kmcn47

dont even know if its the carb, i know its been cold and it really hates this weather, all my filters are clean and i just changed the oil, its been running seemingly fine for over 9 months but i just figured my mpg and its 9.2 anybody got any ideas, it idles real low and wants to stall now that its cold but all summer it ran good, had sharp throttle response and made good power, accelerates fine and everything, i guess what i'm saying is that it doesn't seem like anythings making it run rich or lean but the mileage is terrible, i'm at a loss and upset, this thing is my daily

Ozz1967

iTrader: (4)

On what car is this?

Is the choke installed/working? is the car a factory carb or was the carb put on in place of fuel injection?

Does it only run crappy when it's first started? When you set the choke, whenthe car is first started in the morning, the carb/choke s hould set to high idle, around 2000 ropm. Then when you kick it down or the car gets to operating temps, the idle should return to normal.

The car, once it getes to operating temps, should run fine no matter what the outside temps are.

kmcn47

all factory l69 z28 and its idle funny since i got it, thats not the issue, the 9 mpg is i knew it'd be bad but come on theres no way thats normal

Damon

Two most common contributors are the choke not opening fully (choke heater wire may even be completely disconnected, often the result of the electrical connector that goes to it being mangled) and the O2 sensor being shot and giving the ECM bad readings.

This is assuming that the plugs/wires/cap/rotor are in good shape and that the base timing is set correctly, of course. Bad ignition is often the real culprit when carb problems are suspected.

kmcn47

for bad mileage or the idle problem that i wasn't asking for help on? i'm fairly certain the idle problem is the choke thermostat

Ozz1967

iTrader: (4)

LOL, don't be sore man, it's ALL connected, all of it.

What he's saying, is that once you do a proper tune up and know your cap, rotor, plugs and plug wires are new/good, you can better start diagnosing the problem. It's what I would have suggested next, followed by an O2 sensor, then a carb tune followed by a rebuild and tune.

A properly tuned L69, 3.73 gears (What mine was before the 350 swap), got 22-24mpg on the highway at 70mph on a road trip with the cruise on. I got roughly 19mpg in town.

kmcn47

carb was recently rebuilt before i bought the car, cap rotor, wires are all less then a year old, plugs only have maybe 5000 miles on them (about 5 months) ac delco rapidfire platinums, all the filters are new, new pcv valve oil is still good although i'll be changing it again soon (its been about 5000 miles but you can do that with mobil) and all summer it ran fine up until last week when i let it sit for 3-4 days it ran fine just uses alot of gas, its only a very recent problem that it doesnt want to idle, however its been cold and hurricane sandy is only recently passed so i attribute the weather and sitting through it to the bad idle, could the engine needing new valve seals and piston rings be hurting compression and causing a bad combustion and messing things up?

freaky

the oil companys use a different blend in the winter .gas milage sucks when this happens , also performance drops . i'm going thru the same thing with mine .i'm retuning for the winter .since i have the ebl flash an a tbi .

Ozz1967

iTrader: (4)

Don't worry about the oil blend, I used Mobile 1 5w-30 in my L69 for years summer or winter. It made no difference. How is the dwell? is it still set in the right zone? I periodically have to check mine. Should be 30% on the 6 cyl scale, fluctuating 10-20% up and down as the MCS meters the fuel. Is the choke wire still on? Do you go to "high" idle when the car is cold if you pump the gas once or twice before starting the car?

kmcn47

it started fine today, it just hadn't been run, the idle isn't the problem, it started with no pumping the pedal idled at 2k for a min then slowed down to 1k and i put it in drive it went to 800, pulled the car around the block and into the backyard and it idled fine as it sat running

Damon

Any chance you just got some bad gas in the thing (maybe a little moisture in the gas- that's a problem with today's ethanol blends in wet climates)? If so, your next fill-up should miraculously cure it. If not, time to keep looking.

No offense intended with above comments. Just wanted to make sure the ignition is squared away before we chase carb problems (which are actually far more rare than ignition problems).

kmcn47

mileage has been the same for the year and six months i've owned this car, had to fill up two and a half times to make it home (200 miles) never did the math and figured it out to be so bad though, bad gas i rule out, i mean how much ethanol is blended with 91? from a gulf station no less. no offense taken and i can assure the ignition system is all good, i figured the carb would be too it was rebuilt not long before i bought the car but i don't know how well or by who, like the alignment it got, it still pulls to the left sometimes, the new brake system is legit though, stops on a dime. i guess i could get my 84 chiltons out and go through the carb, the o2 sensor idk the cat idk i know i took the belt off the air pump not long after i got the car, i noticed no difference what so ever. all my sensors/vacuum lines are hooked up, i do have a leak in the hvac controls though don't figure that'd do it, i'm thinking choke thermostat, egr, maybe cat or o2 but how much do the cat and o2 on this car even really do? almost certain the timing is fine, car runs really smooth but i will get a light and test that

8t2 z-chev

Hehe,get under and check the fuel pump fittings for leaks-once i tightened my solid line a little crooked or something and it was leaking a drip every few seconds-car suddenly seemed to be using more gas than usuall,repositioned and retightened fitting-no more leak and mileage back to normal

kmcn47

have to do that, never messed with the pump, so few things go wrong with a mechanical pump that i hadn't thought it could be the problem, wouldn't the engine seem starved for gas if it was leaking fuel form somewhere?

8t2 z-chev

it would have to be leaking really bad to effect the running of the engine.Also the well plugs of 1980s era q-jets commonly seep a little-does car act like it is running a little rich or act flooded when started after sitting a couple hours? The seepy well plugs can be fixed with epoxy(carb has to come apart for repair)

gt4373

How Often Are You Opening Up The 4-BBL

kmcn47

thats the thing, is was recently rebuilt i'd say within a month of when i got the car (actually i have a receipt for the "garage" that rebuilt it)

every couple of days i'll go 3/4 throttle to hear it open up when coming up my hill, i feel that romping on it once awhile is good for it, but even then only for maybe 30 seconds (mashing the gas makes the hill alot shorter)

gt4373

Just For The Hell Of It You Should Recheck Your Mileage Just Runing It On The 2-BBL Just To See What You Get.

kmcn47

that mileage is just running it without getting on it, all interstate back and forth to class every other day, i only "let it out" on weekends when if something goes wrong i'll have time to fix it, this car is my daily driver

Ozz1967

iTrader: (4)

Your cat could be plugged, i've heard where that can cause some serious performance/bogging issue, and could relate to the poor mileage. Also, replace the O2 sensor and the EGR just to be on the safe side.

Was the place the rebuilt the carb reputable?

kmcn47

changed the oil and read the plugs the other day, plugs looked fine, ordinary carbon buildup for year old plugs, sandblasted em, re gaped em and put em back in, gonna see if maybe my method of math is wrong by seeing how far i can go past E, then how far on 1 gal of gas i can go, i'll report back with my findings

Ozz1967

iTrader: (4)

If you just fill it up and then drive it 50 miles or so, then fill it up,again to roughly the same point. I generally use "one click" on the fuel pump when it first shuts off on it's own as my gauge. Then just divide the miles by number of gallons you used. I ould do that rather than risk running out of gas somewhere. Even with extra gas in a gas can, that's hard on the pump.

kmcn47

well when i have the money i also plant to get the MAX mileage on one tank by filling it as much as possible and seeing how many miles it takes to empty.

naf

iTrader: (7)

a much better plan would be to fill it up, record the mileage, drive it for a while, re-fill it, note the new mileage then divide the difference in the two recorded mileage measurements by the amount of fuel it took on the 2nd fill (which should have fairly accurately replaced the amount of fuel burned during your 'drive')

If you're not getting 18+ miles per gallon, figure out why and fix it. Don't waste any more of your precious $ on gas you don't need.

Ozz1967

iTrader: (4)

My factory 305, before I swapped in the 350, was getting 22-24 highway miles, cruise set at 70 mph. Now with the 350, I get around 17-18 with the cruise set at 70mph.

kmcn47

well theres a problem with that when i fill the car i FILL the car, and you can't calculate what amount of fuel is held by the filler neck, i know the tanks should be 15.5 gallons according to what i've read on here, i figure using the trip odometer and running on 1 gal of gas i could get the most accurate maximum mpg reading possible i havent done it yet weather has been horrible, camaro hates snow so do i

Damon

Best you can do is use the same pump each time and stop after one shut-off.

Do that a couple times and you'll have something close to reality. I was surprised my old 2001 V6 Impala was only getting about 17 MPGs the way I drove it (badly) and my new 2012 4 cyl. Malibu is barely able to crack 20 MPGs (equally poor driving habits). Out on the highway the Imp would top 28 and the Malibu is around 31. So it's not just the V8s that can take a nose-dive in economy around town.

kmcn47

9 was my highway mileage in town is probably even lower

Ozz1967

iTrader: (4)

I'm glad I am not the only one to have mentioned this. It's not perfect, no, but it's close.

skinny z

People are tossing out things like "ecm" and "O2 sensor". Does your 84 have an ECM? Mine didn't. Just a good old fashioned non CC Quadrajet and an HEI.

naf

iTrader: (7)

Put all of that 15.5 gallons and filler neck out of your mind. It doesn't matter. When you fill it, drive, then re-fill it, the amount that you put back in is the SAME amount that was used while you drove. At least within a small margin of error-much more accurate than any other cyphering.

Trying to run it empty then run it on only one gallon of gas is not good for the motor AND it's going to give you more error due to 1) the smaller sampling size (fewer miles) and 2) the unknown amount of that gallon left in the tank which can vary trial to trial due to how much it's sloshing around when the pick up goes dry. Besides, it's just kinda silly and not good.

Get in the habit of checking your mileage routinely by topping it off each time and recording the mileage. A drop in mileage is sometimes the first indication of a problem.

Ozz1967

iTrader: (4)

Thats because yours is a Canadian model. For some reason they didn't have the same requirements/options as the US models.

skinny z

I had thought about that although I also thought that 84 might still be non CC even in the states.

Damon

81 was the first year for computer controlled carbs/distributors in the Camaro (last year of the 2nd gen body style) in the US. All 3rd gens in the US had them.

kmcn47

i'm with ya skinny z i really doubt my ecm or o2 sensors even do all that much being as old as the car is, i'm thinking something in the carb, but its kinda hard to pull the q-jet of your dd and disassemble it for inspection, i know the floats on these go bad sometimes, it'll be awhile before i can do anything

HotRodFiend

First things first. Change the o2 sensor. Maybe even wire in a self heating one. If you don't know when it was last replaced it will definitely help somewhat. Work from there. Its a cheap and good start. Mine was replaced 1 year ago and now it doesn't even read below 1200 pms

kmcn47

the problem is the q-jet, cars running rich, really rich, fuel floats leaking and letting more gas then should be through the carb and into the combustion chamber, i worry as though its wreaked havoc on my bottom end, the rod knock and tapping lifter would indicate so. the mileage is the least of my worries.

Camaro305SB

I have a VERY hard time believing that ANYONE with a carbureted V8 is actually getting 18-19mpg AROUND TOWN. I have a bone stock 305 with a rather lean running CCC qjet and 2.73 gears and my winter average is around 14 - 14.5 (including a minute or two warm up in the mornings) and I do not drive it hard very often. Unless the gears are bringing it down around town but still - my 6 speed stock LS1 only gets 18- 19 during my daily commute in the summer(no highway). Hell, my wife only gets around 21-22 in her 04 Grand Prix. 28 on the highway. Is everyone calculating MPG the same way / correctly?

naf

iTrader: (7)

I check mine nearly every fillup. Divide the miles driven by the amount put in. Almost always slightly above 20, near 22 in the Spring/Fall when the AC doesn't run and warm up is quicker. My daily commute is approximately 17 miles with around 7 of those on the interstate. 350 with LO5 heads and cam, ccc-qjet, stock exhaust, 2.73. In the Winter I crank and drive, little other warm up needed as it's rarely below 30 here.

The stock 305 averaged me around 22-23 when I pulled it with 230k miles.

Keep in mind, it's not just the motor and tune, rolling resistance can suffer from improper alignment, tires, tire pressure, wheel bearings, worn steering components, etc.

kmcn47

so anyone know of a write up on tearing down the cc q-jet to fix the leaks?

kmcn47

thanks to naf, and by way of naf sofakingdom i now know what i'm doing atleast when i'll be rebuilding the carb, now alot of people said in this thread "a properly tuned q-jet can get great mileage and driveability" now exactly what tuning is there to do? i've always seen that this carb tunes itself, i can't mess with the idle mixture, so what?

naf

iTrader: (7)

The IAB can be set on the carb so that the MCS dwell (time spent engaged) provides the optimum amount of variance, rich/lean, to respond to changes in mixture. It is normally set at near 50% at idle.

Count turns out on everything as it's disassembled: idle mixture screws, lean stop screw (holds the MCS in place), IAB and maybe rich stop screw (in air horn and doesn't really need to be removed). Install them back to original settings and you should be real close, probably spot on.

Jeffs82TA

iTrader: (1)

I have owned a 1984 trans am with h.o/auto/3.73 combination which I bought brand new. It averaged 26mpg on a trip from MN. to CA. when it was brand new. it's average combined fill ups was about 16-19mpg.

Just curious you say yours runs pretty good, have you check the fuel filler neck after a fill up and looked for leakage in that area.