You need a handheld vaccum pump to test the retard valve , even if you could see it movinng slightly with the mouth only .Alternatively plug it directly to the manifold side nilples as a test.
I doubt the preignition is caused by an inoperative retard valve, most people have it disconnected anyways, including myself. It makes the engine run hotter as a way to burn NoX. So when the engine is too hot the vaccum flow to the retard valve is cut through the thermostat vacuum switch on the right side - back of the engine, where 4 vaccum lines plug into . The thermostat may be broken or connections to it not right .
That said , other possible causes : wrong timing, should be 10 or 12 degrees BDTC , but more likely mixture too lean . Pull a couple of plugs and check the colour . And as said here also, wrong plugs fitted a likely cause .
The workshop manual describes a process involving disassembling it and submerging it in hot oil, 104C, then blowing sequencial through pipes and see how they connect depending on the temperature . I would not bother and would put the retard valve out or work unplugging the vacuum pipe and closing it with a screw . You can also do that as a test and see what changes .
You did not tell us what is the temperature of the engine when you have pre ignition , where is the pointer located in the gauge ? Mine never passes the middle point .
Thank you for the information Rui. Much appreciated. My temp never passes the middle point of ‘normal’ in open road driving and during winter driving sits on N. I live in the Sunshine Coast, Australia, where our summer temp can be well into the 30’s. I plan my trips to not be driving in traffic in the middle of the day. At the middle point M of normal, my twin radiation fans come on and I have never seen engine temp rise above the L.
If I land in heavy traffic in the middle of the day in Summer, I switch the motor off when stationary in traffic.