I’m humbly throwing in the towel and asking for some opinions/guesses on this one. Sorry a little long - but hopefully covering the details... I had been running the same fuel set-up for 4 years with no troubles. It was probably a bit undersized, but never had any issues. Fuel cell to Mallory fuel filter to Summit electric fuel pump, to Aeromotive dead head regulator. Regulated to 6psi, all 6AN line. The car has a 408, 10.3:1 pump gas, 850 Quickfuel carb. I shift between 6k – 6.5k. The car has been primarily more track than street over the last few years. Just a couple months ago, I’ve all of a sudden gone from always being a bit rich on the A:F to really lean. One event all is great, and the next event the car falls off about 3 tenths and a couple miles an hour. I check plugs and really lean, and now I cannot get the car rich. All plugs look uniformly lean. After cleaning the heck out of the carb, cleaning the fuel filter, cleaning the fuel cell, and blowing the fuel line clean, I decided to start with carb jetting ~ knowing it shouldn’t be the culprit since it has been fine for the last 4 years. I kept my 4.5 power valve in (but replaced with new), dropped the high-speed bleeds to .027 (reduction of .006), increased the PVRC jets to .057 (small increase of .002), increased the primary jets to 84 (up 8 sizes), increased the secondary jets to 92 (up 8 sizes). Performing these changes incrementally made no changes to the plugs or performance, and having ALL these changes at once also made no changes to the plugs or performance. Also new bowl and metering plate gaskets, and new gasket between carb and manifold, no cracked or missing vacuum port plugs/caps. With no change to anything with attempted carb tuning, I started changing fuel delivery ~ which I assumed was my problem all along. Started with checking flow of my electric pump. The pump was getting old, and sounding a little different than typical, and checking flow into a fuel jug appeared a bit slow. So out with the old and in with the new. New 10AN supply from cell to new fuel filter, 10AN from filter to new Aeromotive 11203 pump, with 8AN from pump to Mallory regulator, and 8AN return all the way back to the cell. Fuel regulated to 6psi, fuel level is middle (or slightly above) carb view window. Figured that would do it. No change to plugs or performance. I did another test of running the regulated fuel supply line into a fuel jug, and it appeared to flow fine (I think 5 gallons in 1:36). Since it is uniform across all cylinders, I really did think it is/was a supply problem. I thought maybe a leak below the carb like intake gasket, but it would have to be both gaskets, and leak to all cylinders. Seems unlikely. All seems normal when engine is running and I spray carb cleaner all around (where I can reach) around the manifold. I’m a bit stumped. Perhaps I am missing something super simple. Anyone have any experiences or words of wisdom on this? I feel like I’ve been chasing my own tail on this one. Thanks for any help or comments!