ok i have an '86 firebird with a 305 and it is getting about 8-10 mpg. i am not too sure but i think it should get better than this. it idles high from a cold start at about 1200 rpms and then 700 rpms once warm. that seems good. one thing i noticed is that when i go to fill er up, i take the gas cap off and its like opening a bottle of soda. like too much built up pressure or something. maybe i need a new gas cap for starters? ive checked and could not locate any fuel leakage anywhere. also, i am getting much worse mpg on the highway than city which is odd. once i hit speeds of about 60 i swear that needle is ready to take the plunge. trust me too, i am babying this thing big time. so what are the things that might cause this?

thx
Rob

 

I can think of about 20 things right off the top of my head that could kill you MPG that bad. Any other conditions or running problems?

 

My 86 TA does exactly the same thing and I get about 17 or 18 MPG so that's not it!

 

Car runs really well, nothing noticeably strange other than the "87-up" in my gas tank lol. im just looking for pretty general suggestions as i am pretty new to tinkering with cars. so some basic things to check would be good. i tell you what though, the guy i got the car from told me hes got it built up to almost 350ish quality. but still would 10 mpg be reasonable?

 

Something is wrong. My VAN's 275+ FWHP 305 with 3.73 gears and a 700r4 with a 2,800 stall is getting about 15 in town and 19 on the road.

 

I'm going to go out and a limb, and guess that this is a carbed car (LG4); since that somewhat critical piece of info isn't included.

Most common cause of unusually high fuel consumption in those, is the plugs in the bottom of the fuel bowl of the carb, that are put there to cover up passages that were drilled during manufacture, leaking. This leak will also cause a VERY RICH idle; where the exhaust smells real fat and smoky when idling, maybe has that burn-the-eyes funk to it. The excessive gas consumption occurs during idling, as well as when cruising or whatever.

Take off the carb. Remove the 2 long screws that hold the air horn (top piece) on, at the very rear of the carb, just inside of the round gasket surface for the air cleaner. Turn it upside down and remove the 3 screws in the bottom that hold the throttle plate (bottom piece) on. Remove the throttle plate.

Toward the front of the bottom surface of the fuel bowl, with the carb upside-down, you will see 2 "tower" looking things sticking out. Each of those has 2 holes, each of which has a plug spun into it. The bottom of the plugs is exposed to manifold vacuum, and the top of them is in the bottom of the fuel bowl. If the plugs don't seal, the engine literally sucks the gasoline right out of the fuel bowl around the lugs.

The plugs are made of a different metal from the carb; and they enjoy the usual dissimialr-metal electrolysis effect, where the 2 metals set up a little battery between them, and one of them erodes. It always happens to these, sooner or later.

The cure is to clean them up as clean as you can get them. Wire-brush the whole surface area around them until you can see bright shiny clean new metal everywhere. Clean the area off with spray can carb cleaner or lacquer thinner or something; and epoxy over them with JB Weld or Devcon epoxy, use the type that has steel filler in it. Let the carb sit overnight for the epoxy to harden; then put it all back together EXACTLY like it was, and re-install it.

If the carb hasn't been rebuilt recently, you might want to get a carb kit, and do that while you've got it off.

This should help dramatically.

I hope this is more useful than reports of the gas mileage that somebody's somebody's VAN gets. That sort of drivel isn't very helpful TO YOU, fixing YOUR car.

 

if the 305 has been souped up, and runs pretty fast, and you're getting 10mpg in town, driving it hard, in the winter, I can't see anything wrong here.

another thing you assumed sofa, is that it still has the q-jet. Does it? I mean, if the previous owner did hop up the motor, chances are good theres a holly double pumper on the engine. That'll help kill mileage too.


the top arrows are the well plugs, if you have a q-jet.

 

it has:

holley 750 cfm carb with electric choke

edelbrock rpm intake

edelbrock rpm cam


this is what i have been told is in it that is not stock.

 

Well the pieces to the puzzle are falling into place pointing to the MPG problem.

That RPM cam is old technology and IMHO a poor economy choice.

The 750 Holley is TOO much as well.

Engine TUNE will affect MPG more than most people would imagine.

 

The original poster asked if his MPG was normal, I said NO and backed it up with an example of the kind of mileage a 305 is capable of getting.

 

Fast355: That's what he asked; nothing about "normal", nothing about your van, not even anything about what his gas mileage "should" or "could" be. I think he had already figured out that his gas mileage is not "normal", and doesn't need to hear about somebody's van to learn that, and wants to know what he can do about HIS gas mileage, not hear about yours.

Excuse the digression please. Now back to the actual matter at hand.

Ah, a few minor details like ..... telling us what cam and carb it has, can make a difference. What you'd do to a stock setup, might have nothing whatsoever to do with a mystery modded one.

There are few cams that could possibly be any worse than that one, for the application at hand. You should seriously consider ditching that and putting something more appropriate to whatever kind of driving YOU do, in it.

What gear and converter does this car have? What exhaust is on it? (need to know what every piece is, from the heads to the back bumper; at least, what pipe size)

Has the distributor been replaced with a non-computer-controlled one? If not, that's THE VERY FIRST THING you should do. That will probably also make it run cooler.

The carb is indeed "too big", by the usual measures of what's "enough"; but it should still be possible to get it to work reasonably well. However, NO carb will work right on a motor that's either built out of severely mis-matched parts, or that's mis-matched to the CAR it's in (converter, weight, gears). But it sounds like there's lots and lots of things you can do that are a whole lot more important right now than swapping carbs, and will give you more bang for the buck in dealing with your situation.

Before choosing a better cam, or even trying to decide what parts need to go and which should stay, a COMPLETE list of all the mods to this car, or at least all you know of, would be highly useful. A good place to start would be to make sure it's really a 305. Get the block casting number, conveniently located on the top of the bell housing flange right behind the driver's side head. If you can determine what pistons are in it, specifically whether flat-tops or dished, that would be good to know.

Head casting numbers would be the next critical piece of info, after the short block is identified. Hopefully the PO didn't do something boneheaded and put large-chamber 350 heads, like 882s or something, on it. If he did, the motor's compression is going to be down in the toilet somewhere, and no amount of stuff you can do to it from outside, will ever get it to really run good.

Start there, with figuring out what all you've got.

 

Since this question was ignored by someone.

Once again.

Seems like this is the first answer, asking for more information. I have to have some idea to the problem before I can get the correct answer.

I then went to say that 10 MPG is NOT NORMAL.

Now for example this would be the recipe to 10 MPG on a 305 F-body.

a.)Big poorly matched cam (too much overlap)
b.)76 cc combustion chambers
c.)highway gears (with a big cam)
d.)stock torque converter(with a big cam)
e.)front-end out of alignment
f.)dragging brakes
g.)float level too high
h.)improperly adjusted timing
i.)sticking centrifical advance mechinism
j.)improperly connected or malfunctioning vacuum advance
k.)carb not calibrated properly
l.)engine oil too thick
m.)thermostat too low temp
n.)lacks heated air cleaner
o.)no PCV valve (this will throw off a carb calibration more than you think)
p.)vacuum leaks
q.)fuel leaks
r.)improperly functioning EVAP system
s.)NO EGR
t.)too much carb
u.)engine needs tune-up
v.)slipping transmission
w.)TCC not functioning (350-C, 700r4)
x.)tire size
y.)weather
z.)fuel quality

 

IMHO, with a motor built like that, city mileage of those #'s is pretty close. Maybe a tad low, but maybe you're rounding in your #'s when checking your mileage say.

There are ways to optimize it, to get better mileage and performance, as sofa has mentioned. Detailed specs would give us ways to give you better advice.

but hey, who was right with the holley carb eh? huh?

definately check the distributor first, that's the easiest thing to check, and if it's mismatched by a dumb previous owner, then you'll suffer. If it has a vacuum can on the dist, then you're fine, if it's got a 4 wire plug on the base part of it, and no vacuum can, you're in trouble.

 

a couple more things about the car... the distributer is good, the motor has 307 heads, the exhaust is strange and not even legal.... it has headers with a catalytic converter but nothing else, somekind of dynomax muffler with dual exhaust. this car has no computer and some things have been removed from under the hood such as the a/c. most of the dash is not functional, instead there is one of those aftermarket tachs. there is also definately an alignment problem... the front end is tilted to one side. also, when driving i can feel EVERY little bump...HARD. hard like im suprised the windows make it home in one piece. maybe a strut is no good.

well here are some pictures of the car.

notice the slight lean to the left

 

307 heads (the person that did that should be shot), RPM cam, bad alignment - small wonder it gets poor gas mileage.

 

You forgot to mention 750 double pumper.

Not to mention the distributer is more than likely timed by ear.

 

Didn't feel a need to get to 20. . .

 

Ok, ditch the heads. Ditch the cam. Ditch the carb... if you're feeling ambitous, ditch the 305

If it's leaning and you feel every bump hard, sounds like a broken strut. Does it knock when you go over bumps?

You say it has headers and a cat and nothing else, but it has a muffler? Sounds like full exhaust to me! Unless you mean it doesn't come out the back.

Nothing working on the dash means someone just ripped everything out of the engine compartment without knowing what they were doing. Not ALL wires are bad in there.

Car looks neat though, very unique. It has tons of potential!

On a side note, my 67 Riviera with a built up 430ci got 8mpg

 

Absolutely. A 750 CFM carb is way too much for a 305. That cam is old tech and not very good. Those 307 heads should be melted-down for scrap. Get yourself a 4150 series Holley 600 cfm carb, 650 cfm at the biggest. Demon makes a good 600-ish street carb as well. Don't get a double-pumper unless you have a stick and lower rear end gears. If you have an auto, get electric choke and vacuum secondaries. Get yourself a set of heads off an LB9, clean them up, port them a bit and do a nice valve job on them. Call Comp Cams, tell them what you have and get them to recommend a XE series dual-pattern cam. Get one of their roller chain drives while you're at it. Make sure all the smog crap is hooked up and working.

My IROC has an LB9 305 and I get about 25 mpg on the highway. In town it's a lot worse, depending on how heavy my foot is, but to get it into single digit mileage, I have to be driving VERY aggressively.

 

Well, with all the problems you described and his motifications, gas milage should be his last worry. In fact, he should be happy if it runs.

 

I would actually get a pair of the 601 305 HO heads. They have a different chamber configuration(like a smaller version of the old and horded corvette 492s) that is only 52 CCs, flow better than the 081s or 416s in stock form. A comp cam XE 256/262/268 will work well with that combo and decent springs. A tweaked Q-Jet will adjust itself all the way out to 750 CFM, but never overcarb the engine which is a good plus. The tiny primaries will give you excellent fuel mileage when you stay out of the monster secondary side.