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

[Zoomad75]

Could be carb leaking fuel. Qjet right? What about the ignition system? you changed the plugs, but how are the wires/cap/rotor??? If your plug wires have high resistance, your not getting the engry to plugs. Weak spark, incomplete burn and raw fuel goes out the exhaust.

If you do have a qjet, it could be leaking through the casting plugs at the bottom of the main body. Fuel drains out of the float bowl right above the throttle blades. It dribbles into the intake from there. When it's bad it will be hard to start if it hasn't been run for a day or so since the fuel has drained the float bowl. It will foul plugs. My father in law's 77 Pontiac is doing this right now. If you can get it started it will start right up if you shut it down, but if you leave it sit overnight or a couple of days it wont want to start without priming the carb.

Fix is easy, pull the carb. Remove the throttle plate to expose the plugs. Clean the area well of any fuel residue. Mix up a batch of JB weld or other epoxy (must be ok in fuel, JB weld is). Cover the plugs and the seam to the main body with the jb weld and let it cure. Once Cured, put the carb back together and fire it up.

[86burb]

Yes, QJet. Thanks, Rob Z., I'll give those two issues a shot.

Bill