My /6 is a gas pig when it's cold outside. Why?

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GenLee1970

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Hi All. So I've got this issue with my 73 Dart. It's running great and when temps are above 40 deg it returns very reasonable gas mileage, even with multiple cold starts between fill ups. I've only had one reading below 20mpg since April, (19.8mpg) and my average has been 22 for combined city/highway/back roads. But when it gets near freezing, this thing is an absolute pig for the first 5 miles or so. So much so that I get out and check for leaks because I can't believe that the fuel gauge needle has moved so much in so few miles. I don't notice any black smoke in the rearview mirror while this is happening. I've experimented with timing, choke settings etc, to no avail. Do I need to let this thing warm up for a few minutes before I start driving? Timing is currently set at 2 1/2 btdc and it's fully off the choke generally within 3 miles. Any thoughts or is this just the way they were back in the day? I would expect some decrease in efficiency with cold weather but 30% or more seams a little nuts. Thanks

AL
 
My experience with the slant is that they are cold blooded and need warm up time.Like me they don't like to get going in the cold mornings....Just thinking out loud........
 
No definitely a cold blooded animal. My super six Volare wouldn't get **** for mileage in the winter and the gauge took forever to move. I would routinely warm it up for 10-15 minutes before driving to school. If I tried to drive it without a warmup it would choke and sputter and sometimes even die at every stop sign in town. What a pain. I usually got about 17 mpg in the winter. If I warmed it up though I never had a problem the whole day. I also credit the bad mileage though to my tune. I was 17... it didn't have a tune. Lol
 
Could be:
1. Choke staying on too long, or sticking on
2. Heat stove and piping removed
3. Carburetor icing.

Slants and 273/318 2 bbls were bad for these.
 
More advanced timing may help quite a bit. 5 to 10 BTDC should be no issue.

Make sure you are using a 180 thermostat. I used to have to block the rad with cardboard deep in winter in northern Indiana. The /6 has a large block area on the outside, so tends to run cold very easily.

Then adjust the choke leaner a bit at a time until it won't run well dead cold. If you have the heat well type, pull it off the manifold and you should find a small adjustment on the spring pivot inside the heat stove housing, and it will have L and R marked for leaner or richer. Loosen the nut and turn the mark a bit towards L.

Also, be sure the float level is right on spec. That effects all carb operations, including the choke operation. It might be a bit high in your case. If you change this, you may need to reset the idle mixture, and see how it drives in other modes, like the transition from mild to moderate throttle opening.

Mileage is going to be less for sure nor matter what with the choke on. A warm up for a minute will help.
 
There's a flap in the exhaust manifold. If it is stuck the wrong way, the carburetor will not get manifold heat.
 
First, no matter how bad the mileage, I doubt you would notice the tank level drop in a few miles. More likely, the gage reading is changing as it heats up. Higher temp = more resistance = lower indicated level.

One thing often over-looked in choke operation is the importance of the vacuum "choke pull-off" or "unloader". Insure yours doesn't have a torn diaphragm. Remove the hose, push in the stem, cover w/ finger, see if it stays in (after initial outward movement), or test w/ a vacuum pump. If it comes out all the way, the diaphragm is torn (common) and it won't do its job. Which is to pop open the choke slightly the second the engine fires. You need the choke fully closed while cranking. You should also see that happen as you start a cold engine. Without it, you will never adjust the choke to work well. I fooled w/ that on my 69 Dart slant for years in younger days, not understanding it.

You didn't tell us what carb you have. I think 1973's had more complicated cold-start controls, though 1974 was even more (big vacuum amplifier, ...).
 
If your air cleaner assembly doesn't have a heat tube and thermactor valve assembly for an EFE (early fuel evaporation) system find one from a later model engine and install it. Slants are prone to carburetor icing so anything you can do to warm up intake air will help in the winter.
 
nm9stheham said:
Make sure you are using a 180 thermostat. I used to have to block the rad with cardboard deep in winter in northern Indiana. The /6 has a large block area on the outside, so tends to run cold very easily.
Click to expand...

I put a 195° thermostat in the Duster I had back in the day, though it had a 318, not a slant. Not so much for driveability issues, but to actually get something out of the heater in cold weather. My current Dart (with a slant) sleeps in the winter so it's not an issue.

Don't recall the Duster having any particular cold weather driveability issues, or at least any worse than anything else out there at the time. Cardboard over the radiator was pretty much SOP in the winter here in northern N.H.
 
The elevation here is almost 4000 FT and that may be the contributing factor. Ambient temps below 40 F were always a bit of a pain till warmed up. Making sure the choke-stat is properly adjusted is a big part as previously mentioned. When we had blizzards I often used a cardboard T-stat.... As also mentioned you can put in a bit more timing and help it just remember to pull it back in the spring.
 
Thanks for all the suggestions!

Oops, sorry. Carb is a 1920 model. The body is 73 but this poor thing has been butchered over the years. Not sure what vintage the carb, or anything else is for that matter. I'm just glad to have gotten it going and running really well. It doesn't run bad when it's cold outside. It starts right up, doesn't stall etc., it just drinks more gas.

My heat tube/thermactor set up is down at the moment because I can't find an air cleaner sensor. My parts guy says it's factory back ordered, which means they my have stopped production. The flap in the air cleaner works with vacuum applied. I just need a sensor to control it. Will keep up the search. Later model may be the way to go as suggested, thanks : ) Can I just get the sensor or does the flap need to be of the same vintage.

I will try more timing. I had it at 5 once before with no issues. Forgot why I moved it to back 2 1/2.

Though hard to believe, the gauge really does move. I've driven it enough to know how it should react.

I'll try a few minutes of warm up to see if that helps too.

Choke pull off is doing it's job and I recently adjusted it to open a little further by bending the linkage. Seems to run best that way. I've got a spare just in case it craps out one morning.

The adjustment screws on the choke t stat are rusted so I made adjustments by bending the rod slightly. Think I'll spring for a new one. $20 at rock auto, so I can make more refined adjustments.

It was a fairly chilly weekend here and I had a lot of short trips to make, giving the car ample time to cool between starts. I filled it up before heading to NYC on Sunday and got a 17.6 mpg reading for the weekend's short trips. HOWEVER, once on the road she was a champ: 23.2 on the way there with about an hour in traffic and 26.2 on the way back with no traffic. Average speed 55. That was an all time best. I must have clicked that pump 2 dozen times trying to get more gas in the tank because I couldn't believe it only took so much. Nope, it was full. For the record, the car was warmed up when I fueled up at home and I only stopped for 10 minutes in NYC to drop off a passenger so it was an economy run best case scenario of pre warmed engine and highway driving.

Anyway, thanks for your help. I'll play around some more this weekend and keep you posted.

Al
 
Sounds like you're having fun. Advance the timing more.
 
KosmicKuda said:
Sounds like you're having fun. Advance the timing more.
Click to expand...

Lots of fun. Thanks.

I will advance the timing this weekend.

Just curious, if it's ok to advance the timing so much over the factory setting, why was the factory setting at 2 1/2?

Checked my records. I put a new t stat in last year when I bought it. 180 : )

And for the record, when I say this car had been butchered I mean, there is no emissions junk, the a/c and related components are gone, washer bottle and wiring gone, electronic ignition and wiring gone. It's stripped down to the bare essentials, which is kind of cool, but took some doing to sort it all out.
 
As I have understood it..... In '73, the first serious emission stuff was installed, and the timing was retarded to reduce peak combusiton temps to lower NoX emissions. It also worked better to avoid pinging with leaner idle mixtures used to lower HC emissions.

But even at that, the factory timing settings for the /6 were never very advanced. I assume this was just to avoid pinging problems with limited maintenance, bad gas, and so forth in a standard economy car. My '62 FSM says 2.5* and I have it up at 6-7* and could go further.
 
nm9stheham said:
As I have understood it..... In '73, the first serious emission stuff was installed, and the timing was retarded to reduce peak combusiton temps to lower NoX emissions. It also worked better to avoid pinging with leaner idle mixtures used to lower HC emissions.

But even at that, the factory timing settings for the /6 were never very advanced. I assume this was just to avoid pinging problems with limited maintenance, bad gas, and so forth in a standard economy car. My '62 FSM says 2.5* and I have it up at 6-7* and could go further.
Click to expand...

I see. Thanks. : )

Btw, is it common to have to use plus grade (89 octane) gas in an old /6? It'll ping during full throttle (which I don't do often) and also going up long hills on the highway with 87 octane.
 
In addition to the good comments above, you have to understand that the years from about 73 onwards ARE THE WORST YEARS for runnability / drivability if the rig still has the original "smog" distributor, carburetor and "other stuff." These cars were typically so cold blooded it was unbelieveable.

1....You do NOT want a stat "as cold as" 180. You want AT LEAST a 180, and these cars came from the factory with a 190-195

2....The timing advance system SUCKS. Download the 73 shop manual......free......at MyMopar and read the emissions and electrical sections. Consider re-curving the distributor or even buying a performance one. No, I don't know what is available for a slant

3....Check over all the smog stuff. Electric choke control. Choke settings, and make sure it's not gummed up. Disable the vacuum advance smog controls if they have not been. Your factory timing setting WAS FOR ONE THING..........smog control, and nobody cared "how it ran" except the hapless owner

4...Carb.......I "ain't" expert on slant carbs. I would consider getting an older "pre smog" carb. Actually, anything built 68 and later Federally is a "smog" part, but it gets worse and worse as the years went on.

5...Carb "snorkel" heat.......These cars originally came with an air cleaner bonnet heat system. Again, read the manual, make sure that is working.
 
On the octane.....Not really.... it ought to run on anything. Is it doing this at lower and medium speeds, and does it not do this if you are at 55 mph and floor it, for example? Pinging will be more common at lower engine speeds, if you (or the trans if it is an auto) do not downshift, and the engine starts to lug. And, it will do this more easily with advanced timing and a lean carb. The Dart Lite's and Feather Duster's in '76 were more prone to pinging with their extra lean carb.

1) Your 26 mpg on that run kinda makes me think your carb is on the lean side, especially with just a standard 3 speed manual or auto trans. I would check the float level when running (look up 'wet float; setting) if this is a Holley 1920, or at least do a regular float setting for either brand of carb. Too low of a float setting will make it lean.

2) Also, what trans do you have? If a manual, you can downshift sooner and avoid this. Or if an auto trans, the downshift linkage to the trans may be misadjusted, delaying downshifts and causing the engine to lug and thus ping.

3) Very slight vacuum leak will lean the mixtures but this would not tend to happen with open throttle settings.

4) The engine's timing marks may be in error and causing you to have more advance than you think. This would give you better economy but more chance to ping.

5) The power valve (economizer valve on the 1920) may not be working and so you end up with a lean mixture when you run at open throttle. That would help gas mileage too, but cause lean running and pinging under hard throttle. The car would also be doggy at hard throttle.
 
67Dart273 said:
In addition to the good comments above, you have to understand that the years from about 73 onwards ARE THE WORST YEARS for runnability / drivability if the rig still has the original "smog" distributor, carburetor and "other stuff." These cars were typically so cold blooded it was unbelieveable.

1....You do NOT want a stat "as cold as" 180. You want AT LEAST a 180, and these cars came from the factory with a 190-195
Click to expand...
Yes, 73 and 74 were the worst years for sure. The 195 t'stat was used to elevate engine and heads temps and keep fuel in suspension better and help combustion in general and thus help with HC and CO emissions, but it made the engines more prone to pinging/detonation, especially with leaner carb settings for the emissions, and helped cause dieseling on shut down and harder starting at times. I would actually prefer to use a 180 t'stat for these reasons.
 
I had a /6 in my '68 for a decade. It was a smooth running motor stock, but I had Doug Dutra rebuild it using the smaller of his two spec'd cams, his Dutra Duals bigger valves, ported and significantly decked head. This deepened and broadened the midrange power significantly with only a mpg or 2 penalty in fuel mileage.

Slants and V8's love more initial timing, but are not tolerant of excessive total timing. It wasn't until I advanced the timing until the rpm stopped increasing(maximum vacuum) and limited the amount of total mechanical advance that it really woke up the motor. With my modifications, the initial timing ended up at 22 initial and 32 total. I then hooked the vacuum advance up to manifold vacuum and it advanced the timing even more at idle(but this does not increase the idle).

Once set-up like this, the car was dramatically more powerful off idle, would spin the tire out of the hole and got those 1 to 2 mpg back. This is the same technique that I use on all my V8's with distributors. The concept being that as you depress the gas pedal, the timing falls back to your initial settings reducing the tendency to detonate or ping while giving you more power right of idle and through the midrange. Max power output is not affected as the max timing is set by your total timing.

I now have a stroked 5.9 Magnum in the car. If driven like a normal person, it gets better mileage in the city and on the highway, runs and sounds way WAY better and is much more fun.
 
Wow, thanks for the added replies.

I went up in my loft and found the old dual snorkel air cleaner form my 70 Charger (running a chrome edelbrock on Charger now) and robbed the manifold heat sensor from it. Bench tested per 70 Charger FSM and Chiltons and it worked ok and worked on the car like it should. Flap is up when engine is cold down when warmed up. Can't buy one of those anywhere so it was a score.

I also spent time working with the choke cam, filing the steps and screw end for smoother movement and it seems to come off the choke sooner (within 2 miles versus 3)
I also got a new choke t stat from rock auto, actually an nos tomco electric unit, j.i.c.

The choke on it now is the old school well type situated in the exhaust manifold.

I had rebuilt the carb last year and had a ***** of a time getting the float setting right without the float guage. As I recall the wet fuel level was just shy of spec (1 or 2 32" lower) but it ran good and didn't flood so I wasn't gonna risk taking it apart AGAIN and ruining the bowl gasket (and having to buy another rebuild kit just for the gasket). I had the bowl off too many times to mention.

This car has absolutely no emissions junk on it. Vacum advance goes straight from ported vacum to distributor and functions as it should. Egr is blocked off. Distributor is brand new points unit (repl by previous owner). Basically it's old school. Carb has no emissions solenoids or anything.

It's supposed to be cold tomorrow morning so I'm going to see if the things I've tried per your suggestions so far make a difference.

As for the pinging, yeah, full throttle would induce it and would climbing a hill on the highway, typically in the 2,100 to 2,500 rpm range. 2.76 gears, 205 75 14 tires and auto trans. I tried messing with the timing and put it as far as slightly after tdc and it ran good, believe it or not, but still pinged. I did consider that the timing cover or damper may be wrong but I have nothing to compare it to. all my Mopar buddies have V8's as does my charger.
 
Well the low float is very likely the cause of the pinging; every carb circuit (except maybe the accelerator pump) will be lean. 2/32" low is a fairly significant amount for a float to be low. The 1920 float can be checked via the wet float method without taking the carb off but changing the float setting does require the bowl to be removed. Not sure why to NOT find a gasket to change the float setting and get to this.

Checking the timing marks is like any other engine.... use the piston stop method to find true TDC on #1 and see where the 0 mark lies in relation to that.
 
nm9stheham said:
Well the low float is very likely the cause of the pinging; every carb circuit (except maybe the accelerator pump) will be lean. 2/32" low is a fairly significant amount for a float to be low. The 1920 float can be checked via the wet float method without taking the carb off but changing the float setting does require the bowl to be removed. Not sure why to NOT find a gasket to change the float setting and get to this.

Checking the timing marks is like any other engine.... use the piston stop method to find true TDC on #1 and see where the 0 mark lies in relation to that.
Click to expand...

Yep, checked the wet fuel level today. It was 5/32 low. Yikes!!!! I'll be looking for a gasket after I'm done here in case the one on it now doesn't survive another removal and will bite the bullet for a whole kit if need be. Cheaper than running plus grade gas or replacing pistons.

Yeah, thanks. After I logged off I recalled the piston stop method from a Mopar Action article.

Thanks for all the input so far everyone. It was 25 deg this am and i took it out for a spin. Seamed to do better as far as dramatic gas usage. Ordering a gasket today and will retest as soon as I get the float adjusted and running.

Al
 
Scored some float bowl gaskets from Mikes carburetor parts, sorry for got the link. Keep you posted : )

Al
 
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