V8 60 ignition timing question?

[1938 Coupe]

I am hearing pinging as I power up under load. I have tried to adjust the vacuum brake which is new without it making any difference. Is the ignition too far advanced and how do I retard the timing. I did notice the plate with lines on it and a screw in the middle on the lower right side of the motor under the water pump. Do you adjust that and how? Up or down or turning the screw?
Any suggestions would be appreciated.....thanks!

 

[flatjack9]

Loosen the screw, move the screw and plate up to advance and down to retard.

 

[mrfliboy]

Picture worth a 1000 words

 

[Bruce Lancaster]

Screw at center line is normally quite close to specified 4 degrees; rotation of distributor viewed from front is CCW, moving screw with that is retard, agaibst is advance.

Ruler method for 3-bolt distributor, stolen from a Ford patent drawing, is down in this post:

http://www.fordbarn.com/forum/showthread.php?t=62659&highlight=helmet+ruler+method