'84 Mustang With Awful Gas Mileage

I have a 1984 Ford Mustang, 91 K miles, 5.0 L engine with manual transmission. After the car was running very rough, I rebuilt the 4 barrel, Motorcraft / Holley 4180C Carburetor. Car runs okay (now rough starting when cold), but my mileage has dropped from approximately 14 mpg to 7 mpg. As I drive this car to work the same way each day, nothing else has changed.

As I didn't want to go back and rebuild the entire carburetor, are there any tips any one has or something I can check or change?

Thanks.

 

My first thought would be the choke is stuck in an intermediate position = cold starting problems because it doesn't fully close. Then poor mileage because it doesn't fully open.

I would pull the breather and check the choke when the engine is cold and hot respectively.

Hope this helps,

Bob

 

does it emit black smoke? how does it run at speed? is the carb leaking externally/internally that you can see? i tend to agree with bob. i would check my mounting bolts and gaskets and screws in the top of carb. also, what about the air mixture? if you have a vacuum guage, you can set the air jets to obtain maximum vacuum. LUCK.

 

Yeah this is going to be one for the old timey's here but from my experience with carburators :
Once they start giving you trouble you might as well buy a new carb altoghether , a rebuild sometimes will not cut it you'll be adjusting and playing with it forever, why don't you buy a new one and a performance one if possible ?
You can buy an Edelbrock Performance for about $ 350 check out this store
www.rockauto.com

 

Lou, I am deeply offended by being called an "old timer", but you are RIGHT! Takes me back to my '55 Chevy days, trying to adjust that darned WCFB on that power pack 265. Adjusting that puppy will be by either tach or vacuum or ear (ear being the best final test). That is AFTER you plug up all your vacuum leaks, seal the base really good, and check the choke mechanism, as mentioned earlier. This being said, a new performance carb will work wonders, especially on a Mustang (even if it isn't a '65 or so Cobra) Good luck with it and let us know the culprit.

 

Rough starting when cold and massive fuel consumption, I'd have to think check the choke first thing
Could be stuck, out of adjustment, or even inoperable

 

I worked at Ford in '84 during the transition from carb to fuel injection.
The early '84 5.0 was the last caburated Mustang to be produced.
IMO the carb car sounded soooo much meaner than the injected but the injected was much quicker.
Anyway back to your problem.
Yes, everyone is correct regarding choke staying open but don't overlook the coolant sensor. If the sensor thinks that the engine is still cold (even when at operating temperature) it will tell the throttle body to run rich dumping tons of fuel down the carb in a permanant cold state.
Remember it isn't a real carburtor because it has an electronicly controlled throttle body instead of jets.
It can also trash your catalytic converter with all that raw fuel.

I don't think of myself as an oldtimey but I'll take a carburator over fuel injection any day of the week and if you rebuild 'em right they'll run like new for a long time.
Just my .02 cents.

 

i'm with mackey. a carb with wires doesn't do it for me!! old school-tried and true!CHANDLER-wcfb GREAT CARB.for mileage. i'm a Q-jet man.(yeah i know) anyway, the best carb. i've bought is a edelbrock 650 with elect. choke. runs great right out of the box. GOOD LUCK! is 64 OLD??

 

Thanks for the responses - I pulled the carburetor back off and checked and retightened all screws and made sure there was no leaking around gaskets and seals. I put the carburetor back on, noticed the choke was closed when the car was cold and it opened fully when the car warmed up. I made sure there was no leakage around the carburetor while the car was running. Also, no black smoke was exhausted while the car warmed up or after it warmed up. As for the air/fuel mixture, I did not change these from the factory settings as I didn't want to drill out the seals covering the mixture screw.

Two questions - first is there a way to check coolant sensor? I did have a hole in my radiator and the car had overheated just before I rebuilt the carburetor.
Second, if I do decide to buy a new carburetor, how do I determine the CFM necessary? Thanks.

 

As far as I know, and I'm real, real, confident on this, the '84 HO did not use the electronic feedback carb
But the choke was redesigned for '84

 

So if I understand Slickshift correctly, the coolant sensor has no effect on the carb? If this is the case and the car runs great when warm, should I assume it is time for a new carburetor? If this is the case, most new carbs I have priced are listed in CFM, not by model & year. How do I determine the correct CFM - 500, 650, higher?
Thanks.

 

If I remember right Holleys use an internal power valve that actuates when the vacuum drops = loaded condition. Did you replace those when you did the rebuild on it? The other thing would be the float setting could be checked.

 

As I understand it, and have found no evidence to the contrary, the 5.0 HO in the '84 Mustangs with the Manual Tranny did not have the dreaded feedback carb (from personal experience the '83 and '85 absolutely did not)

The Automatics however, had the CFI (looks like a carb but has the big injectors inside)

So, yes...I think your H2O sensor is merely a temp sensor

Your engine originally came with a 600 cfm 4180-C Motorcraft/Holley carburetor

 

Hey........,my bad.
I guess it's been too long.

So the manual trans Stangs (84) had a real carb and the automatics had throttle-body of the same production year......?

They sure were fun to drive!!!!!!!
varoooom......cha.....whaaaaaa.........

 

Ahh-yes, another Quadrajet man.
We're a dying a breed out hear among all these tuner car dudes.

I love the Q-jets! .
You can watch your hood suck in when those secondarys open up.
('67 Skylark, 350 w/Q-jet & dual exhaust.)

 

Yes
For '83/'84/'85 the 5.0 'HO' Mustangs with the autoshifts had CFI, also called TBI, the Center Fuel Injection or Throttle Body Injection
...it can get confusing...and there were non- HO 5.0s with the electronic feedback carbs also at the same time in other cars
I think they left the Stangs alone so we could mess with them
...really
Those Four-Eyed Square Light 'Stangs helped put the muscle back into affordable Muscle Cars
"The Boss Is Back" indeed

 

Not to beat a dead horse, but $3.00+ gas is killing me. When I rebuilt the carb, I used an after-market kit and changed all the o-rings, gaskets, power valve and needle assemblies. I adjusted the float level (dry only). Again, when the car is cold and I push on the gas, it chokes out because of the amount of fuel and funs fine when warm.

Is it just time to break open the wallet and buy a new carb?

 

I wonder if you have two problems at once:

1. Have you made sure you truly ARE getting a strong squirt of gas from the powervalve or accelerator pump when you give it gas? Have you tried it with the engine off, and looking down the throat when you pull on the throttle? You would think if you really were getting too much gas that it would actually run BETTER, when cold, as you need more gas when it is cold. That is why I am wondering if you might have too LITTLE gas when you go to throttle it. Unless you get that big rush of gas when you tromp on it, the car will fall flat on it's face.

2. The other problem you might have is in regard to your mysteriously high gas mileage. Maybe the dry-set float job you did is not right and is constantly spilling over gas. Another possiblity is that some crud (all it takes is one spec of something) is trapped between needle and seat from your rebuild work.

And surely you have confirmed the choke is okay, after all the posts here.

And also, if it is cold out, you can't really count your gas mileage fairly if you are idling your car more now than when the weather was warm.

 

your head must be spinning from all this thinking. i know mine is! the only thing i can think of that hasn't been covered is the air cleaner. not usually a big deal, but with your "runs terrible cold-good hot" have you changed the air cleaner with the cold air(vacuum operated) doors to a newer style? if so, did you block off the vacuum port that supplied the vacuum to the cleaner? really sounds like a "cold start" only situation. the engine temp. changes the supply route on some engines to open and close doors to allow heat/no-heat to the carb. JUST A THOUGHT! maybe something to check.