backfiring and sputtering under load

I have an early 1970s lincoln 460 in my 1977 f250 truck. As far as I know it has stock bore and stroke,as well as the heads, the spark plugs are the big plugs 13/16s size, edelbrock performer 460 manifold, 750 cfm holley street avenger carb with vaccum secondaries, the cam is a competition cam part number 34-225-4 high energy with duration at .050- 212 degrees, lobe lift .2795 and lobe seperation 110 degrees. The problem I am having is when I set the timing with idle at 850 rpm it runs smooth, but when I give it some gas around 2500 rpms it starts to backfire through the exhaust, and when it rains the engine will accelerate bearly and backfire more through both ends, carb and exhaust. I have replaced the plugs wires distributor cap and rotor. Last night I was messing with it and was able to trun the idle mixture screws both of them together all of the way in and the engine was still running. The timming I have it set at 20 degrees BTDC anything below that it sputters and falls on its face under throttle. Please help.

Sounds like you are running extremely rich. How much vacuum are you getting? If it's too low, you may be getting gas from the power vavle at idle. Are you getting any smoke from the exhaust? (Gray/black usually means rich) I'm not too good with holley carbs, but I know for a fact if you can close both idle screws and still have it running you have a problem. The gas to keep it running is coming from somewhere. I would check your float level. I believe with the holleys you take the sight plugs out of the side of the bowls (big screws on the side of the carb) and adjust the large screws on the top until fuel just starts to trickle out of the holes. Also, since you are getting carb backfires, you may have blown out the power valve. Also check the fuel pressure if you can. Hopefully someone else can chime in..my knowledge of holley carbs is not so good.