The early carb is fine when they are right. At least they can be made to be right, the later electronic carb, when they are wrong there is no telling how to make it right.
I have been chasing issues with my early carb but it seems to be down to choke adjustments, to slow to open, and I think I got water in the gas, can not blame these on the vehicle. We got warm here today and I went up in the field and put a wrench on the crank of my turbo Monti, That might be my easiest fix to swap it over.
7 MPG is way low.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.