Is my 340 Overcarbed? Avg 5-6 MPG

Once you get your timing dialed in, you still have to dial in your jetting. Drop
primary jets a few sizes and drive around easy and check A/F ratio. You want it
to be approx 14/1. Jet accordingly. Once you get that, you need to tune power
valve RESTRICTIONS for wide open throttle(wot). Your powe valve restrictions
are the two holes behind the power valve when removed. They need to be opened
up a little at a time until you achieve desired wot A/F ratio in in short bursts. I would
add a couple jet sizes to secondary side before dialing in primaries.