Are you holding the accelerator pedal to the floor when cranking?
If not, it may be exactly that there is insufficient engine vacuum affecting the vapor valve diaphragm in the J reg.
Older engines that have some accumulated wear will be less able to develop a strong vacuum until the engine has been run a few minutes. This lies in the fact that the piston has not yet expanded and the piston rings are therefore not inflating properly when the engine is stone cold.
Sometimes simply making sure the accelerator pedal is always depressed fully when cranking will make all the difference.
Make sure that there are no vacuum leaks anywhere that will steal the vacuum.