Gas mileage for 1968 mustang is very poor

i finally got my mustang street legal and while drivng i realized that the gas milage is HORRIBLE. i estimate it at around 3 MPG. my question to you is can a leak in the feul pump cause it to be that bad? and another important detail to add is it also spits out TONS of exhaust when you give her some gas.

Which engine? Which carb? Could be the carb needing adjustment or a rebuild, could be a bad diaphragm in the fuel pump letting gas leak into the crankcase, but your oil would be very diluted and smell like gas. Have you done a compete tuneup? What all did you replace?

Sounds like a carburetor problem. Is the choke opening all the way? Could also be a sticking float or a leaking float needle valve. A leaking fuel pump is unlikely. You would
see gas pouring out on the ground.

it is a 289 with an edelbrock carb i dont know the model it is a four barrel. i did a complete tune up and rebuilt the carb a couple weeks ago.

It would help to know the engine, tranny, rearend ratios, engine history and any and all mods, car history, and tire sizes. Then we’d have something to work with.

Back in '68 a HS buddy of mine had a new '68 Camaro. It got terrible mileage. It had a 396CID with a high rise and/or duration can, a high rise manifold, a 4-barrel big enough to swallow a swallow, headers, glasspacks, a high-ratio rearend, and big wide tires on aftermarket rims. His dad owned a performance parts and service business. He had to put traction bars on it or we’d have lost all our fillings. That sucker wound up like a slinky.

Another friend a few years later had a '68 that got 25mpg. It had a totally stock 327 bolted to a totally stock three-speed automatic spinning a totally stock pumpkin turning totally stock wheels. And we were in North Dakota, where there aren’t any hills, curves, stoplights, or stopsigns. Well, I exaggerate… while I was there the town of Grand Forks put in their first stoplight.

Everything in your question is in the details. Without them, we’re wandering in the dark.

When you rebuilt the carb did you carefully adjust the float level? If the fuel pump were leaking into the crankcase you would have gallons of gas in the oil by now at 3 mpg.

TONS of exhaust means a much to rich mixture. Either the carb has a bad part, or is way out of adjustment. A float that has a hole in it doesn’t float and is just one possible. Since setting the points, timing, etc. is easier to do I suspect the carb is the problem.

One relatively easy thing to do is take off the linkage for the accelerator pump on the front of the carb. Then run it and floor it and see if the exhaust is the same or much less. You will get a hesitation if you floor it quickly, but if you squeeze down on the gas it should run OK without the accelerator pump. If there is a big improvement then the accelerator pump is the problem.

Have you ever had it running with this combination of parts (carb esp) and gotten “good” gas mileage?

Two things to check with gas mileage that poor are a choke not opening, and a carburetor with an overflowing float bowl. I had a '68 Firebird with the latter problem, and it was getting about 4 mpg.

The Edelbrock carbs are simple and do not use a power valve, etc so if the 3 MPG is correct you need to revisit the carburetor and make sure the jets are in the proper holes, the choke is operating correctly, the floats are not sticking, and the mix needles are correct.

Those needles are interchangeable but even if in the wrong spot it’s difficult to see them causing a problem this severe.

Dig up a 2nd carb for test. Or put the stock 2 bbl intake back on car.

I have to assume the spark plugs are black, right?

they are new

^Doesn’t matter. If the engine is running as rich as you’re suggesting it is, the plugs would look like they were covered in coal dust: black, sooty, dry. If the “white” insulator is very sooty, that is a strong indicator of very rich conditions in the last few seconds of engine operations.

Pull the plugs, and let us know what you find. You can even try posting the photos!

Assuming the carb was rebuilt correctly the possibilities are near endless. Bad vacuum for vacuum advance, weak coil, poor connection on coil wires, bad distributor cap or rotor, worn points, bad dwell angle or point gap, bad condenser, timing off, mechanical advance sticking fora start

Remove the air cleaner.
Start the engine and allow to idle with the choke open.
Look down inside of the carburetor bores.
You should not see any gas dribbling from discharge tubes or anywhere else. If you do, back to sorting the carb out.

If you hear a subtle crackling sound while the engine is idling and with the air cleaner off that can be a sign of gasoline dribbling out.

Is the choke fully open when it’s warmed up?