so first im french so sorry if you have hard time understand

my rebuild 350 run very perfect with the quadrajet carb and at full throtle all the power is there but if am at 25-50% and floor it to 100% the engine die for a second and then all the power is there and everything work perfect
if i goes from 25% to 100% in 3-4 seconde there is no problem the engine goes to 100% without any hesitation
i dont know exacly what to chek , the air filter is new and its a 3x14 inch so i dont think my problem is air starvation and im not a pro on carburator i dont wanna mess all the carb because everythink else work perfect

 

You might check accelerator pump shot, But a disclaimer, I know only Holleys. But that would be my first thing to check. Good luck!

 

Sounds like the secondary air valve spring tension is too low.

 

Is this spring ajustable ?

And for the fuel pump would i have touble at full throttle always ?

 

Yes. It's very easy. Loosen the set screw, which is on the passenger side immediately underneath the air valve shaft, using an Alen wrench (I think it's a 3/32" but I could be wrong); using a small straight-blade screwdriver in the slot that's in line with the shaft, adjust it until the blade has exactly zero tension (just barely begins to open by itself under its own weight), tighten the spring about 5/8 turn in the clockwise direction; then while holding it there, tighten the set screw.

What the air valve does is, as the engine's demand for air increases due to the secondary throttles being opened, it also opens, but more slowly, and only opens to the extent that there's air flow demanding it. If the tension is too low the valve just slams open instantly and lets in too much air, and it goes horribly lean. That's the feeling of "dying".

5/8 turn is about the stock setting for one of those carbs on a 350. You can experiment with it a little and see what your car likes. "Ideal" will depend on your cam, gears, torque converter, car weight, etc. etc., so a certain amount of variation can be expected. It will tend to make more power with the tension lower (blade opens easier) but have that "bog" at first, but drive more smoothly and predictably with higher tension. Not hard to find a good trade-off.