Very poor fuel economy

When I got my '77 Aspen (318 auto) it didn't run all that well. Went through the usual maintenance, replacing all fluids and consumables. New spark plugs, ignition module, cables and even a brand new Firecore distributor. Set it up with a timing light at 10* BTDC and had a pro double check on my installation.
Decided the old Carter BBD had seen better days and replaced it with a reman unit to keep things simple. Well, I now get around 5 mpg in town. Terrible for a daily driver!

First place to look would be the carb and it is running slightly rich but when setting it up I used both a Colortune spark plug and my vacuum gauge and it seemed to run well. Leaning it out certainly didn't help, car would immediately start running rough and eventually die. Vacuum was a tad low at 16 inHg but remained stable (worn rings?)

I plan to eliminate most of the EGR stuff as it may be causing a vacuum leak and it's not required anyway but I'm a bit lost where all that fuel might be going. It's not leaking from the tank, that's for sure!

Carter BBD is a horrible carburetor at best, and it gets worse as they age. Re-manufactured ones don't really help the issue.Just throw it to the bottom of the ocean as fish habitat, where it belongs.

Make certain your choke system is working correctly. Make certain you have no vacuum leaks. Make certain your distributor is installed correctly and the plug wires are in proper sequence. Make sure your ignition timing is spot on.

And what does your vacuum gauge say? (Thank you for using one!)

Which Ignition system is on your Aspen? Is it a Lean Burn? If the egr is functioning, removing it may cost you more gas mileage! Was the car designed with a converter that is no longer present?

Have you looked at all the fuel lines for signs of leaks? Old hoses & new gas don't mix well.

A reman carb does not necessarily mean it was rebuilt correctly, Ive seen some really wore out carbs that have been rebuild that still have worn throttle bushings and poor quality work. These carbs are rebuilt from 30 year old (or more) cores. Slapping a kit and a new float in them does not mean they are rebuilt, which is all a lot of reman shops are doing these days.

I don't know how things were set up over where your at, but here in the states, a 77 would probably have a lock up trans. They were really bad at being set up incorrectly and some would lock up as soon as the car shifted into drive. Coupled with really high rear end gears (2:45 was not unheard of) and the trans locking up at 20-25 mph, people would tend to step harder into the gas to maintain city speeds, making poor city gas mileage. Gene

Did a simple pressure test on the engine btw, seems ok given its age and vintage*
I think:



That's

1 - 121 psi
2 - 139 psi
3 - 133 psi
4 - 121 psi
5 - 124 psi
6 - 142 psi
7 - 121 psi
8 - 133 psi


*low comp smog era engines

Lock up and lean burn came in '78 AFAIK. Asked my mechanic about the sure grip rear end and he told me it had a 2.60 ratio. Not sure what to make of that, could be either 2.75 or 2.45 I guess.

It does have a catalytic converter, single exhaust setup that's next on my list. Pretty rusty, had it welded for the time being but wants replacing really.

I'm aware of the Carter's reputation, thought I'd give it a try first though. That way I didn't have to worry about getting linkage adapters and whathave you. Cost 140 $ from Rockauto and looks okay. Fuel economy or lack of was more or less the same with my original carb btw.
Guess I'll go back to checking the ignition first.

Ignition kit from Jeg's that I installed, kept the 4 prong BR though. Haven't messed with the adjustable distributor yet as it's such a PITA to get at. Should be okay for a stock application really.





Also planning to replace the presumably original nylon timing gear, maybe the harmonic balacer too.

Years ago I bought a 72 Dart that got horrible mileage, went thru all the stuff you did but the engine had a fresh remanufactured carb on it when I bought it.

Turns out that reman carb had jets 8 sizes too big. Once I put the right jets in mileage more than doubled.

I have one of those Colortune plugs! I would put in a new timing chain set with the 3 keyways and adv it 4 deg. your compression readings are OK (thanks for converting from bars to psi, I was worried for a bit!) so the longblock is OK. 73/74 was a bad period for discombobulated timing/jetting as Ma was trying to meet gov't regs plus there was the fuel crisis of that era but '75 & up was much better so I think there is something else going on. I would do the tchain plus possibly a new mileage cam to build some DCR, confirm no vac leak. blockoff or unhook/cap the vac lines to the egr thingamajig. that carb may have no ported nipple so tune the dist for manifold. confirm the choke operation is good. holler how it turns out

Intake manifold is probably carbon plugged at the heat riser passage Most carter stink fuel mileage wise. Make sure the car is heating up enough that will really make a difference in gas mileage around town. Don't be too hasty in tossing the emission stuff especially the egr The convertor can also be a real source of problem so toss it probably plugged any way.

I currently have several carters and a new cat that would probably fit that car Just a fyi. Run a 195 stat and check the heat passage. My Dart I had 113K and would stall and get terrible gas mileage Took off intake cleaned heat passage rebuilt carter 2bbl and avg over 16 mpg in town 22 on road

My first thoughts were a plugged cat and a stuck heat riser flap in the exhaust manifold.

Interesting comment regarding the intake manifold. I was thinking of swapping in a clean pre-EGR manifold that I picked up for next to nothing. Using a heavy cast iron intake when I also have an aluminium Performer (318/360 4-bbl spread bore) might be a daft idea but at least I have a carb for it.
Haven't used my vacuum gauge to check for exhaust back pressure, which I understand you can also do. Probably worth a shot.

Stock, the car should do way better than what you're getting. SO, I'd not start swapping parts. Buying and installing new this and thats is a sure way to spend a lot of money with possibly no return. The last few comments have been solid. First, get the stock car to run the way it was delivered, then make changes to increase performance.

R.

I agree with above. What octane is your fuel there? Sound like the enrichment circuit-spring could be the problem on the carb(making it run rich all the time) or float to high and running over, about the only way you can lose mpg that much.
Should add, undo the choke and see how it runs, too cold to open all the way.

Do a 10 second check.

Pop the hood. Go to the passenger side. Grab the heat riser flap on the exhaust manifold and make sure it turns properly.

If that thing is closed the engine will not be able to breathe.

Had a friend back in the day with a 318 OD 4 speed Aspen wagon. On a long trip we pulled down 27-28 mpg with a stock motor. Don't be too quick to ditch the Carter 2 barrel. Make sure the floats and the metering rods are adjusted properly. Make sure the heat crossover in the intake manifold is not plugged - I've cleaned a few 318 intakes out that were choked solid with carbon from the EGR. And check out the heat riser and cat converter as others have mentioned.

I doubt that's much of an issue. The cheapest fuel available would be 95 Research Octane (RON) that's ca. 91 octane US. 93 and 95 octane US (98 & 100 RON) are also available at most filling stations but on a low compression engine that would be a waste of money.

I'll check the heat riser flap tomorrow. How does the intake crossover affect mileage btw? Using a valley pan that actually blocks the crossover passage seems to be a popular solution to fuel evaporation in the carb. Do they also get terrible fuel economy?

If clogged, it keeps the choke,if stove type, not electric, from opening. How far down do you have to push the gas pedal to maintain, say 35 mph.

I have the traditional divorced choke setup:


Not sure if the "electric assist" function works (the cable going to a little metal plate). Ordered a replacement but hasn't arrived yet. I did however disable the choke on the old carb without seing any major changes in fuel economy.

Never really thought about how the accelerator pedal feels, it's responsive enough though. Certainly no excessive travel.

If you're not having to floor it to get moving, maybe carb is flooding. Look in carb when running, see if fuel is drizzling from the boosters. If not see how far throttle opens before fuel drizzles and how much. You're dumping a lot of fuel to only get 5 mpg. What is the longest time you drove the car?

Drove the car home several hundred miles with the old carb, a gearbox that wouldn't shift into third and a sticky kickdown linkage. Gearbox and linkage now serviced, still chasing an overheating issue but that seems to be down to a faulty gauge. Goes right up to 100% within the first ten minutes before the (new) thermostat opens, then settles at around 70%. No actual signs of overheating though, checked with an infrared thermometer.
With the rebuilt carb I covered around fifty miles but not in one go.

That's a great way to ruin fuel mileage with the stock type choke.

Ok, time to wrap this up now:

initially planned to rebuild the engine when a friend offered me a '72 318 which he rebuilt himself. Swapped that in and since it already came with a 4-bbl Performer intake I put on a 500 cfm Edelbrock AVS.
Sent the radiator away to have it re-cored at the same time and ditched the factory a/c.

Had the 904 overhauled too but of course the linkage didn't fit the new carb, which again led to rather disappointing results. Ordered the Bouchillon Performance solution but for the time being the stock setup seems to be working okay after a few mods. Not perfect (yet) so I'm still undecided whether to stick with it or install the cable conversion.
Installed a new exhaust too. Couldn't fit duals but the larger diameter single exhaust seems fine. Hated the Flowmaster muffler though and swapped that for a Dynomax: much better!
Installed a cheapo AFR gauge/O2 sensor which is a bit hit and miss. It does show that the carb is jetted surprisingly lean @15.5 AFR for part throttle cruise.
Maybe I'd better check the spark plugs as well...

Fuel economy improved significantly but only when I replaced the gas cap did I finally see 21-22 mpg on a 50 mile test run in town.

Was it all worth it? Well, at around 5.50 per gallon it's eventually going to pay for itself and while the engine and carb swap wasn't strictly necessary the transmission and exhaust definitely were. Doing away with the non-functioning a/c also meant losing considerable weight up front and it cleaned the engine bay up nicely. You simply can't get R12 over here and I found the pricey Sanden compressor conversion in my other car slightly disappointing and hardly ever seem to use it.

temp gauge still doesn't work but I added these:

That open air filter doesn't help economy or power.
The engine now sucks in hot underhood air.

The original airfilter housing on your other engine is a much better design, feeding the engine with cooler/fresh air.

15.5 AFR during cruise is ok.
As long as that number goes down when you accelerate or put more load on the engine.

I would also install a vacuum-meter to monitor the condition of the engine and maybe tailor your driving habits.

Nice (economy) upgrade none-the-less!