318 two-barrel getting 426 hemi gas mileage!

[Dibbons]

Driving my '72 Plymouth Satellite Sebring Plus up from La Paz, Baja California Sur, Mexico to visit friends and family in California. This vehicle still has the original factory stock 318 two-barrel (Carter) motor (numbers matching). Gas mileage for the first 950 miles from La Paz to the USA border in San Diego is averaging 10.7 MPG. My average speed averages about 60 MPH, but I go faster on some straight stretches of highway. Something must be wacko here.

The facts:

History

The motor was froze up before installation, so a year ago I had to do the following:
a) freed up the pistons using various penetrating oils
b) changed out all the engine gaskets and freeze plugs
c) new timing chain and cover, new water pump
d) new distributor, cap, rotor, points, condensor, wires, plugs
e) I personally rebuilt the carb (choke is wired open don't need it where I live in the desert)
f) exhaust manifold heat control valve wired open
g) new gas tank and fuel filter
h) I believe a new fuel pump
After all the above, I installed the motor (4/15) and it fired right up.

That was a year ago. After that I ran a compression check and it was pretty even.

Presently the vehicle condition is:
a) starts up immediately, both when cold (with 2 or 3 squirts from the accelerator pump) and when warm (just touch the key and it fires right up)
b) when starting it cold, a quick puff of smoke is seen from the tail pipes (clears out after about one second) don't notice that puff when restarting it warm
c) transmission was rebuilt a year ago, stock rebuild with a shift kit and performs flawlessly
d) differential is stock and open (no sure grip) 8 1/4 with around a 2:71 economy ratio (just changed the gear oil 90W)
e) tires are new and just finished a wheel alignment last week
f) timing with vacuum line plugged is about 2 1/2 degrees BTDC, dwell is within factory specs
g) oil and filter just changed, cooling system flushed and refilled, new air filter
h) new dual exhaust system that exits in front of rear tires with glass pack mufflers

Engine performance:

a) engine idles smoothly at 700 RPM and purrs like a kitten, which indicates to me the compression is good and even between the cylinders
b) driving stop and go between the stop signs/signals in town, seems to run and shift just fine
c) on the highway downhill it purrs like a kitten, on level roadways it seems to miss intermittently
d) under acceleration (more than half throttle) and whenever under load (going uphill) it misses quite a bit and sounds like crap. Does not sputter like when one runs out of gas, it misses (misses a beat). It accelerates enough to pass slower traffic, but all the missing/popping gets on my nerves.

So, taking the above into consideration, I should at least expect 15 MPG, maybe more. So 10.7 MPG is way off. (I drove a new 1971 383 automatic Road Runner back in the day with a 3.23 open rear and that consistently got 12.5 MPG) Thank you for listening.

 

[BeatersRus]

id highly suspect the points,
everything you describe sounds like points not set right or already burnt up.
in fact id swap over to an electronic ignition if it was my car

let us know how you make out,cheers.
ps,great looking car too

 

[dieseldazzle]

Have you checked you total timing not just your initial timing?

 

[Dibbons]

I replaced the vacuum hose on the vacuum advance after setting initial timing and the timing did not move, stayed as set at about 2-3 BTDC.

I cracked the throttle and the timing mark went up and off the scale on the timing cover, so I know it advanced at least ten degrees.

Don't have a timing tape on the harmonic balancer yet so could not determine total timing.

I pulled the vacuum advance hose off to feel for a vacuum signal from carburetor port at idle and felt no suction. Then I cracked the throttle a little bit and felt good vacuum on the hose. Don't know if 1972 emissions settings for that year limited vacuum advance at idle to nothing, or if that is a symptom of something amiss.

There is a factory speed sensor built into the speedo cable that changed timing based on road speed. That sensor is disconnected for now, but I don't believe that would cause the miss I am experiencing.

The funny thing about the ignition system is that the previous owner told me "the transmission was slipping so much that it was burning up the ignition points". That did not make any sense to me, until I figured out the real problem he was having with the points when I went to replace/adjust a new set of points in the old/original distributor before starting the motor for the first time. Come to find out, I could not set the new points in the old distributor because the cam was worn down so much the points would no longer open at any setting. That's when i purchased the new distributor (made in Taiwan) at Pep Boys that included new points, rotor, cap and condenser. With all of those new ignition parts, I was suspecting the carburetor, but maybe you guys are right and the ignition system still holds the secret to the problem.

I did check the voltage at the coil with ignition on, reads a correct 5 volts.

 

[YY1]

Do you have a vapor return system?

My 73 with the same 318/2 is getting about 13.5, which I think is low.

Problem #1 was the distributor wasn't returning the advance, so it was advanced almost all the time after initial RPM increase, and just came back down when it felt like it. Caused pinging but hard to find.

I'm currently on what I feel is problem #2. When I take the gas cap off, the built up pressure almost blows it out of my hand.

I suspect something is amiss with the vapor return lines, manifold or canister.

I put a vacuum pump on the line from the canister to the manifold in front of the tank. and it almost held a vacuum.

Line might be plugged or heaven forbid, the manifold is toasted.

The car runs great, starts as you describe, one or two pumps cold, none hot.

At first I thought the accel pump shot was too large, as it was quite a bit more than book spec, but I adjusted it down and it caused a lag. Back to where it was and no more lag.

Book calls for 0* (TDC) for my initial timing. Any more and it pings.

I also recommend swapping to electronic. ignition- parts are cheap, and it's an easy conversion.

Not sure if any of this helps.....

 

[Dibbons]

Vapor canister is missing for now, but have a 1973 NOS version to install soon. Going to shop for electronic conversion today. Also have a 1973 NOS Carter Carb I will intall later.

 

[dodge1972]

Vapor canister is missing for now, but have a 1973 NOS version to install soon. Going to shop for electronic conversion today. Also have a 1973 NOS Carter Carb I will intall later.

Voltage at coil when starting should be battery level (12.6) when running typically 9.0 via ballist resistor. If you have a dual ballist resistor, you maybe on the wrong side. Also you should be able to advance your initial timing to 8 to 10 degrees. Your overall maybe too high depend upon distributor setup causing your motor to ping. If have a ping, try using mid grade fuel and retard timing until your engine stops pinging under strong acceleration. That a good starting point. Your cab float level maybe to high or sticking because of old fuel during storage. The worst case is your timing chain was incorrectly installed. Good Luck

 

[BeatersRus]

the reason i suspect the points is because of your description of the problem.
either the points are not set to dwell spec or they are burnt.
runs fine until it gets a load.

heres you a cheapie tip as well,for the electronic conversion process.
the Pertronix units will slip right into that brand new distributor you have and nobody will even be able to tell.
the last one i bought was 79 bucks too.

 

[Bb70charger500]

I had my 74 charger 318 2 bbl 2.71? Gear I was getting about 19 mpg out of it . Unless I was standing on it then we would get in the 11-12 range . Hope you get it fixed soon . The points are a pain I'd just upgrade them

 

[Dibbons]

Update: Installed a Petronix conversion kit in the aftermarket Cardone distributor I had purchased a year ago at Pep Boys. I had to remove it and replace it with a new set of points and condensor when I found out the rotor would not fit all the way down in the slot. The Cardone (made in Taiwan dizzy) uses a smaller/narrower indexing slot/tab and the Petronix rotor would not fit in the slot, too bad. After my new points were installed, the dwell was off so far that it advance my timing from 2 degrees BTDC to about 10 degrees BTDC. i was in a hurry to hit the road again, so I just left the dwell at about 22 (should be 28-34) and left the timing at 10 degrees advanced. Filled up with premium just in case that timing advance was going to cause pinging. Oh, by the way, the old points, which had been installed a year ago, had metal transferred from one side of the contacts to the other side of the contacts, and of course the contacts were a gray (burnt?) color. Also changed the year old fuel filter.

On the highway, it seems the miss was greatly improved, or perhaps eliminated entirely. However, looks like my gas mileage is still in the dumps, I will do the math tomorrow and then check in to share the results of today's MPG.

Today traveled from San Diego to Corcoran (near Fresno), heavy traffic but it was moving above the speed limit all the way.

 

[nstyle73]

I had a similar problem on the acceleration and uphill scenario and mine was the coil. After swapping the coil, the missing was fixed but I had to tune the accelerator pump to completely remove the hesitation upon highway cruising tip in. A little kickdown linkage adjustment finished it off. 318/4 with electronic ignition and 2.71 that would get 16-21 on the highway.

 

[1968rick]

You have not mentioned your vacumn at idle...all the suggestions given to you will affect the vacumn.....do you have a strong smell of gas after you park the car? so bad you do not want to park it in the garage. If so the problem is a very rich running carb...caused by the step up valve or as some call it the power valve not responding to vacumn. I have had that problem on older carbs and the solution was to remove the spring from under the step up piston. Did not even notice it was disabled.

 

[dangina]

just wanna say the car looks great!!! I see you decided to put "plymouth" over the rear tires

 

[Dibbons]

Made in north now, 1462 miles, from La Paz, Baja California Sur to Sonoma. Mileage is now better than initial 10.7, got some 11, 12, and 13 MPG figures depending on driving conditions. Some of the improvement must be due to the new points, condenser, and advancing the timing from about 2 1/2 degrees BTDC to about 10 degrees BTDC (and using premium fuel to ward off any fear of possible pinging). The "tune-up" was performed in San Diego.

Today went to Sonoma Raceway and ran a couple of time trials against my step-daughter who was driving a 2004 Toyota 4Runner with a 4.0L V-6 rated at 270 HP @ 5600 RPM. She beat me by about 2 seconds. I ran 18.37 @ 75.37 MPH and 18.41 @ 75.79 (shifter in "Drive" and not hitting 3rd gear until right past the finish line). Her 4Runner ran 17.05 @ 66.11 (she stepped on the brakes too soon) and 16.38 @ 82.10. According to an ET calculator my 1972 SSP auto was putting out 125 rear wheel horsepower from the original 318 two-barrel.

 

[Dibbons]

Update for return trip: NOS carb and vapor cannister

I feel fortunate to have found a 1973 NOS Carter Carburetor and 1973 NOS vapor canister to match for my return trip south, that will replace the factory original 1972 Carter Carb and matching 1972 style canister. The original '71 vapor system used two purge lines, one with a "t" fitting into the PCV hose and a second with a "t" fitting into the distributor vacuum advance line. The '73 system uses a dedicated purge port in the base of the Carter Carb located right between the two idle mixture screws. I will need to plug another "extra" vacuum line in the '73 NOS carb that was meant for ported EGR system (which is located on the driver's side of the carb next to the driver's side idle mixture screw)>

The current '72 carb has had a non-functioning accelerator pump ever since I rebuilt it myself, don't know if I should have polished the accelerator pump chamber a little or if I installed the actuating spring incorrectly. I will install new gaskets in the NOS carb before installation and check the float level, of course. Hope this further improves my gas mileage, will keep my fingers crossed.

Fotos #1 and #2 are of the '73 NOS carb
Foto #3 is the original '72 carb (installed)
Foto #4 is the original '72 vapor canister on the left and the '73 NOS canister on the right

 

[mopar73dge]

To compare, I have a 1973 Charger with a 318 two b, It has a 340 spec cam which gave it a nice sound, and more response, more hp over the stock cam. The rear has 3:23 and I have been getting 18-19 mpg at an average speed of 60 mph and top speed about 70. this was on a 65 mile trip. Like someone mentioned, check with a vacuum gauge, that will tell all. and check the color of your plugs and tail pipe soot, the cleaner the tail pipe the better. I installed a NOS carter also, its nice and runs just like it did when I bough the car in 1980 with the original carter still on it, again a swear by a vacuum gauge, Set the timeing as far as possible with out hard starting, and pinging. I take a distributor wrench with me and test drive the car stop adjust drive again, also checking the vacuum gauge, keep that around 20-22, also you will know if your timeing is adjusted to far when you rev the r.p. m. up to 3000 in park and the engine seems to shake which means you are too far advanced. good luck! MIke.

 

[69b3rt]

I rebuilt the same carburetor in my 72 Satellite also. I got the same results as you with the accelerator pump not squirting well with new pump. I wonder if 10% ethanol gas can shrink pump seal that quick. I didn't want to spend too much time on working on this (I thought) simple carburetor.

 

[mopar73dge]

If you continue to add marvel oil to the gas will that help the old carburators? anyone have input out there?

 

[Dibbons]

Good news update! After installing new points, the '73 NOS carburetor and NOS charcoal canister (and new gas cap) I have some new MPG figures on my most recent fill-ups of 15.9, 15.0 and 16.1 MPG. That is more like it. And today, for the return leg of my road trip, I installed a Pertronix distributor to replace the Cardone brand points type distributor (Summit shipped it to me overnight). This looks much better than the first leg of my trip which was between 10.7 to 12.7 MPG.

 

[mopar73dge]

Good news update! After installing new points, the '73 NOS carburetor and NOS charcoal canister (and new gas cap) I have some new MPG figures on my most recent fill-ups of 15.9, 15.0 and 16.1 MPG. That is more like it. And today, for the return leg of my road trip, I installed a Pertronix distributor to replace the Cardone brand points type distributor (Summit shipped it to me overnight). This looks much better than the first leg of my trip which was between 10.7 to 12.7 MPG.

That seems normal for the average, 318 my last road trip over the weekend included some highway some in town and back roads a short burst up to 90 MPH and 160 miles total trip length. I averaged about 17 MPG so your doing well, granted I have the 3:23 rear and 340 spec cam spec , tail pipes were clean of any residue and spark plugs normal color, I will leave everything alone as far as adjustments I am Happy with the tune up adjustments I have, thanks for the reply enjoy the car! Mike