noob needs help with mileage - only 10 MPG!

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Renato

Renato

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I've got a 1966 Valiant with a 225 slant six and the Torqueflite 3-speed auto. The engine is not original, and according to the casting numbers it is a 1980 production year engine. Anyway, I've had this car only about 2 months, its my first Mopar, my first slant six, and my first classic car. So I'm truly a noob here.

But 10 MPG!?!?
That's with a decent mix of in town and highway driving. I was expecting mileage in the 20 MPG ballpark.

The car runs great, so I'm not sure where to begin troubleshooting this. My guess is the carburetor is key to this lousy fuel consumption, but that's only a guess. The car starts up fine with the engine cold, but once the engine is warmed up it does idle rough. Other than that, there are no problems with drive-ability.

And I've been putting premium gas in the tank...

I'd greatly appreciate any help or hints here.
Thanks, guys.
 
Renato said:
I've got a 1966 Valiant with a 225 slant six and the Torqueflite 3-speed auto. The engine is not original, and according to the casting numbers it is a 1980 production year engine. Anyway, I've had this car only about 2 months, its my first Mopar, my first slant six, and my first classic car. So I'm truly a noob here.

But 10 MPG!?!?
That's with a decent mix of in town and highway driving. I was expecting mileage in the 20 MPG ballpark.

The car runs great, so I'm not sure where to begin troubleshooting this. My guess is the carburetor is key to this lousy fuel consumption, but that's only a guess. The car starts up fine with the engine cold, but once the engine is warmed up it does idle rough. Other than that, there are no problems with drive-ability.

And I've been putting premium gas in the tank...

I'd greatly appreciate any help or hints here.
Thanks, guys.
Click to expand...

Besides carb and fuel leaks are there any vaccum leaks? Some times some general maintance that hasn't been done will wipe out your mileage. A tune up with the right heat range plugs might help too. What shape are the plug wires in? Correct gas cap so the fuel isn't evaporating?
 
if it is running rough at idle, im going to say the floats need attention. it may be dumping a little fuel.
 
Very first thing is a general tune-up for diagnostics purposes

Does this still have adjustable lifters, that is, mechanical? (Not familiar with newer slants)

If so adjust the valves

Run a compression/ leak down check

Check the timing, and I don't just mean initial, or "idle" timing

Check that the mechanical advance advances, and that the vacuum advance works, and check how much advance you are actually getting.

Hold the throttle at an RPM to simulate "highway cruise" and listen and look down the carb throat. If you hear a "popping" or can look down the throat and see gas dripping into the carb, you have a problem, float level too high, etc.

Also, what exactly are you using to check MPG? Speedo? You sure it's correct? You have a GPS? Use that instead.
 
The absolute best I ever got out of my slant was 15-16 mpg mainly highway in a '74 Duster a904 with 2.76 gears, dropped down to 8-10 when I swapped in an 8.8 with 3.55, pulled it shortly afterward. It was a 77' super six. It had flow issues with the head and never ran properly.
 
Thanks for all the replies, guys.

There are no leaks (as far as I can tell).

I had a tune up done about 4 weeks ago, and the car got a good bill of health. They checked each cylinder and compression is good.

I've been checking mileage just using the odometer...
 
Slants suck on gas mileage, my 450hp 5.9 360 gets about 2 more mpg than what my slant got, which was 17. One of the reasons I axed the thing.
 
Run regular unleaded fuel. The /6 does not need premium. It may even run better & improve the mileage. This is of course if the motor is stock.
 
warwagons said:
Run regular unleaded fuel. The /6 does not need premium. It may even run better & improve the mileage. This is of course if the motor is stock.
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x2 Premium is higher octane which contrary to popular belief burns slower. This fights preignition.Pull a plug and see whats going on. If its sooty black its too rich. Float problem or stuck choke.
 
I had a 225 with no visible gas leaks, turned out the fuel pump diaphragm was bad and leaking into the oil pan. A lot. Might want to smell inside the oil filler cap. 10 mpg is pretty bad unless you're mud bogging
 
Cowboy said:
dropped down to 8-10 when I swapped in an 8.8 with 3.55, pulled it shortly afterward. It was a 77' super six. It had flow issues with the head and never ran properly.
Click to expand...
must have had some kind of issues- I had better mileage...ok, about the same mileage...with my 340 and 4.10s.
 
I was originally getting 10mpg back when I had a remanufactured Holley 1920 on there. Swapped it with another remanufactured 1920 and it jumped to 12mpg. At that point I said screw it and went with a Super Six with a NOS Carter BBD and it jumped to 18mpg. I threw on an HEI ignition and a 2 1/2" exhaust and I've been 21-22mpg ever since.
 
PoisonDart74 said:
Slants suck on gas mileage, my 450hp 5.9 360 gets about 2 more mpg than what my slant got, which was 17. One of the reasons I axed the thing.
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By that logic, you should swap the 360 for a full-blown race hemi. Bet your mileage will jump to 20+.
 
my 65 valiant average 17-18 with a burnt exhaust valve on cylinder 1 and points. Something to check there is a flapper valve on the exhaust manifold just below the intake. If it sticks shut you have a very large exhaust restriction which will severly hurt fuel economy. The board over at www.slantsix.org is less active than here, but there are a lot of people over there with slant specific experience.
 
My 69 Dart 225 got 21-22 mpg hwy regularly. Adding electronic ignition (Crane) didn't noticeably help. It always idled too lean. Most likely the problem is in your carb. Do you have the Carter BBD 2 bbl that was common in 1980? Those are fairly rugged. Most likely the float has sunk or the inlet has debris so the needle won't close off, both of which can make the bowl overflow into the throat. If you have any of the emissions stuff from 1980, that could make it run bad unless in tip-top shape. Make sure the choke opens fully when warm. Down the road, consider adding a rich/lean indicator, which requires welding an O2 bung in the exhaust. That is the only way to really know how the carb is functioning.

Also check your ignition. Do you have points or the electronic stuff from the 1980? I assume the mechanics checked the spark timing. Adding HEI ignition is not hard or expensive and would take care of any questions on spark.

Finally, valve timing could make it run real bad. If you have the hydraulic lifters of the 1980, no need to adjust valves, but do verify the lifters aren't gunked up. If the timing belt is loose, it could even skip a tooth. Read up on how to check that by rotating the engine fwd and back while watching the distributor rotor.
 
BillGrissom said:
If you have the hydraulic lifters of the 1980, no need to adjust valves, but do verify the lifters aren't gunked up.
Click to expand...

solid lifter in all slants from 1960-1980 (us)
hydraulic lifters in slant 1981-1987 (us)
I don't recall when production ended for foreign markets and what valvetrains were used.
 
I'd change the float because they get saturated then heavy. Also, as mentioned above check the total timing. Make sure the distributor diaphram is not leaking.
 
Oklacarcollecto said:
slantsixdan would be the best slant 6 guru I know of. Maybe send him a pm if he doesn't show up on this thread.
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I agree...I've gotten acurate part #'s from slantsixdan that the Chrysler dealerhip said didn't exist...very sage advice from slantsixdan.
 
Start with a tune up and a new carb and make sure you have no vacuum leaks, Check the the tranny a slipping will kill gas mileage, and don't forget about driving habits I always get bad gas mileage in under powered car cause I'm always given it.
 
Goin through this same issue on the 74 swinger my kid just bought. Going to keep a close eye on this thread. My built 440 gets almost 10mpg downhill with a slight tailwind.
 
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