new here need sbc timing/vacuum advance help
I have a fresh 421 sbc brand new everything including hei distributor. Howards hydraulic roller cam close to 600 lift w 1.6 lifters (still makes good vacuum based on specs for sure and I can verify it does) Holley Ultra Billit 770 carb on Vic jr manifold. 10.77 to 1 using gapless top ring.
OK> problem is setting timing at 10 at idle of 950 w no vacuum advance hooked up and goes to 34-35 at 3500 to 4000 THEN I hook up the vacuum advance and it jumps to 38 to 40 at idle?!?!?
Also getting some popping/backfiring off idle when throttle is applied? ?? WT (HECK)? Howards said to set lifters at 1/4 turn pressure and I set them at about 1/3 turn from zero lash ( could they actually be staying slightly open from this tiny amount extra and be creating the backfires? )
New to this whole sbc gig so any help would be great for sure.