My 340 getting 6 mpg

Bigger Air bleeds . More air less fuel. Easy to change on the road by removing the air filter. If you change over to removable style.

also square the carb so your not into the PV at idle. Idle off the secondaries and leave the primaries square

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I agree completely about taking a look at the air bleeds first. For me though, I would be too chicken to change them out unless I could stuff a blue towel into the bores before pulling the bleed out to make sure it does not accidently drop into the butterflies... then I somehow jerk the throttle and let it through. Ugh!