396 backfiring under load, need some expertise...

So my car has a new engine in it. I've put about 300-400 miles on it so far. It was running good and strong but it is a little rich on bottom. It's a 750 double pump holley with 78 primary jets and 86 secondaries. It hasn't been tuned yet. I had changed the spark plugs 2 days ago. It is running rich as the plugs are dark. It has never backfired though this whole time until yesterday when I took it out and got on it a little bit. At idle, revving it doesn't make it backfire. I can cruise it and it won't backfire. But the second I give it some throttle while driving it sends out multiple backfires out of the exhaust. I was thinking it could be a powervalve? I already checked the valve lash and the valves were in spec, it has a solid lifter cam. None of the valvesprings are broken. I'm thinking either bad spark plug wires, blown powervalve, or blown headgasket? You tell me what you think...Oh...and this is a very aggressive backfire. It will accelerate then start popping loudly out the exhaust and lose all power. No backfire while in nuetral revving it.

So far I've checked:
-Valves in spec.
-Valvesprings are not broken
-Cam is not flattened, all rockers push down evenly.
-New sparkplugs(2 days old)
-Spark plugs are black but a couple had some white patches(could be a sign of water getting into chamber)
-I haven't checked to see if oil is in the coolant yet, but there is no water in the oil...
-Fuel pressure is good
-The timing was set at about 36 degrees total. The distributor was tight so it shouldn't have moved.
-Spark plug wires are in the right firing order
-I know it is running rich on bottom...but the car has been running fine like this for a month until now...
-I have a edelbrock air/fuel ratio meter hooked up and it reads 12.5 to 1 through all the RPM's. I haven't had a chance to fiddle with jetting yet.

Thanks for any ideas.

A bad plug, especially since you just replaced them. New ones can be bad. When the pressures go up at full throttle the plug can't handle it. You ruled out the obvious causes, a flat cam lobe or wrong valve setting, if you are sure about these two things, the next probable cause is a plug. Or a plug wire.

Thanks. I have some new plug wires which I will put on soon. I don't think it is a faulty spark plug as it was running really good after I changed them. It was a couple days later that this started happening. But I guess it could have fouled them already being that it is running rich... Could it be anything else? And could these symptoms be happening from a blown powervalve?

Change the plugs again, install the new plug wires and try it once more...if you are positive that the cam is good then replace the power valve. Holleys from my experience are notorious for blowing them and they can produce the system you describe under load. One other thing, might seem far fetched, but I've had it happen to me with a Holley 750, the base plate of the carb may be warped and not sealing to the intake...this too can cause backfire at load

Back off the fuel pressure to 5 psi. I know they say Holleys can run higher, but what is it with this obsession with some of you guys to run the pressure right up to the max??????????????? If you run lines that are large enough to handle volume, 5 psi is all you'll ever need.

69elko, I'm not pickin' on you. It's just that your thread has provided me with an opportunity to rant about excessive fuel pressure.