If your 280S has dual Zenith INAT 35/40 carburetors, I can offer a few
First, they should be properly synchronized and adjusted.
Second, the Zenith 35/40 carburetors have an electric choke mechanism where
the car's choke plate defaults to about one-quarter closed with the engine
off. When the car is started, electric heating elements warm bimetallic
springs to fully open the choke plates. If the electric heating elements
are not working or are misadjusted, the car continues to run at one-quarter
choked, using more fuel than it should.
It's also possible that the choke plate was overadjusted to compensate for
worn carburetors (leaking air from worn throttle plate hinge openings), a
cracked base (vacuum leak), or aging engine compression.
My 1968 280S was a great car, a real beast on the road once I got the
carburetors figured out. It took awhile to understand the Zenith
carburetors, but it turned out to be far more simple to adjust, synchronize
and tune than I had originally thought.
Weber makes a conversion kit for the Zenith INAT 35/50 dual carburetors, but
I really liked the original setup once I got it adjusted. I remember
mileage being in the 12-18 mpg range.
With two four-barrel carburetors on an inline six-cylinder engine, you're
running almost one carburetor barrel per engine cylinder at low speeds, and
over a barrel per cylinder at 60 mph and above. Great for performance, but
expensive for heavy-footed driving.
If you want to check some carburetor settings, send a note to rugbygynw at
yahoo dot com.
Low mileage from a gas engine could be due to:
Too low an operating temperature - below 80 degrees C. replace thermostat.
Too rich an air / fuel mixture - carburetor problem.
Worn out spark plugs & ignition wires.
Retarded ignition timing, check ignition timing and adjust.
All that said, the mid '70s 116 280S version sold in the USA was
certainly no economy champ, 13 to 16 MPG (US) overall as I recall.
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