My 340 getting 6 mpg

The difference between cruising at 13:1 AFR and 15:1 isn’t that drastic with regard to MPGs. But if you’re really interested in getting mileage out of it you’ll end up cruising at 15:1 (or maybe even leaner). But you can’t do that without timing, you have to stick a bunch of timing in it at your cruise rpm and vacuum. That lets you close the butterfly’s down drastically and gets your cruise just off the transfer slots and tickling the booster. Then you can jet the main down to get the cruise afr you want. But you have to put the timing in it to do that. For wot you get the fuel back with the power valve channel restrictors on the primary side. If it has a secondary power valve, plug it and jet the secondary up 10 sizes (starting point) from the primary. Then once you have a basic mpg tune up in it, you can start worrying about dialing in the fuel curve with air bleeds and stuff like that.