Putting this here since"Jeeps with the Buick heartbeat" are usually my 231 odd-fire. I'm at my wits end here with this motor. I have suspicions but I don't know how to confirm them. Here's the situation Since I've owned my Century it's had this issue where under load, especially at WOT at low rpm, the engine will backfire out the carburetor primaries (it's a dull grey backfire not burning) and the car will stumble on its face until throttle is let up. Sometimes, it wouldn't do it at all and I would drive 50 miles with no hiccups. Other days like today I can barely get to the gas station without treating the throttle like a baby. I've done so much work on this thing I don't know what else to do short of have the entire motor rebuilt from the ground up. Here's a list of things and their effects -Buick 231 Odd-Fire v6- Replaced plugs and wires multiple times - passenger plugs consistently lean, number 2 white white white Brand new rebuilt Delco HEI distributor with a Blue Streak coil Had the 2GC carburetor rebuilt Added fuel pressure regulator Blocked off EGR and all other emissions Rebuilt TH350 transmission New PCV valve Confirmed timing 12' at 900 rpm idle No change so I started getting a little more crazy - I thought maybe intake manifold leak between ports SO Replaced intake manifold with a 4 barrel Offy and a Slayer 450 carb - old pan gasket was NASTY Runs GREAT but still backfiring with lean plugs on 2,4,6 Jetted up bit by bit from 55/60 to 65/70 - 10 sizes up is quite a lot, engine liked it every time I went up Found a 4 barrel snorkel air cleaner hooked that up for Thermac in the cold mornings - engine liked that on the highway a lot Every time I would jet up it became harder to backfire I climbed a pretty long hill and almost teared up when it did it without jerking until today it just fell flat on its face and barely accelerates at all. STILL HAPPENING No mechanic has been able to diagnose this issue or is to lazy to do so and just wastes my money. They all just say "it's a problem with the carb" Well I have a brand new fuel delivery system so obviously that wasn't it. Here's my diagnostic data Vacuum at idle 18-19 in, perfect vacuum response across range, quick little shake between 18/19 Fuel pressure 6.5 (per carb manual) and stays that way during backfires Sparks appear strong Compression is good 140ish one cylinder was at 128 still within spec per factory manual Rockers all moving smoothly nothing seems stuck No external vacuum leaks that I can find. No coolant/oil consumption or leaks. All ports were pretty carbon-caked up when I had the intake off, cleaned those up Any ideas? I'm too deep in to this thing to "put a 350/455 in it" so don't suggest that please - been down that road. Everything I'm reading either says it's running lean - but I've already jetted up 10 sizes - or something with a sticking valve, worn timing chain, piston issue all of which would require some serious work.