Mpg 318

-
S

shanedy

Member
Joined
Dec 16, 2010
Messages
18
Reaction score
0
Location
pa
Have a 1971 swinger 318 5.2l. Engine rebuilt. New plugs plug wires electronic ignition PVC valve replaced. Rebuilt carb which is a 2 barrel holly original carb. Seems to run ok but I do expect alittle more performance. Only getting 10 mpg on highway I think 15 to 18 would be right with today's gas. Any ideas as to why such poor gas mileage. Vacuum at ports are good also
 
Ignition type, timing advance and total timing have a lot to do with this.
What are your details?
I get about 17 with a four barrel, but I use an A/F guage to tune with and run HEI ignition. (which adds about 2 mpg right off the top)
 
Something is not right , I get over 20 with a 340 n, you should be getting at a MINIMUM 15 mpg , my old 1970 440 New Yorker with a 440 got 12 MPG on the highway , the 75 440 New Yorker got 16 mpg highway .
It's not the crap gas of today , what gears you running ? What shape the transmission in ?
 
yes,i have a warm 340 and it is 16 - 19 in all round driving,depending on how bad i am.
..i think if babied on the highway it would be 20+ (imperial Gallon)
..tuning,gears,converter,tire size,cruise speed...lots to consider
 
Before you change a thing, you need to diagnose the bad mileage problem. First, verify that you are measuring it correctly and go from there.
 
Ignition is a summit distributor with vacuum advance. Using the flame thrower coil. I'm at 2 degrees advanced total advance I don't know. Using auto lite plugs. Some tell me I should be using champion instead. A faint smell of gas is present only when driving not when sitting. Seems gas is not fully being ignited. Carb / ignition don't know thanks for your input
 
Transmission was also rebuild. Checked mpg 3 times with each right at 9-10.
 
Run more initial advance. With the vacuum advance disconnected and line plugged. Try to advance it until it starts to be difficult to start. Then back off 4*'s. With the vacuum advance now hooked up, you want a max of 52*'s total. If you can not adjust the vacuum timing with a 3/32 Allen wrench located in the vacuum advance canister, back total down to 52*'s.

Re tune the carb as best you can. I myself can not stand Autolite plugs and only run the Champions in my MoPars.
What gears and tires are you running?

Is this engine a 318 or a 5.2? There NOT the same.
 
Thanks for the reply. I will try adjusting the timing. The engine is a 318. Thanks again!
 
Is it orginal drive train, cause if the rear gears, tires or speedo gears are off you might be getting the wrong info to calculate your mileage from.
 
RustyRatRod said:
Before you change a thing, you need to diagnose the bad mileage problem. First, verify that you are measuring it correctly and go from there.
Click to expand...

x2

And also make sure that there are no fuel leaks from the tank to the carb. Check all lines and tubes for leaks....
 
To calculate the mpg I travel where I know the miles driven. Pretty accurate. No leaks from tank to carb. Checked that being I was smelling gas. Thanks for the additional info
 
Pull a couple of plugs, if you are smelling gas most likely it is running rich.
 
I have 318 on my duster. 904, 3.23 gears, second smallest lunati cam, bad headders, its recentry rebuilded about 9.0:1 comp,

HEI ign and one of those cheap summit 600cfm carb

Car gets about 15 mpg on mormal driving 40% town 60%highway.
Longer trips i can get 20mpg if i drive really carefully.


Check timing, then carb tuning, plenty of good info about both in fabo.
whats your idle vacuum and, if you can do it compression test that new motor

And DONT use cars speedo/trip meter to calculate your mpg.
Fill tank full, drive normally and calculate trips you drive from google maps or other similar places, after second fill, you have accurate result.

Ofcours if your normal driving leaves your tires smoking then that 10 mpg is really good:)
 
How about doing a compression check wet and dry to see what we are working with?

Start with the basics and work your way from there.
 
Speedo accurate? I always use a GPS when checking mileage. Thin radial tires inflated to the max, synthetics everywhere?
 
inkjunkie said:
Speedo accurate? I always use a GPS when checking mileage. Thin radial tires inflated to the max, synthetics everywhere?
Click to expand...

good point. How accurate is your speedo.


Go on the highway and get up to 60 mph on your speedo. Now hold it steady at 60 and time how many seconds it takes to go from one mile marker to the next. If it is not exactly 60 seconds, then you need to correct for the spedometer inaccuracy.
 
Your car would have to be running pretty rich to be eating up 6-10 mpg.
What colour is your tail pipe and plugs.
Other things would be install a vacuum gauge, tach and even air fuel ratio meter to diagnose and to tune.
 
krazykuda said:
What about the Carter 2bbl?
Click to expand...

BBD's have treated me very well. ive rebuilt quite a few for friends and other people, havent had one come back yet. those damn holleys though, they have burned me more than once. ive only had one that actually ran well. one out of 6. their limited adjustability doesnt help much. ive converted everything I own(ed) to 4 barrels, thermoquads, couple 4160 holleys, and a 4175 holley, and quite honestly, i havent looked back. mileage increased, driveability was better, and cold starts are no problem. obviously theres more things that could be wrong, but with the OP stating hes running a holley 2 barrel, id question it, even if its rebuilt. air/fuel gauge or vacuum gauge, and a plug reading is where id start. (NGK GR4's, get some, you wont regret it)
 
did the compression range 125 to 135 plugs are black, all 8. carb is the holley 2210 im petty sure. Will look at the float being its running rich and adjust it a little along with the timing. Thanks to all!
 
got 20 mpg with a 318 2bbl automatic. My brother got 24 mpg with the OD 4 speed in a Dodge Aspen R/T.
 
my recommend

go to a junk yard. find a car/truck with 318/360 with electronic ignition. 1970's

grab the dizzy and the electronic ign box. mark the dizzy before you take it out.

bolt it into your car lining the mark to where it was in the other car as a starting point.

put in your fresh plugs
i bet money it runs better.

then take that summit distributor and fling it as far as you can throw it
 
You must log in or register to reply here.
-

Back
Top