I have a 1974 Plymoth Valiant Brougham with a slant 6 225 & a single center barrel carb. My timing was all screwed up when I purchased it & now it's set to factory specs, but I'm only getting about 11 mpg. What kind of gas mileage can I expect with this car? We fixed just about everything there is to fix on the engine & I'm thinking the only problem left is that maybe I need a new carb. Is there something else I should be looking for besides a carb problem? From what I'm seeing out there it looks like this vehicle only got 13-16 mpg anyways. Welcome any feedback.

Thanks
Rod

Could be any of many factors at work here, or a combination of multiple factors. If the timing was screwed up, that could indicate the previous owner was trying to compensate for some other problem. So, a quick list:

•distributor vacuum and/or centrifugal advance mechanisms not working properly. If you have a timing light, rev the engine and watch the timing mark, first with the vacuum advance hose connected and then with it disconnected. You should see the timing advance as you rev the engine. With the hose connected, when you snap open the throttle, the mark should dip (retard) then pick up and advance.

•valve adjustment. Have you checked and adjusted your tappet clearance?

•emission control hookup. The car will run better and get better mileage without the OSAC valve connected (just run a vacuum hose from the carb directly to the distributor, instead of stopping in between for the valve on the air cleaner housing), but if it or other emission control devices (PCV valve, thermostatic air cleaner, EGR valve) aren't hooked up correctly, mileage will suffer.

•vacuum leaks at the carb, carb-to-manifold junction, and manifold-to-head junction. Check by carefully spraying carb cleaner at these locations with the engine idling. If the idle changes noticeably, you've got a vacuum leak that needs to be fixed.

•choke! Is it sticking partially closed?

•Thermostat. Is the engine warming all the way up to operating temperature?

•fuel pump. Does the oil smell strongly of gasoline?

•speedometer/odometer. If they're out of calibration (as for example because different-size tires or a different-ratio rear axle was installed without changing the speedo drive pinion, or perhaps the wrong drive pinion was installed as a replacement), then mileage will appear different than it actually is.

• Manifold heat control "heat riser" valve. Is it stuck? Grab the round counterweight (when it's cold!) and see if it turns freely and returns by its own spring tension.

•general tune-up. How old are the spark plugs, distributor cap and rotor? Plug wires?

•carburetor. At best, it's a thirty-two-year-old original tired pony. At worst it's a "remanufactured" hunk o' junk with some years and tampering owners worsening matters. How does the car run? Pull a few spark plugs and take a look at them. Are they fairly clean with a light tan to medium brown coloration to the firing ends? Or are they crudded up and blackened?

Parts store "remanufactured" carburetors are junk. I have brand new ones if you need one.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the wealth of information. We've probably done only about half of the things you mentioned here. As far as the car goes, I inherited it from my great aunt so we actually do have a mechanical history of what's been done & the one thing that seems to stand out the most is she had the carb changed & ever since then it never seemed to be running properly. That's why I was leaning towards a carb problem, but the first thing I was thinking was what you were thinking, is that the timing was compensating for another problem.

Thanks
Rod

"had the carb changed" means there's probably a "remanufactured" piece of trash on there. Not your aunt's fault, but you will want to correct the situation.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If you can't get 20mpg+ on the highway, you likely still have an issue. Dan's got it pretty well covered! My 74 Valiants both would do about 24-25 on the highway, but they had a high (2.76) rear end gear.

there is a possibilty that the stator in the torque converter siezed. i had that happen to my dart and it would only get 10-12 mpg. the only indidcation i found was that it took a high throttle setting to maintain speed. you should only have to press the pedal a small amoun to maintain speed. if you have press it around halfway to hold 55-60, i would guess thats the problem.

zedpapa

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1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!

When I bought my dart I was getting about the same 11-13... I did a lot of the same stuff Dan has told you, the osac valve, changed to a 9r governor on the dist and did a good quality tune up, changed timing to 10 BTDC... Also leaned out the carb as much as possible, I got up to about 17/22... I've already seen better mileage since the 2bbl upgrade too...

Also remember! The way you drive has a huge impact on your mileage... The best I've seen is 25mpg and that was getting into it softly and keeping it at 55-60 mph.

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1973 Dart Swinger Super 6 with 71k
1976 Datsun 280ZX with Mopar I-6

How many miles does this engine have on it. The factory timing chain is probably toast by now and stretched pretty far.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

I have had a rebuilt engine put in my dart, but i have had the same issue on both engines. on a 95 degree day on the highway the engine will get up to 190 degrees, but on warm days it sits around 150 or 160 no matter how hard or long i drive the vehicle. any guesses?
i'm using a carter bbd for a super six that is on its last legs. do you have any of these new or with mating surfaces ground and throttle shaft rebushed?

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God took you away, then brought you back to me.