340 Gets Terrible Gas Mileage

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Sport340 said:
After I drive it a bit and come back to start it, I find I need to put pedal to floor while I crank it to start.
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the carb needs adjustment. at the very least the idle mix is rich. when it's warm (and it hasn't been sitting for 30-60 mins) it should fire off as soon as you turn the key...with no need to pump the throttle.
probably need to check ignition timing and make sure the vacuum advance is working too.
 
That seems normal for a vintage era "performance" engine. I think I looked it up once my 70 dart Swinger 340 is quoted as 13 mpg highway...that's with 3:91 rear. I knew guys back in the day with Chevelle 396's etc that said they never passed a gas station and that was in the 80's...with gearing keep this in mind, I have an 86 Mustang GT convertible with 2:73s, is it street able and highway drive-able? yes, does it launch hard at the track? no in fact its a 15 second car. Thing is its a cruiser and fun on the street and will do 140 on the highway.
 
Sport340 said:
Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
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What gears are you running? Engine in good condition and is it tuned properly
 
Sport340 said:
Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
Click to expand...



67Dart273 said:
Be honest do you keep your "foot out of it?"
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Get a vacuum gauge installed in the car and drive for highest vacuum all the time... It will teach you how to drive for best economy...
 
ch1ll said:
Make sure your vacuum advance works, better yet check your whole ignition system. And read your plugs.
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Do you have vacuum advance on your distributor?
Is it hooked up?
If hooked up, does it work?
You can see if its working with a timing light.
The reason I ask these questions is I've seen more than a few cars where the owner didn't understand how important vac. advance was to gas mileage, and engine life. Sometimes they'll say " it doesn't run right with the advance hooked up" if true, they've got some thing or things adjusted wrong. You need to find a local tuning expert. Doesn't have to be a Mopar lover, just someone who understands carburetors and distributors. Looks like a great car! A stock 340 with 3.23s should get about 12mpg in town, and with a very light foot I've gotten over 20 at 55mph.
 
Sport340 said:
Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
Click to expand...

there is something really wrong. What state do you live in?
 
sccachallenger said:
Do you have vacuum advance on your distributor?
Is it hooked up?
If hooked up, does it work?
You can see if its working with a timing light.
The reason I ask these questions is I've seen more than a few cars where the owner didn't understand how important vac. advance was to gas mileage, and engine life. Sometimes they'll say " it doesn't run right with the advance hooked up" if true, they've got some thing or things adjusted wrong. You need to find a local tuning expert. Doesn't have to be a Mopar lover, just someone who understands carburetors and distributors. Looks like a great car! A stock 340 with 3.23s should get about 12mpg in town, and with a very light foot I've gotten over 20 at 55mph.
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Thanks for the post. There is a vacuum line coming off the distributor and it hooks up to something on the firewall. It is the top line in the picture. Not sure what that thing is that it is hooked up to. So I'm not sure what the source of the vacuum would be.
IMG_1069.JPG
 
AJ/FormS said:
You got hilly country there?
and are you cruising in them thar hills?
Cuz that will eat gas; see post #26 re: "topography"; which is like Elevation Change, except not, lol.
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Memphis area is very flat so not cruising in any hilly areas.
 
I would be looking at that valve in post 37. Not sure what it does, but it connects to the VA, so I presume it has some effect on VA operation. If it prevents VA operation because it is faulty, then that can kill mileage.
 
My 340 gets about 8-9 miles / gal. so I feel your pain! I also have a street/strip cam, Headers, etc. 727 transmisision. Rear end has 323 gears. I have a Holley Sniper EFI sytem that is pretty well tuned but might be a tad rich.

Part of the reason you are not getting any better gas mileage is the 3 speed transmission. My car turns about 3200 rpm at highway speeds (70 mph). I'm going to swap out to a 5 speed Tremec which will allow crusing at a lower rpm as well as more fun shifting gears.

Nice looking car by the way! I love the look of 18" wheels on these cars. Here's mine.

hqUTBm9.jpg
 
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disregarding the gas mileage, how does the car run? smooth, lots of power when you put your foot in it? No hesitation , misfire, etc?
 
The 73 is a lo-compression engine.
With a standard 340 cam, the CCP is gonna be pretty low, which means the low-rpm torque will be low. which means it will take a lotta throttle to get it moving, and more a than usual amount of throttle to keep it moving. And that tranmslates to lousy city mileage.
The usual cure for the soft bottom end is a higher stall, and for a city-car, more rear gear....... which now make for lousy hiway mileage.
When a performance cam is installed, this can, and usually does make it worse.
There are only three cures for this sad situation;
1) more cylinder pressure so you can use less stall, and less gear, or
2) more stroke, or
3) less weight.
4) A finely tuned ignition advance system can help but there is nothing that can touch decent cylinder pressure. So once again, start with a compression test

Just so you know;
not very many years ago, I had a hi compression, alloy-headed 367, with a Final-drive ratio equivalent to 1.97, that at 85=2100 got 32mpgs on a certain measured day-trip. The cam was a Hughes HE2430AL that was 270/276/110. with an Ica of 61*, the pressure was over 190psi. I ran that on 87E10 full-time, with full advance. The cruise timing was close to 60*.
The point being; it is possible to get both power and economy. Just not with a cammed-up/stalled up/geared-up, 340; yur gonna need a different line of attack.
Just so you know-2;
A long time ago, I put a 318 cam into a hi-compression 340, with the early 318 heads even,installed into a 65 Canadian-Valiant wagon that was a gangbusters city car.
 
Just for comparison, in my E-body I have a stock cam 69 340, ProFlo4 EFI, 727 trans with a GVOD, 3.23 gears and TTI headers and exhaust. Running to Carlisle from northern Virginia back in July some stops at the beginning and two days running to the fairgrounds and dinner with A/C on part of the time managed 15mpg hand calculated. Filled up on the way to the fairgrounds the last morning and then home after the show got 16.5 mpg hand calculated.

This was with tent and everything else in the car for the weekend and yes I did keep my foot out of it! So yes it is possible to get decent fuel mileage.
 
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jcolman said:
My 340 gets about 8-9 miles / gal. so I feel your pain! I also have a street/strip cam, Headers, etc. 727 transmisision. Rear end has 323 gears. I have a Holley Sniper EFI sytem that is pretty well tuned but might be a tad rich.

Part of the reason you are not getting any better gas mileage is the 3 speed transmission. My car turns about 3200 rpm at highway speeds (70 mph). I'm going to swap out to a 5 speed Tremec which will allow crusing at a lower rpm as well as more fun shifting gears.

Nice looking car by the way! I love the look of 18" wheels on these cars. Here's mine.

Thanks for the post. You have a beautiful car. Do you get any tire rub with the 18" wheels? I am getting some in the rear. Not sure if a new pair of shocks is in order or if I need to go with air shocks? In terms of the gas mileage I may just have to put up with bad mileage but I believe I should be getting better than 6.

I have noticed that the car gets into 3rd gear quickly (35 mph) and while I have not noted rpms (it does have a tach) at different speeds it feels like the rpms are higher than you might expect which when I first drove it I felt would limit the top end. Which is fine I don't plan on going 100 mph in it!

hqUTBm9.jpg
Click to expand...
 
str12-340 said:
disregarding the gas mileage, how does the car run? smooth, lots of power when you put your foot in it? No hesitation , misfire, etc?
Click to expand...

Thanks for the post. That is a good question. Given the 4 barrel, performance cam and headers I expected more performance from it. Its not a dog but I think I should be getting more out of it. Maybe a slight hesitation when starting out from a dead stop. Let me know what you think.
 
AJ/FormS said:
The 73 is a lo-compression engine.
With a standard 340 cam, the CCP is gonna be pretty low, which means the low-rpm torque will be low. which means it will take a lotta throttle to get it moving, and more a than usual amount of throttle to keep it moving. And that tranmslates to lousy city mileage.
The usual cure for the soft bottom end is a higher stall, and for a city-car, more rear gear....... which now make for lousy hiway mileage.
When a performance cam is installed, this can, and usually does make it worse.
There are only three cures for this sad situation;
1) more cylinder pressure so you can use less stall, and less gear, or
2) more stroke, or
3) less weight.
4) A finely tuned ignition advance system can help but there is nothing that can touch decent cylinder pressure. So once again, start with a compression test

Just so you know;
not very many years ago, I had a hi compression, alloy-headed 367, with a Final-drive ratio equivalent to 1.97, that at 85=2100 got 32mpgs on a certain measured day-trip. The cam was a Hughes HE2430AL that was 270/276/110. with an Ica of 61*, the pressure was over 190psi. I ran that on 87E10 full-time, with full advance. The cruise timing was close to 60*.
The point being; it is possible to get both power and economy. Just not with a cammed-up/stalled up/geared-up, 340; yur gonna need a different line of attack.

Thanks for the post. It does seem like it takes a lot to get it going which is what I think you are saying in your first point. You noted 190 psi, what does that refer to?
Thanks
Just so you know-2;
A long time ago, I put a 318 cam into a hi-compression 340, with the early 318 heads even,installed into a 65 Canadian-Valiant wagon that was a gangbusters city car.
Click to expand...
 
Bewy said:
I would be looking at that valve in post 37. Not sure what it does, but it connects to the VA, so I presume it has some effect on VA operation. If it prevents VA operation because it is faulty, then that can kill mileage.
Click to expand...

Yes not sure what that valve does so you are right this would be something to look at.
 
To hell with trusting the speedometer and your math. Use a GPS if you suspect a problem.
 
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