Poor SL-6 Gas Mileage

-
mydart270

mydart270

myDart270
Joined
Mar 18, 2014
Messages
190
Reaction score
7
Location
Brattleboro, VT
i am new to the forum but love Dodge Darts, esp SL-6s. Here is my question.

Why am I getting very poor gas mileage? I fill the tank and after short ride, its down to 3/4 full. I am checking the following:

(a) plugs, must ck color today, gap =.030, AUTOLITE 66 plugs
(b) my timing is 5 deg BTDC, 42 deg DWELL
(c) manifold heat control valve is free, must ck if it rotates freely upon rev at idle
(d) carb is HOLLEY 1920 (not the best but adequate)
(e) must ck compression today, rebuild engine at .060 oversize (car was cheap, TX car, no rust, looks like heavy use initially, immediately pulled eng for rebuild and started restoration)
(f) valves set at .010 IN, .020 EX (cold adjustments)
(g) eng has less than 500 miles after rebuild, was fighting hi temps after 15 mins driving, new re-cored rad solved problem
(h) drum brakes not dragging
(i) maybe fuel gauge sender in my gas tank is goofy, anybody know the resistors to ck EMPTY, 1/2 FULL, FULL?
(j) 13 in tires (original size)

What else can I should I be checking? Thx a bunch.
 
fill it and fill a gas can write down the mileage.Take it out and drive it until it runs out of gas . see how miles it went and divide by 18 to get an accurate picture of what your mileage is. Or,just fill it and go exactly 100 miles and fill it again.Divide the 100 by the number of gallons you had to add .Either way will give you an accurate picture of what is happening.My money is on the sender or a bad ground at the sender on the tank or the voltage limiter in the dash.You say you were running hot,well maybe you weren't. The gas and temp gauge will both malfunction if the limiter is bad or starting to fail
 
Check both the vacuum and mechanical advance in the distributor if either is bad it will severely affect economy.
 
valiantwagonguy said:
fill it and fill a gas can write down the mileage.Take it out and drive it until it runs out of gas . see how miles it went and divide by 18 to get an accurate picture of what your mileage is. Or,just fill it and go exactly 100 miles and fill it again.Divide the 100 by the number of gallons you had to add .Either way will give you an accurate picture of what is happening.My money is on the sender or a bad ground at the sender on the tank or the voltage limiter in the dash.You say you were running hot,well maybe you weren't. The gas and temp gauge will both malfunction if the limiter is bad or starting to fail
Click to expand...
Easier to just fill the tank, drive a few days, then refill it and divide miles driven by gallons put in on 2nd fillup...
 
I've owned my 65 dart for a while now with a 225 /6 one barrel Carter and automatic tyranny and 2:93 gears. I did a few changes on my car/engine : I run NGK nickel V groove plugs 0.035 Gap . Modified a Mr gasket 10" air cleaner to fit my little one barrel with K&N filter. Opened up my exhaust system just like the dart arrival in the latest issue of mopar muscle magazine. I bought a "had saver " vacuum gauge from my local speed shop to adjust my timing and the face on the gauge has acolor setting :red = bad yellow = ok green = good blue = best for the fuel/air mixture screw and idle. I set the valve lash when engine is hot and idling at it lowest point . I also put 2 3/8's thick fiber base gaskets under the carb to keep the heat from percolating the fuel in the carb on hot days easier starts just add longer bolts and a swivel from a 1/4 inch socket set on the carb rod. It purrs like a kitten
 
Lean the carb a tad and see if it affects diveability. You running manifold vacuum to the distributor? the "economizer" valve in the 1920 can get gummed up. Im not sure how you clean it.
 
Last edited:
I would guess the gas gauge does not read right.

The gas gauge gave up to the ghost a few year ago in my old truck, I just fill up every 100 miles, never ran out. I just divide the amount it takes to weigh the tank back down, to how many miles driven.
It stays about the same 14.5 MPG, not bad for a V8.
 
Nice in a truc wish I got that mileage!. I have 87 318 in a W150 barely break 10! And the gas gauge is gone. I have 32 gallon gas tank fill it every 200!
 
halifaxhops said:
Nice in a truc wish I got that mileage!. I have 87 318 in a W150 barely break 10! And the gas gauge is gone. I have 32 gallon gas tank fill it every 200!
Click to expand...

Its in a w150 it gets 14.5 every tank.
If it makes you feel better I have a 2017 6.4 hemi that gets 10 mpg driving the same roads.
It sucks fuel down so fast it just sits in the driveway, it has only 3800 miles on it, a super thirsty vehicle.
 
Sounds like you are not getting a full tank when filling up. Try keeping the nozzle in for a few seconds after it clicks off and put more into it at a slow rate until full. I actually have to hold the nozzle in place on my 64 Valiant otherwise it wants to fall out. Also it is my experience after a fill up that the gauge Stays high and it takes 30-40 miles before it starts dropping. I checked my original 1964 sending unit so I reused it. I've heard too many bad stories about "new" replacement sending units. Sounds like you do not have much history with your new car so it's an unknown for you.

After I completely rebuilt my 225 it seemed to run a little on the hot side but seemed to settle down after break in. After about 100 miles of just buzzin around I reset the lash, adjusted the carb and thinking I probably went with 8-10 degrees of initial timing. I used all new ignition parts recommended by SSD (points) and the NGK long reach plugs.

Almost 2 years and just short of 4000 miles I finally pulled a plug for the first time last week and it looked good. It runs so good i just knew i had no issues. Drove it 320 miles each way to Carliske and got 23.8 MPG from A to B. A being a gas station 1/2 from Interstate here to point B, a gas station right at the Carlisle turnpike exit. 3.23 rear with manual 3 speed.I did run the valves before leaving and only a couple were out.

I'm driving this hulk about 6 hours each way to the /6 race in KY this Friday so I'll change the oil and check the dwell tonight just to be sure.
 
dazedand confused said:
I've owned my 65 dart for a while now with a 225 /6 one barrel Carter and automatic tyranny and 2:93 gears. I did a few changes on my car/engine : I run NGK nickel V groove plugs 0.035 Gap . Modified a Mr gasket 10" air cleaner to fit my little one barrel with K&N filter. Opened up my exhaust system just like the dart arrival in the latest issue of mopar muscle magazine. I bought a "had saver " vacuum gauge from my local speed shop to adjust my timing and the face on the gauge has acolor setting :red = bad yellow = ok green = good blue = best for the fuel/air mixture screw and idle. I set the valve lash when engine is hot and idling at it lowest point . I also put 2 3/8's thick fiber base gaskets under the carb to keep the heat from percolating the fuel in the carb on hot days easier starts just add longer bolts and a swivel from a 1/4 inch socket set on the carb rod. It purrs like a kitten
Click to expand...
hey many thx. Will order NGK plugs on Wed and try. What number?? Will ck out fuel sender also, could be screwed up. Timing currently at 5 BTDC, will try 10 BTDC.

Can you show me a pic of the 1/4 in swivel used for the carb throttle?
 
They were not just any NGK spark plug but ones with an extended reach recommended by SSD. Don't have the number handy but should be able to post it later. Note that they are not listed as a replcement for a /6.

Also in general, if your engine has spark plug tubes you need to remove the plug gasket.
 
KosmicKuda said:
They were not just any NGK spark plug but ones with an extended reach recommended by SSD. Don't have the number handy but should be able to post it later. Note that they are not listed as a replcement for a /6.

Also in general, if your engine has spark plug tubes you need to remove the plug gasket.
Click to expand...

Ngk zfr5n. I dont know the stock number. I run them in my 71 val.
 
Me too. According to my GPS the speedo is dead nuts accurate. (195-70 x 14 radials replacing the bias ply 6.50 x 13 originals)
Original sending unit checked out OK so it's in the new tank. (18 gallons!)
 
finally got mine reading accurate for the 3rd time of changing rear gears. And my gas gauge works. I'm getting 15.5 in my van. Got rid of the points and went with the pertronix kit AND E3 plugs, What R YOU RUNNING. don't know if they helped but sounds good. Opened up the exhaust flow also.
 
Points can work...but i use them drive a ferd tfi module. It was their electronic ignition from the 80s into the 2000s.

Ferd used a hall effect distributor to drive the module. Module sets the dwell and uses a high ouput coil.

Well, module doesnt care if it sees a - square wave or the - signwave from my points. Just pull the condenser and voila.
Basically, pulls all the current out of the points and the tfi does the hard work. Mounted it on a heat sinc. Any thing with points can drive it...

Super reliable, (ferd did mount them on distibutors early on but changed it because heat was killing the modules) and i got modules from junk yard. Keep one as a spare. Just incase. And use an accel 75kv coil.
 
Rim Size needs to go. Switch to 14 inch rims 215 70 r14. Better braking and easier on the RPMS
 
You must log in or register to reply here.
-

Back
Top