing light by raising the rpm with the vacuum disconnected and plugged to the vacuum advance. You point it at the timing mark with the rpm raised to specification and you rotate a knob on the timing light which will make the timing mark move back towards TDC. Once it reaches top-dead-center you look at the number lined up with the pointer on the knob and that tells you how much advance it took to get the timing light mark back to TDC, then you measure the advance again with the rpm raised and the vacuum line hooked up and compare it to the specification.
Sometimes you can have an engine with poor performance and fuel mileage because of a failed diaphragm. So, then you replace it and then the engine pings and the customer is not happy. Then you have to find out why it's pinging. Maybe two diaphragms have failed. On modern engines the number one reason for pinging is a failed EGR valve. The timing is usually computer controlled so it's usually not the problem. First thing I check is that that EGR valve is working properly.
The pinging could very well be from the winter blend fuel. Here in CA when they changed it each year my '94 pickup used ping on a warm day. The knock senser should sense that pinging vibration and retard the timing but it wasn't, or it wasn't able to retard it far enough. I would check your base timing, mechanical advance and vacuum advance using an adjustable timing light. Never buy one that is not adjustable. It lets you measure how much advance you are getting. You can back off the timing 2 degrees at a time until the pinging stops or try a higher-octane fuel.
