Odd thing happens under full throttle=can heard almost like a backfire noise

It has been a long time since I fooled with carbs, but.
Maybe the secondarys have a temporarily lean situation when the butterflies open in the secondaries.
I used to have some Yamaha 500's(4 cyl 4 stroke singles).
The stock carbs had a whole separate antibackfire circuit- when the throttle was suddenly opened- without the circuit the air would temporarily "beat" the fuel and it would be too lean to fire in the cylinder- but once the too lean mixture made it to the exhaust- it would ignite and you would get a slight backfire pop.

Well- it was a long time ago- but that is how I remember it.
I would try to do something to richen up the midrange!
How is the midrange adjusted on that carb? My memory of car carbs-late 70's dual feed dual pump Holley 650 on a 71-Cuda 383 is my last car carb memory(came with a Holley vacuum secondaries and actually worked better/faster with the stock carb- but I was young and thought I could make it faster)

Yeah maybe you have a temporary lean situation-take a while for the fuel to catch up with the air.
Charlie

Accelerator pumps take care of this on some carbs- but only when you really hit the throttle- not with normal vigorous less than full throttle acceleration.

Are there still Western Autos?
When I was a kid living in Gramercy,LA( South Louisiana-40 miles upriver of NOLA- FROM 1958-1970)- there was a Western Auto on Main Street.
It was a general hardware store auto parts store.
I would buy 22 bullets there-still remember it was 39 cents for shorts about 50 cents for Long Rifles.
My Winchester 190 would take shorts(21 or so) Longs(about 19) and Long Rifles(about 17) so I always bought the cheaper shorts.( I bought the winchester mail order in about 1963 for $42.50 delivered-think I must have lied about my age(I was 12) or had my parents fill out the mail order form)

Memory LANE-Western Auto also sold guns- seem to remember tham occasionally selling surplus Mausers(7.92??)-the rifle-Argentine usually-really cheap.

I would try bigger jets-see what happens.
Carbs are easier to fool with than EFI-but I love EFI.
There just wasn't any way- other than seat of pants and ears-to tell if you had the jetting correct.EFI does all that automatically.
If was fun playing with carbs points distributors but the more modern stuff just works sooooo well.

What is the stock HP rating of that 454 -with EFI-maybe 300 hp 450 lb ft ?? It should make about as much peak power with the carb, but it will be thirstier-not that anyone who buys a 454 is much concerned with mpg??
It should be a beast once dialed in.
The PO spent $$ rebuilding it, but then cheaped out(ran out of $$ maybe) and installed a worn out carb.
It should run hard once you dial it in.
It must have originally had a throttle body FI- maybe it had a mass air sensor in the air box snorkle ?
Did it originally have some sort of O2 sensors in the exhaust-something that reported back to the COMPUTER-ECU- TO ALLOW IT TO DIAL IN THE FUEL??
Guess it would be tricky costly to try to revert to FI??
I LOVE THE WAY MY 98 5.7 RUNS-still runs great despite 217,000 miles-22000 of them are mine.
I wouldn't object to having 454 power torque- but the gas cost would dent my pocket book!!
No one who buys a
Your truck LOOKS GOOD in the pictures- is it slightly lifted or is that stock 3/4 ton 4x4 setup??
Body looks good-it must not have been a mid west NE truck- or it had a very careful owner.
Where are you and where did it come from??
Suburban are real bargains now-gas goes up in price-Suburban become cheap to buy.They are the best bargain you can get--you get full sized utility for maybe $2500-$5000- in a easy to repair vehicle-get parts anywhere and parts are generally inexpensive.

Charlie