I have the Doug Roe Q-jet book and have been studing it for some time now.I know that 4M and M4 Q-jets primary rods and power pistons aren't interchangable because they have different lengths.Could the power piston,spring and primary rods be changed from one carb to the other and if not why?

 

You mean between carbs of the same generation? Sure.

 

I'm not sure what you mean by same generation.What I mean is there a difference in the two carb bodies that would prevent swapping the power spring,piston and prim.rods as a unit from a 4M to a M4 or vice versa?I know you can't mix the componets among themselves.

 

Primary rods and pistons from carburetors made in 1974 and earlier (4M series) are not interchangeable with those from carburetors made in 1975 and later (M4M series). Primary jets and secondary rods and hangers are interchangeable across all years.