I have a /6 installed in a rat truck we are building for my son. The engine has a fresh overhaul and ran sweet as could be until about a month ago. It started running like the timing had jumped a tooth on the timing chain. We pulled the timing cover and it was all ok and timed correct on the chain. The pin was still intact on the cam and was solid. We pulled it off to double check. We changed the carb to a different one. We switched the plug wires, coil, rotor and cap for new ones. We installed a petronics electronic ignition. We have check for vacuum leaks and found none. It still backfires through the carb and is hard to start. The wires are installed correct with #1 cylinder one top dead center with the rotor at #1. The firing order is correct 153624. The compression is good on all cylinders and the cam seems to be operating correct as the rockers are all going up and down. I'm stumped and a couple of friends have all been stumped as well. A /6 should be a no brainer to make run but it has me scratching my head. Any ideas?