Thread: 396 backfiring under load, need some expertise...

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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.