454 fine tuning
 

Hi everyone, names jay. I have a few issues with my truck, maybe you can help. I have a 88 k3500 w/454 and a th400 tranny. I recently did a full rebuild which included removal of my tbi. My combo is a Holley 770 street avenger, edlebrock performer intake, Heads off a 72 corvette(i dont know the specs), Stock cam, Stock crank, dished pistons, and a set of thorley headers. My ignition is a MSD 6a, Blaster 2 coil, and a ready to run distributor. I curently have it timed at 14 degrees Advanced. That said, my problem lies with my carb, i think. Im getting a lot of random backfires out of the carb when i give it gas. Not always, just every now and again. I even had a fire burning in the carb while it was running! So i figured it must be to rich. But every thing points towards it being lean. I know its not the float level, if anything its to low. My air/fuel gauge(even though not totally accurate) is on the lean side. Then i thought it was timing, but it seems to have the same problem no matter where its timed. i read something about power valves having a tendancy to blow out, except for carbs after 92 that have blow out protection(which i have). What the hell, why not try it anyway. I was told my power valve should be 1 to 1 1/2 inches hg below the average engine manifold vaccum at idle while in gear. Well, when i tested it my average vac is at 15! Does this mean i need to go from the stock 6.5 power valve to a 13! That cant be right. That would make it backfire more, right? What should i try next? Is there something really simple I'm overlooking? Why is it acting like it is too rich but seeming like its lean?