I have a 1970 mustang with a 289 in it. This 289 has domed pistons and a QFT four barrel carb. I am getting pretty bad mpg, about 6-8 mpg per the few tests I've done. I took to the forums and most guys seem to be saying that they are getting around 16-21 mpg. Is this number exaggerated and are the guys full of crap? Or is there something wrong with my carb? I also have a four barrel edelbrock I was considering putting on the engine. Is there anything I can do to get up my gas mileage? Thanks
Gas Mileage
- Thread starter Stangboi4444
- Start date
there are a lot of things that affect fuel economy, in no particular order;
excessive cam
very low gears
carb flooding
wring ignition timing
driving style
clogged air filter
among others
in your case, lets start with the carb, do you smell gas when the engine is running? is there a vacuum leak somewhere? have you checked the float levels? how does the engine run at idle?
answer these questions and we can work from there. remember we want to deal with one system at a time here, lets start by getting the carb right first.
excessive cam
very low gears
carb flooding
wring ignition timing
driving style
clogged air filter
among others
in your case, lets start with the carb, do you smell gas when the engine is running? is there a vacuum leak somewhere? have you checked the float levels? how does the engine run at idle?
answer these questions and we can work from there. remember we want to deal with one system at a time here, lets start by getting the carb right first.
There are way too many unknowns to answer here.
I've had old carbureted cars that got 25mpg (2.8L 1984 Camaro with a 2-bbl) and I've had them get 4mpg (460 1975 Thunderbird with a 4-bbll Motorcraft).
Depending on what gears, transmission, other engine work done, how you're driving, compression ratio, and so on and so forth, your mileage will vary.
Well first off the transmission is a C4 transmission. I have not checked the float levels but I do know that the carb is a duel fuel bowl. The Car runs well at idle. At first the idle was to high, but I turned it down and it seems to be better now. As for the vacuum leak, I don't think so; however, when I put in this 289, I had forgot to put in a vacuum hose from the transmission to the carb. I did put this in though after driving it a while.
basically a cam with too much lift and duration, as well as too much valve overlap.
as for the carb, if i recall correctly with the dual fuel bowls, you have a small brass plug in each bowl about 2/3rd the way up the bowl. these are called sight plugs, some may be clear by the way. if they are brass, then remove them,one at a time, and with the engine running see if fuel dribbles out, or runs out freely. if it dribbles out, then you are ok, if it runs out your float levels are too high.
as for the carb, if i recall correctly with the dual fuel bowls, you have a small brass plug in each bowl about 2/3rd the way up the bowl. these are called sight plugs, some may be clear by the way. if they are brass, then remove them,one at a time, and with the engine running see if fuel dribbles out, or runs out freely. if it dribbles out, then you are ok, if it runs out your float levels are too high.
Ok. Based on what I've read about this carb, it is meant to be extremely tune-able. As for the sight plugs, there are little windows on the side of the bowl so that one may see the fuel levels. I can't remove them. Should the fuel level be half way up this window? Also, on this model of the carb, it is very easy to change fuel float height so that is no problem.
the fuel level should be right near the bottom of the sight glass, higher than that and your engine will run very rich.
if changing the fuel level doesnt help, then we need to get into the carb and see what size jets you are running, and what power valve you are running. again too large on the jets and to high a vacuum on the power valve and the engine runs too rich.
if changing the fuel level doesnt help, then we need to get into the carb and see what size jets you are running, and what power valve you are running. again too large on the jets and to high a vacuum on the power valve and the engine runs too rich.
The engine may be a bit small for the carb. If so, setting float level lower than the inspection port is possible. If cam lift/duration were too stiff, engine idle would not be smooth as described. If fuel were leaking internally, gasket, crack, etc., idle would be poor. If float setting is externally adjustable, like the old Holleys, it's easily tried. imp
that ill be a good start. i think that your carb is set up wrong for your combination though, and we can change a number of things to get it tuned for your engine. we shall take this one step at a time so we know where we are at all times. that makes reversing a change easier since we will know what change was made and what effect it had.
120mm
Active Member
Does it make good power?
I had a Mustang with a bad vacuum advance that made crap power and ate headers, that got horrible mileage. Went through everything and then everything got better when I replaced it with a centrifugal advance distributor.
I had a Mustang with a bad vacuum advance that made crap power and ate headers, that got horrible mileage. Went through everything and then everything got better when I replaced it with a centrifugal advance distributor.
I lowered the floats on the fuel bowls and it sounded better. Took it out and it felt good to drive too. Once I got in there to change the fuel floats, I definitely heard a vacuum leak so I will have to fix that. The 289 makes good power and I have a MSD ignition system so I don't think that is it.
so we are getting there slowly but surely. so now find that vacuum leak and fix it, then see what you get for fuel economy on your next tank, it should go up. how much though depends.
you may find no that other issues may crop up on your that you didnt think you had a problem with earlier, so be aware of that as well. its all part of the tuning process.
I have yet to actually fix the vacuum leak; however, I have identified it coming from the vacuum hose from the transmission. Why would a vacuum leak cause worse gas mileage? Also, I have been further attempting to tune the carb. I have some sort of magniflow mufflers that are very loud ( I didn't put them in). Should the exhaust end up sounding very throaty? Like a deep rumble? The only two things on the carb I am adjusting is the fuel float level and the screws on the side of the meter block that control the fuel air mixture.
120mm
Active Member
then you are still too rich, but closer. get the idle right here, both speed and mixture. you want the idle speed around 750-800 rpm in drive with the vacuum advance disconnected and plugged to avoid it screwing with your adjustments. then to get the idle mixture screws set right, start by< with the engine OFF, turning each screw in until it just touches the seat, then back then out 1 1/2 turns. that will get you a baseline you can always return to if necessary.
then start the engine and let it idle. turn each screw in turn out until the idle starts to run a bit rough, that should be no more than about 1/2-3/4 turn normally, so turn that screwdriver easy. then turn it back in until it runs a bit rough again, that again should be no more than 1 turn. do this until your get the smoothest idle you can. then go to the other screw and do the same thing. this may take a bit of time do be patient. and be aware that your idle speed may change slightly as you do this. also be aware that you may have to go side to side until everything is right. as you gain experience you will take less and less time setting the idle speed and mixture. remember your ears are a great tuning tool at this point, so remember to use them.
Have you looked at the plugs? The color can indicate all sorts of tuning problems including too rich or too lean.
In my experience with C-code and A-code cars is that 15-20mpg in mixed driving is normal. I got 12-15 with my S-code.
Tuning matters, as does everything already mentioned, but I think foot pressure has a bigger bearing on actual mileage than any other :-D
In my experience with C-code and A-code cars is that 15-20mpg in mixed driving is normal. I got 12-15 with my S-code.
Tuning matters, as does everything already mentioned, but I think foot pressure has a bigger bearing on actual mileage than any other :-D
What was S code, FE block? Having become an anti-carburetor convert, I have been totally amazed by fuel economy with SEFI. Have a '72 Ranchero sitting out back, thinking about building a first for me: EFI Cleveland. Did a 390 in the '90s, quite a challenge to mate the EFI distributor bowl to the FE distributor base, but worked well. Old firing order meant nothing, the 5.0 HO distributor worked fine with wires switched around appropriately. Used the Ford Racing MAF conversion kit with 24# injectors, '93 Cobra PCM. imp
Similar threads
