The 261 I just installed in my 1-ton gets 10+ MPG running over the mountain pass to the next town and back. It has 2 Zenith 228 carbs and a 250 degree Crower cam. Stock advance curve. With the camper on making 8500lb. gross it gets 10- over the same route. Engine has only about 250 mi on it so it may be a little tight yet. The only time I've gotten way sub 10 MPG was pulling an 8,000 lb. trailer through mountainous terrain. Then it was ~9.3
There's definitely something wrong with your setup. Perhaps it's too rich as has been suggested or more likely the ignition timing is off. Setting the timing for maxium vacuum will get you way over advanced with all the problems that brings on. Static time it to the ball in the flywheel as a starting point.That will get you an initial spark lead of 5 degrees. Also check the plugs for gas fouling from a too rich mixture.
