Quote: A 1 1/2" primary carb(850) on a small-inch low-rpm MPG project ? ...
Sure, why not? May not have "razor-sharp" performance, but it leaves room for "growth"!! It's mostly about the NEEDED amount of airflow, NOT the TOTAL rating of the carb, Doc. Jetting, float height, choke unloader settings, acc pump settings do have an effect on performance (model depending)..BUT.. you CAN fit the TQ on a 318 and have decent performance!! Personally, a 360 TQ model would be about right for the '18.
HYPER - ...
WE are talking about a low-rpm MPG application ... NONE of what-choo mentioned above really has anything to do with that.
The primary vent SIZE is what matters ... and a 1 3/8 (I would think) would work the bestest there.
I have a 1 3/8 on my Discoverer motorhome(413 power) ...and it has performed GREAT. NO NEED to get into the secondaries uNder normal flat-road cruising operations at around 60 mph.
... 
