MISERABLE Mileage.............

-
pastortom1

pastortom1

Well-Known Member
Joined
Nov 29, 2005
Messages
3,466
Reaction score
60
Location
Zephyrhills, Florida
My buddy FINALLY gauged his mileage by filling the tank and checking it at the half way mark.........I almost CHOKED...........(which is probably what the CAR is doing).............. 9.3 miles per gallon with his Super Six !

I've tuned enough motors to know the obvious things that need to be taken care of, but I have NEVER heard of this lousy mileage, even on a clunker.

This car is a Volare wagon, 1978.......he says there is a Cat under it........I wasn't aware that they used cats as far back as that, but I suspect it's pretty well plugged for one thing............the timing is a bit off for sure (sometimes it deisels a bit when you shut it off)........I will lay odds that the choke is set too rich and not opening all the way, especially since he just installed a 160 degree thermostat in it and it runs way down by the first line in the cold........as I remember, those chokes needed proper temp to open all the way.

Air cleaner is OK........has electronic ignition.........probably needs plugs and wires, cap & rotor, etc...........

Anything you guys can add here? Any little particulars for the Super Six you can think of that I should look for?.............At $3.50 a gallon, this guys goin' broke fast................... :help:
 
well the restrictive exhaust is a killer. My super six wagon (63) is getting 19 ish mpg (up from 14-15) but i put a 2 1/4 " pipe all the way back with a turbo muffler. Make sure that your valves are adjusted properly, tune up, proper plugs, and make sure that the 2 bbl isn't running rich. I'm going to recurve my distributor ( worth 2 mpg) and make sure the vaccumm advance is working. Vaccum advance is important for mpg. Check that the trans kickdown is functioning correctly and make sure all the electrical connections are solid (aka bulhead connector). That should keep you busy ...... for a while.
 
I have alllways gotten the most terrrrrible mileage from my /6's.
 
I averaged 12 -13 mpg over the last 3 years in my daily driven Super Six /6.

225 slant
mild performance cam (unknown specs)
super six with way-too-rich 318 sourced BBD carb
2.25 " exhaust all the way back
904 with stock T/C
2.76 rear gear.

It ran really rich at idle and most of my driving was city driving around LA. Lots of idle time. The exhaust would leave a black stain on the rear valance. If a white car was parked behind it when it was warming up...well..that water/soot mixture would really make a mess.

I flogged that little engine and manually shifted my auto at higher RPM than auto shift points. If I was nice on the highway (65 mph and not standing on the gas) I got around 19 mpg. Most of the time on the highway (when it wasn't backed up) I would tool around at 80 (or get run over by others).

Overall I was pretty happy with the gas mileage, especially how I drove it. I know it could have been better if tuned correctly and if I could keep my foot out of it. I had v-8 envy.

Now I'm hoping that I can get my 340 to get similar "bad mileage" as the slant six. I'm around 9 mpg right now without vac advance distributer (yet) and a poorly tuned orphaned Holley 4010 750 DP. I try to get in a couple acceleration runs per drive.

I have a new Quick Fuel Q-series 650 vac secondary carb that is going on at the end of the week. I think it will meet my goal if...I can just keep my foot out of it... The good news is that I no longer have v-8 envy. Maybe Oil company profit envy...but not V-8 envy.
 
linc's advice is very sound and complete.

One other thing I would check is the heat riser in the exhaust manifold. Not too big a deal during the summer. But very important in the colder/winter months.

With those long intake runners, the /6 has a habit of condensing fuel into liquid in the intake if it isn't kept nice and warm. The liquid fuel will pool and run into the cylinders making the car run crappy, and affecting fuel mileage.

Be sure and do your valve adjustment when the engine is thoroughly heated up. I did mine just the other day with the engine running and fully warm. It definitely made a difference.

Please let us know how you fare!

Chris E.
 
did you make SURE that the manifolds aren't leaking? Did you plane them out or mill? My super six had some vac leaks initially and it was running ok but I had it tuned to run with the leak and yeah it got about 8-9 MPG. after truing up everything ( see this post http://www.forabodiesonly.com/mopar/showthread.php?t=33620 )
I'm getting 15ish around town and 21-22 on the hwy.

oh and keep a close eye on the 160* tstat, It's great if the system is up to the task but it may end up overheating....
I know, you are thinking I'm crazy but it's because of:
1. tstat opens at 160
2. coolant flows from the rad into the block and from the block to the rad.
3. "cool" coolant hits the tstat and it closes.
*4. if the rad can't cool the coolant enough before it makes it back to the tstat, the tstat will never close and the engine will get hotter and hotter.
 
Couple of things if you are still fighting this. Was just reading the bbd manual a bit yesterday. Make sure your carb is good. A careful rebuild may be in order and make sure the throttle shaft is not worn and needs rebushed.

In the first few pages of the bbd manual it mentions to make sure you have a good bowl cover gasket on the carb. The bowl needs to be sealed for the vent to work correctly. Says that an airleak here can cause a mileage complaint. Would be worth checking the mating surfaces are clean and straight too.

Also mentions making sure the bowl vapor vent adjustment must be made to specs. Says if the vent opens too far or hangs open it will allow an external vent to the bowl resulting in poor mileage.

Anyway, hope you figure it out as from all I've read on the / board that is very poor mileage and something is out of wack. Once tuned the good mileage, from what I understand, is from the car running in general operation on just the one barrel which is smaller than the single barrel carbs in size. If you put your foot in it the other barrel comes into play. You will have more power, but fuel mileage will suffer. Seems a popular tip to hook up a vac gauge and drive using that. One specifically for showing you fuel mileage range you are in. Driving with an eye on that is said to help. You should be getting much better though.

Let us know how you got on.

Cheers
 
My `74 Dart daily-driver had a bad carb and was giving me horrendous mileage.... Luckliy I found an NOS Holley on e-Bay and all my problems went away. Best $$$ I spent on that car.
 
There absolutely no reason or benefit to using a 160 t-stat, get a 195 back in there then go through all the obvious tune up things. Also, if you are not required to have the cat for emissions testing or a visual inspection get it out of there.
 
the cat' converters appeared in 74. That is most definately part of the mpg issue and causing it to run hot as well.
A worn out distributer will vary the reluctor or point gap costing performance and mpg too.
 
If you don't need the cat chop it off, for "off road" purposes only of course :wink:

If you do need the cat, drill a 1/8 to 3/16" hole in front of the cat, use a metal tapered adapter to hook a vacuum/fuel pressure gauge in to the hole. Run the engine at 2000 rpm and look at the gauge, anything more then 1.5 psi and the cat is plugged.

Test the vacuum pod on the distributor.
Check the timing chain for slop.
Plot your timing curve with a dial back timing light. Do it twice, once with the vacuum connected and once disconnected.
Put a 190 or 195 thermostat in it, a 160 stat just causes extra engine wear by not letting the oil get hot enough to burn the crap off.
Check the clutch fan, a stuck fan clutch will cost mpg at hwy speed.


What carb does it have? Has it been rebuilt recently? Don't buy a "reman" carb, rebuild what he has.

There are many other "tricks" to getting better fuel mileage, but you MUST start with a solid foundation.

TopHat
 
The carb is the original 2 bbl for the super six........It does not appear to have been rebuilt...........problem is that this is an every day driver, and we can't wait for a rebuild.....we'll need to buy one already done from somewhere.

The cat is still on the car, but is short lived.........we may go off road shortly.

From what I'm reading here, some guys have had good results with a new or rebuilt carb...........the 160 can be swapped out, no problem (will this affect mileage at all?)..............and we've yet to change the wires, plugs, cap, rotor, and reset the timing.

How about a recommendation...........we've unhooked the gizmo on the air cleaner (forgot what it is called.....supposed to adversely affect mileage).....checked carefully for proper vacuum connections and leaks.....replaced the heat riser tube to the air cleaner..................WHEN we do the ignition components, what would be a good timing to start with?

:read2:
 
Definately needs work. I just overhauled my 273 2V, punched it out .030, installed 10 to 1's with a mild isky cam and ran dual 2-1/2" X-pipe thru turbo mufflers and am getting 21 MPG on the highway. That's Denver......6000 feet.
 
pastortom1 said:
...........the 160 can be swapped out, no problem (will this affect mileage at all?)..............
Click to expand...

The higher temp t-stat will help mileage. Back in the 70's they actually started putting 205 degree t-stats in cars to try and improve the mileage.

Back in 75-76 I was working for Standard Thompson, they were the primary supplier to Ford, GM and Mopar for t-stats and we were suppling to all three a unit that started to open at 203 and was fully open at 207 degrees.
 
As for the timing, Mine is set at 7*BTDC and runs quite well, at 10* it had some pinging issues at partial throttle. The books (depending on what one and what year) will tell you anything from 0* to 5* BTDC for the slant. I agree with tophat, check the vacuum advance pod on the dizzy, if you aren't getting enough advance at cruise, your mileage will suck big time.
and for the rebuild, buy a kit from the local parts place and do it yourself, it will take about an afternoon including cleaning time. The only things to be careful of are the 2 check balls, one under the accel pump and one in the venturi's, search for the bbd manual at the slantsix.org site.
It sounds like you are on the right track, replace the usual maint stuff and check mileage again, then on to the harder stuff :)
 
I don't feel so bad, in light of this. My 408 stroked Duster with a 3000 stall and 4.10 rear drove to the strip 45 miles away at 55mph, made 3 passes, and came home, earning 11.2 mpg for the trip. Not so bad in light of these Super Six numbers...
 
Rub it in, rub it in.............. (sounds like a song I've heard).

We'll git 'er dun pretty soon here..................:read2:
 
Shoot... I guess I have to go look for a replacement bowl vent fitting for the top of my 318's BBD. Also, anybody know how to change the PCV filter? Mine seems like a single unit and I can't see any way to break it open to change the filter.
 
You must log in or register to reply here.
-

Back
Top