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Fuel consumption issues


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I just bought a 1980 720 4x4. It has only 35k miles on it, so there are a lot of parts that have not been replaced. After putting 600 hundred miles on it, the fuel economy dropped to around 6 mpg. I added berrymans b12 chemtool halfway through the trip. It is not a fuel leak. I replaced some vacuum lines and heater hose with no results. Tried spraying carb fluid on the lines with no result. It is also idling high, which has been happening since the belts were replaced on the day I got the truck. Possibly a carburetor issue?

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6MPG is virtually impossible from a 2 liter L20B. Can't see how it would even run. Black cloud out the exhaust???





Some things to check that can in combination cause poor mileage. Generally most likely at top


1/  Fuel leak, if not the tank maybe a hard to see line or hose. Is there a fuel smell around the truck any time and after filling?

2/  Choke ON or stuck on, not shutting off

3/  Carb flooding. Float set too low. Needle valve dirty/not closing. Fuel pressure too high. Secondary jet in the primary jet position. Power valve ON at all times. air filter VERY dirty.

4/  Fuel pump diaphragm torn, gas leaking down into crank case.

5/  Ignition timing extremely retarded.

6/  Front brake caliper stuck, e brake on, drum brake dragging?... does any or the wheels get hot to the touch?

7/  Speedometer drive gear changed with replacement transmission. Mileage is reading wrong.

8/  Thermostat stuck open, engine runs too cold.

9/  Too thick engine oil

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I have not smelled any fuel, anywhere. I changed the fuel filter and a few of the lines anyways. Air filter looked fine; replaced it anyways. Thermostat was replaced within the last month/1000 miles.

I ran the engine at around 3k for a minute, let it die and checked the spark plugs. The one furthest from the radiator was black, middle ones brown, and closest to the radiator white.

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