914 Popping and backfiring out exhaust, Need help! Options

[SolarRT99]

I'm at my wits end with this. Background on car. Originally a 74 1.8 car. Now has a 1.7 engine block with an A&A 1911cc piston kit. All new bearings, scat c45 cam with new scat lifters. Rebuilt 1.7 heads with hp springs, shims and elephant feet adjusters. Pertronix ignition with flamethrower coil. Dual Weber 40's. SS 2.0 Heat exchangers with Sebring exhaust. New cap, rotor, plugs, and wires. I have set the valves 3 times and it still has a pop out the exhaust at idle. Carbs were rebuilt last year. I think 3 and 4 aren't firing completely even though it idles fairly well. I crawled underneath with my nc temp gun and 1 and 2 are at 500+ degrees at the exhaust pipe right off the engine, 3 is right around 350 degrees and 4 is around 220. Could it be a bad carb? I tried new plugs and wires and nothing changed. Any thoughts or ideas appreciated.
Thanks

pics here

https://drive.google.com/drive/folders/0Byp...d28?usp=sharing

[SolarRT99]

Well after a 10 minute jaunt around the neighborhood, was running well with good power. Towards the end the valves got loud and sound like they really need to be set again. Is this common with a new build, cam/lifters? Also the valve covers are leaking, come to a stop and get some smoke from underneath, brand new victor cork gaskets. I tacked them to the cover with some silicone to hold in place, but still leaking. Oh well, will have to wait until next weekend again. (IMG:style_emoticons/default/sad.gif)