IT'S A '75 MOTORCRAFT 4350 4V ON A 460 AUTO AN E350 MOTORHOME !!!
If anyone has any ideas, I could sure use the help.

I know this Carb has a bad reputation but I'd like to keep it original and get it working.

I did the carb rebuild, she's tuned, vacuumed up, idles great, purrs like a little lion and runs good on the primaries but a little doggish for the 22' motor home I'm asking her to power.

While having my son-in-law drive it, (it's a 1975 E350 ...with the doghouse open) I can see that the secondaries are not opening either under firm acceleration or punchin' it. I can open them mechanically by twisting the SECONDARY THROTTLE SHAFT LEVER (cam)with a screwdriver (it twists kinda' hard)....and get an immediate powerful response.

The THROTTLE AUXIARY LEVER on the choke side releases properly with engine warm and the linkage on the accelerator side operates freely with engine off and choke PLATE wedged open. The linkage to the upper AIR VALVE ("flap") that goes to the AIR VALVE DAMPER PISTON (little "plunger) on the accelerator side operates it freely at all times.

I know the secondaries work on Venturi vacuum but does the AIR VALVE PLATE (upper flap) have to open first or SECONDARY THROTTLE PLATES ....or simultaneously ???

I've read that a lighter AIR VALVE DAMPER SPRING {under the "diaphragm"}(THERE IS NO DIAPHRAGM ON THE MOTORCRAFT 4350) solves the problem.

I SHORTENED THE DAMPER PISTON SPRING BY 1/4"....it definitely reduced the force needed to operate the air valve but did NOT make the secondaries
open.

I've heard you can trick the secondaries to open with a cheater screw on the driver side linkage but really would rather learn how these things work and then make it work right.

And this is weird ! .......WITH ENGINE OFF when everything is free'd up and I open the secondaries by rotating the cam's on the end of the shaft, I can feel the blades stick a TINY BIT. When I open them by pushing on the accelerator side linkage or with a screw driver down her throat, they open perfectly smooth.