Flat acceleration off idle is due to the AFM if everything checks out,
including even the AFM Bentley test (which does not test spring
tension).  I just went through this exercise.  It can sometimes be fixed
by reducing the spring tension if the flapper moves without binding.
Remove the lid and loosen on notch at a time. Worth a try before you
spend $$$.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Driven since 1969)
'74 MGB GT (Restoring)

Assisting on Restoration:
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Parts cars:
'68 & '73 MGB, '67 MGB GT