Fuel mileage stinks and I want to know why

[86burb]

Hi folks:
My 86 3/4 ton with the carbureted 350 has seen the mileage go from 14 to 10.5 over the last year. No changes to truck, engine, or driving style. Last month it failed to start until I put in new plugs (previous ones were only 8 months old). Removal showed fuel soaked plugs.

Any recommendations on fuel mileage or fuel metering improvements? Shouldn't I be getting 15 mpg?

No a/c, manual transmission, no cat (8600 gvwr), factory stock, dual exhaust, 4 bbl.

Thanks,

Bill