A couple of years ago I bought a Q-jet from Mark at Qjet Power for our '69 Chevy C10 w/ stock 350 engine. It's been a great carb, but lately has us stumped. We've had a wideband O2 gauge installed for the last couple of years, which is very helpful in setting idle mixtures and seeing what's going on as various issues have cropped up (like a crazy vacuum leak, etc). (it's almost too much info some times) When it's running like it should, the jetting built into it is perfect - 14.7-15.0 at idle, 16 at cruise, and 12.5-13 under load, transitioning into the power circuit right when you'd want it to. Symptoms: The A/F mixture ratio when returning to idle is erratic - that is, while driving and you come to a stop sign, sometimes it will be right at 14.7-15, and the next time it may well go as lean as 19-20+, with rough running and engine dying. Pump the pedal and restart it, and the idle may be normal, lean, or slightly rich. Next stop sign, it may be normal or lean again. It rarely goes rich, and then it might be 12-13, most often it leans out. Above idle / transition (cruising down a road or highway), it's fine - generally right around 16, with great transition into power and secondaries. History: 1. A year ago we had problems with the old fuel tank sending dirt to the carb, so now the truck has a new, clean aluminum fuel tank, an electric pump regulated to 5psi, two fuel filters, and we've taken the carb apart a couple of times, spraying out all passages and double-checking float height, etc. Since the new fuel tank I haven't seen any dirt, dust, or anything inside the carb. 2. We had a freaky issue with the intake gasket allowing a large vacuum leak, and replaced the gasket with a thick Cometic gasket - forgetting to open up the holes for the exhaust crossover passages, which meant we had to reset the choke. 3.Ignition is a GM HEI unit with relatively new plugs and wires, maybe 15K miles on it. 4.When it's idling correctly, it pulls 19 inches steady of vacuum and is rock steady. I haven't gotten a vacuum gauge on it when it's acting up yet - it behaves fine when at home (of course). 5. Engine temps are always a solid 195, Voltage is steady 14.5V when running. 6. The only vacuum ports are: Brakes, PCV, Distributor. 7. Transmission is a 700R4. I'd think something is intermittently plugging something in the idle circuits, but for the life of me I can't find anything in there. Maybe Carb icing because of no exhaust crossover - in West Texas? I'm grasping at straws now - Any ideas?