Congrats on using an EXCELLENT carb.
Comments/suggestions:
- Edel AFB/AVS springs will work. Use this method to test for best springs. Loosen met rod piston covers, twist slightly, retighten so that you can see the piston, but piston is still captured. Go from softest to stiffest spring. Warm up engine, idling, in gear; pistons need to stat DOWN & not move. Change to stiffer spring & re-test. If piston moves, spring is too stiff. You want the stiffest spring that just stops the pistons moving.
- if ^this does not help, try a larger pri jet or a met rod with thinner power step; you can file them.
- you have gone to 1/4" larger primary barrels/venturiis. That reduces signal strength AND atomisation quality. It can also change the a/f distribution in the intake. Many of these AVS carbs had snivs riveted to the pri booster to re-direct air; a real band aid fix. It might only be one or two cyls running lean, no way for the home mechanic to really tell.
- do everything you can to get the engine running cooler; heat is the enemy of detonation.
- the springs for the centri weights in the dist may have stretched, allowing timing o come in too soon. You could try heavier spring, remove metal from the weights.
- remove the carb & check the gasket against the base for leakage. I doubt you have an air leak. While removed, check how much T slot is showing below the pri blades; should be 0.040" or less. More than this will require bypass air & enlarging the IFR.
- how much throttle opening is being used when you get the pinging?
- float level correct?