Ive recently adjusted timing on the 340. 15 degrees initial 38 total. demon sizzled thermoquad ld 340 intake during this adjustment i plugged the vacuum advance. Motor fresh rebuild stock j heads, comp 284, 3000 stall 323 gear in 72 Cuda. When pressing pedal hard out of the hole i have nothing let up a bit and here we go, while driving 30mph punch gas four barrel opens with the sound of valve ping,My timing light is old. Thanks any help appreciated.

Back the timing off until the detonation goes away.

Back the timing off slightly and see what happens. Never heard of the valve ping term though

you should be around 30-34* degrees total advance w/o vac can hooked up.

what kind of cranking psi does the motor have?