black plugs is a good indication of a blown PV. I have backfired many times and never have I blown one (i have the adpter for my mityvac that tests them).

You could buy a PV plug and install it and if its the same sympton, its nt a blown pv.

that being said, my six pack does (and always has burned my eyes). they are hard to tune when you get a big cam in there (over 484/282 or 240 at .050)

What you really need to do is set the idle where it will run then remove the carb and look at how far open the throttle is. its probably open too far causing you to run on the main circuit because the transfer slot is open too far.

upping the timing to 16-20 dbtdc will speed up the idle so you can close down the throttle and get back to the idle circuit.