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Mysteriously bad gas mileage
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Author:  Seattle07 [ Fri Jul 06, 2007 1:29 pm ]
Post subject:  Mysteriously bad gas mileage
I have posted here a couple of times, and every time you guys have helped me solve my car troubles; very helpful forum.

So heres the problem: I recently installed super six 2 BBl setup on my 75 Valiant, and after discovering it was running way too rich (8-12 mpg) I lowered the jets down three sizes to .083. I tuned the carb again, and the car runs noticebly slower with the smaller jets, but other than that runs great. Now my gas mileage is 14 mpg. I have set my timing at 5 degrees BTDC, and i know my vacuum and mechnical advance is working. This 14mpg was city driving, and by city driving I mean driving in Seattle with LOTS of stop signs and lights. Would a properly running 2 BBL setup give 14mpg in these driving conditions, or is something wrong with my car? I keep hearing figures like 18-21 mpg with the super six, and just want to make sure my car is running properly.
Author:  Aggressive Ted [ Fri Jul 06, 2007 2:12 pm ]
Post subject:  Super Six
What rear end ratio do you have? Sounds like the mileage a truck would get with a 4:11 rear end. I know of a few that are in the 12 to 14 mpg range.

With all the hills we have you could be lugging it down alot causing the power valve to come on more than it should. How do the plugs look? are they running white? light cinnamon? or dark gray?

My 77 Volare with the Super Six would get 22 average city and 28 on trips over the mountains with a 2.76 gear ratio.

My 74 Swinger drops to about 20 in town and 23 plus back and forth to Everett from Black Diamond. However, it is a single barrel with a #57 jet in a 1920 Holley, and 2.76 gear ratio 8 3/4 Posi. It is a new motor still breaking in.
No EGR on this motor, just the PCV, gas vapor recirculator on the carb vent and hot air in the snorkel. Pretty simple set up.

Both cars have 30 degrees timing (mechanical and initial combined). The Volare was set at zero initial and 30 mechanical, the Swinger has 20 initial and 10 mechanical. If I can find some MOPAR springs I will recurve the distributor closer to the Volare set up.

Let us know what rear end ratio you have and what color your plugs are running. Sounds like your still way to rich. Maybe a better two barrel carb is in order. Sounds like you snagged a lemon.
Author:  Seattle07 [ Fri Jul 06, 2007 6:02 pm ]
Post subject: 
Yeah compared to your numbers 14 seems a little low. My ratio is the stock 2.76 (thats stock for an A body 7 1/4 right?). How do you know your total timing advance? I only know how to set my initial advance with a timing light.
What timing advance combo is best for mileage?

as for the carb, my spark plugs are white, with a little bit of cinnamon. I've rebuilt the carb, and everything works. The mixture is lean enough that I can get it to ping on hills if I push it.

what is up with this car?
Author:  sandy in BC [ Fri Jul 06, 2007 6:09 pm ]
Post subject: 
'Well..the motor is new...it takes a while to break in...its a step or 2 too lean.....the rest sounds good....
Author:  Sam Powell [ Fri Jul 06, 2007 7:25 pm ]
Post subject: 
Do a search here for threads on this topic. There are lots of them, and many suggestions for things to check. One of my favorites, because it is often overlooked is to make sure the charging system, and the primary ignition is not losing voltage. Check at the alternator, it should be 13.5 volts, and then check at the high side of the ballast resister. It should b no less than .5 volts down. If it is down more, you will lose some efficiency in the engine. If there is more drop than .5 volts, then work your way back through the system and find where the voltage drops, and change out that wire and connector. I am sure you can do better than what you are doing now. When my car was stock it went from 13 to 18MPG when I changed out the primary ignition wires. I was losing a full 2 volts from alternator to ballast resister.

Sam
Author:  Rug_Trucker [ Fri Jul 06, 2007 9:37 pm ]
Post subject: 
Check the tag on your rear. Mine is factory with 3:23's. My mileage is better than yours.
Author:  carpdar [ Fri Jul 06, 2007 11:44 pm ]
Post subject: 
Replace your wiring harness if you always have had poor mpg. If you just noticed that the mileage went to hell after your modifications then it could alot of things. The wiring harness will also make your engine run ALOT smoother.

thanks,

Aaron
Author:  Seattle07 [ Sat Jul 07, 2007 1:39 am ]
Post subject: 
Yeah, replacing the wiring harness seems like a good Idea. Where can I buy a new one?
Author:  Sam Powell [ Sat Jul 07, 2007 6:45 am ]
Post subject: 
You can get a new harness from Year One, and they are high quality,but expensive. I just rewired it myself. It is a simple, one wire at a time proposition. Just run the diagnostic test I mentined first. If there is no voltage drop, then save your time and money, and do that at a later date. In general, new wires work better than old ones. I don;t know why. It seems as if the wires themselves develope high resistance. I know when you strip the insulation back the old wire is often dark, and corroded, but you would think the core of the wire would still carry current as well.

Often the biggest drop in the system happens at the ameter. Those can develope high resistance in them, and really draw down the entire electrical system. And then there is the bulkhead connector which has been open to the elements, and heat from day one. I simply replaced everything on ewire at a time. Unwrap the harness, and take out only the wire you intend to replace then. I soldered all the new connectors on.

Sam
Author:  Aggressive Ted [ Sat Jul 07, 2007 2:19 pm ]
Post subject: 
I did as Sam suggested, one wire at a time. I found new stock connectors and installed them as well. It made a huge improvement!

The most important and simplest is doing the bundle going to the coil and alternator.
Sam's suggestion is a good one!
Author:  Aggressive Ted [ Sat Jul 07, 2007 3:15 pm ]
Post subject: 
Quote:
Yeah compared to your numbers 14 seems a little low. My ratio is the stock 2.76 (thats stock for an A body 7 1/4 right?). How do you know your total timing advance? I only know how to set my initial advance with a timing light.
What timing advance combo is best for mileage?
2.76 is not necessarily stock, some cars came with lower rears, 3.23, 3.55, etc.

I use a timing light and check it at 2000 rpm and 2500 rpm and 3000 rpm
to see what the initial and mechanical comes out to. Unplug the vacuum advance when you do this. Write down the timing numbers for all three. Then do it again with the advance plugged back in and you can capture the total. It will show a higher number if your advance is working. That is your overall total.

I followed Slant Six Dan's advice and file/painted notches in my damper every 10 degrees so you can use a normal timing light. You can also use a dial in type timing light with just the stock timing mark.

Let us know what you get for all 6 tests.
Author:  SlantSixDan [ Sat Jul 07, 2007 3:56 pm ]
Post subject: 
Quote:
2.76 is not necessarily stock, some cars came with lower rears, 3.23, 3.55, etc.
Actually, 2.76 was indeed stock with automatic transmissions starting in '68. 3.55 wasn't available after '62 in the North American market. 3.23 was stock with 225/auto through early '62, and with 170/auto from '61-'64, after which 2.93 with automatic and 3.23 with stick shift. The '75 in question almost certainly came from the factory with 2.76. It's possible someone replaced the rear axle, but even a 3.23 would not cause 14mpg.

Seattle07, I don't think your problem is anywhere near the rear axle. I think it's probably under the hood. The carb should not have required jetting-down to solve rich running; it is likely the carburetor is causing a significant part of the problem. You would probably do well to swap on a better one. One critical key question: What kind of mileage were you getting before you swapped on the Super Six setup?
Quote:
I followed Slant Six Dan's advice and file/painted notches in my damper every 10 degrees so you can use a normal timing light.
Instructions and details on that are here.
Author:  SlantSixDan [ Sat Jul 07, 2007 4:09 pm ]
Post subject: 
Quote:
You can get a new harness from Year One
Yeah, but they don't make 'em, they buy 'em and mark them up. You may be able to get a good-quality new harness from Evans Wiring, http://evanswiring.tripod.com/id3.html (contact them if you don't see what you need on their page)
Quote:
I just rewired it myself. It is a simple, one wire at a time proposition.
One of the advantages of pre-'80s cars is that DIY repairs like this are possible with simple, basic tools and parts, together with actual wire.
Quote:
In general, new wires work better than old ones. I don;t know why.
It's because over time, the magic electrical smoke seeps out of the wires. If a short circuit develops, all the magic smoke comes out at once in a big cloud, and things stop working in a hurry. :twisted: Seriously, it's because the factory didn't use very good quality wire to begin with, and they tended to pick just-adequate gauge sizes, so that when the inevitable deterioration occurs, performance suffers.
Quote:
Often the biggest drop in the system happens at the ameter. Those can develope high resistance in them, and really draw down the entire electrical system. And then there is the bulkhead connector which has been open to the elements, and heat from day one. I simply replaced everything on ewire at a time. Unwrap the harness, and take out only the wire you intend to replace then. I soldered all the new connectors on.
That's definitely the way to do it. Also, although I choked on even the bargain-hunted $35 price I found, it will pay anyone working on an old car's wiring to buy a bottle of Stabilant 22A (if you don't turn up a good price with a web search, you can get it through auto parts stores as Standard Motor Products #SL5, or NAPA #CE1). This is not a contact cleaner; there are plenty of those out there. This is a contact enhancer fluid, and you may read about it here and here.

Potential lower-cost source here

Field notes here, here, here, and here (last one is a long page but an interesting read, search on the page for the word stabilant )

I was a skeptic until I tried it. My bottle is still almost full -- you use it a tiny drop at a time -- and it has already performed a couple dozen miracles for me around the house, office, and car.
Author:  dakight [ Sat Jul 07, 2007 4:34 pm ]
Post subject: 
Evans quoted me $135.00 for an engine harness. I ended up doing my own but $135.00 is not a bad price. You'll probably need to send him the old harness to copy but you'll get back one that is better than factory new.
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