Horrible gas mileage - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Rain City · 03-30-2009, 07:03 PM

I've been getting very crappy gas mileage in my 1981 350/th350/holley carb.

I just drove to work and back and used about a little under a quarter of a tank (24 mile round trip)

Is this what you guys experience with similar trucks?

I put air in all the tires this weekend, and took my tailgate off this morning.

Pic of my engine...

Things I am considering....

PCV Valve
New Spark Plugs
Spark plug wires (??)

Any thoughts? Is this normal should I get used to it??

Rain

flamingbig10 · 03-30-2009, 07:32 PM

You should be getting a little better gas mileage, are they high way miles or city miles, I check/ replace what you listed and also the air filter, fuel filter, also cap and rotor

lurch115 · 03-30-2009, 07:50 PM

i'd be checking to make sure the needle and seats aren't bad and causing a flooding condition, but not enough to stall the motor...what speeds are you driving at to and from work??? and what size tank??

pokethesmot · 03-30-2009, 08:29 PM

on myth busters it said you get better gas millage with the tail gate on and up

factory2wdblazer's · 03-30-2009, 08:46 PM

how big is the carb?. carb maynot be set right i know alot of people like to just open them up, or could need a tune up, or maybe timing,.......with a overdrive transmission you would have better gas mileage......

71swb4x4 · 03-30-2009, 08:59 PM

Is this verified with a gas fillup? Don't go by what your gas gauge says. This era of gauges seems to go down slowly between half and full and then go down very quickly between half and empty.

79Silv4x4 · 03-31-2009, 12:07 AM

Quote:

Originally Posted by 71swb4x4

Is this verified with a gas fillup? Don't go by what your gas gauge says. This era of gauges seems to go down slowly between half and full and then go down very quickly between half and empty.

Agreed, until you post an actual MPG, we have nothing to go on.

piecesparts · 03-30-2009, 09:33 PM

What is your timing set at, what is your plug gap, what gear ratio do you have, what size tires are you running, do you have a 4X4, and have you set your carb by using a vacuum gauge, are you running a custom cam profile? These are all things that can affect your approach to economy. If the carb is too large of a CFM rating, it will flood the engine and give poor performance in daily driving and suck your billfold dry. You should be running a carb that is around 600 to 650 CFM for you size of engine with a truck for weight considerations and if you are running it hard,then look for a bigger unit.

Rain City · 03-31-2009, 11:44 AM

Quote:

Originally Posted by piecesparts

What is your timing set at, what is your plug gap, what gear ratio do you have, what size tires are you running, do you have a 4X4, and have you set your carb by using a vacuum gauge, are you running a custom cam profile? These are all things that can affect your approach to economy. If the carb is too large of a CFM rating, it will flood the engine and give poor performance in daily driving and suck your billfold dry. You should be running a carb that is around 600 to 650 CFM for you size of engine with a truck for weight considerations and if you are running it hard,then look for a bigger unit.

Just got this truck about a month ago (my first running project) and I am NOT an expert by any means...

I'm thinking the carb might be an issue, given this issue that I recently came upon and fixed... http://67-72chevytrucks.com/vboard/s...d.php?t=335690

How do I set the carb or know what to set it at etc..??

Don't know what timing is set at
What should my plug gap be?
Gear ration? stock I think.
Size tires? Not sure the exact size
Car is 2wd
Custom Cam profile? Not sure...

I'm going to replace the spark plugs today, and get a dual spring throttle return (recommend by many forum member) - and see what happens.

UltraBlue71 · 03-30-2009, 09:45 PM

buy a honda or a toyota! j/k tune er up, check the timing, is a choke sticking open? Pull those plugs out and look at em, clean em up and check the gap, if ya have an ohmmeter you can check the resistance on your wires, look the cap and rotor over, spray some carb cleaner down your carb, If you give up on her and part her out, I got dibs on the valve covers and aircleaner, they are "tight" as my eleven year old says! Keep your foot out of it, opening up the secondaries with that hi-rise intake isnt going to help much either. Thats one ole poor boys way to get you started anyways!

gchemist · 03-30-2009, 10:18 PM

Ditch the throttle return spring. Install a dual spring setup. I've seen springs break at the worst time. I've had it happen to me!!

Install a taller air filter. A SB should have at a minimum 4". A 4.5" is great!!

OrangeAnimal · 03-31-2009, 09:24 AM

To me it sounds ok. I am getting about 10mpg in town. 350/TH350 with a 600cfm edelbrock and shorty headers with "RV" cam. stock gearing. I am sure it would get better as I have some trans issues. I would suspect somewhere in the area 12-14 in town would be about right. Probably about 17-18 on the highway provided you are not going 70 running over 2000rpms.

enaberif · 03-31-2009, 12:29 PM

I'm not sure what the point of the dual spring is but I'm running a single spring no problems.

Tailgate put back on its a long time myth that helps and it doesn't.

PCV valve? Shake it. If it rattles its fine.

I notice an extremely long spring stretched on the left side. What is this for?

As recommended purchase a 4-4.5" air filter

When was the last time you checked your distributors cap and rotor? Or the springs?

I notice your vac line to the vac advance is running off the carb I suggest tapping into the manifold source to give it a constant source of vacuum.

Definitely change the plugs. Wires shouldn't be needed to be done.

impalamark · 03-31-2009, 12:40 PM

wondering about that long over stretched spring hooked near the front of the intake . I'm hoping that's not your throttle return spring . if so I'd recommend getting one that fits properly for
safety reasons because it is not safe . And I'm told can hurt the gas mileage by allowing the throttle to stay open slightly
mark

enaberif · 03-31-2009, 12:44 PM

Quote:

Originally Posted by impalamark

And I'm told can hurt the gas mileage by allowing the throttle to stay open slightly
mark

I'm used to driving a lot of old cars so if I think the throttle is partial open I just kick the pedal up hahaha which yes I have noticed has happened a few times on my truck.

Can the OP take a picture from the front of the left side of the truck I'm looking at your PCV valve and curious how THAT is hooked up too.

Rain City · 03-31-2009, 01:12 PM

Quote:

Originally Posted by enaberif

I'm used to driving a lot of old cars so if I think the throttle is partial open I just kick the pedal up hahaha which yes I have noticed has happened a few times on my truck.

Can the OP take a picture from the front of the left side of the truck I'm looking at your PCV valve and curious how THAT is hooked up too.

OK so I bought a dual throttle return spring, but when i hooked it up, the throttle couldn't move because the springs were so tight on it. Literally I the gas pedal wouldn't budge, so I ended up with this

Also, I found a random hose that was not hooked up to anything.. umm..

And by request above, here is a pic of the front of the left side of the truck (I don't know where the PCV valve is so I dont know if this helps)

and... "I notice your vac line to the vac advance is running off the carb I suggest tapping into the manifold source to give it a constant source of vacuum." - Not sure how exactly to do this, any help?

Driveway Dreams · 03-31-2009, 01:16 PM

My guess is to check the Fuel Gauge. Like was said before, they are not known to be 100% acurate. Fill it up, put a 2 gallon fuel can in the bed, and run the truck till its EMPTY. Then you know 2 things. If your gauge is pretty close to acurate and you can find a true fuel milege for your truck.

By the way, your pics from the left are actually from the right. Right and left are as determined from sitting in driver's seat.

enaberif · 03-31-2009, 01:17 PM

hahaha you don't have a PCV valve! You have to valve cover air breathers which is bad!

On the left valve cover a pcv valve will have a vacuum connection which is usually fed off the carb somewhere.

I have never seen a throttle spring like that ever!

Driveway Dreams · 03-31-2009, 01:19 PM

Quote:

Originally Posted by enaberif

hahaha you don't have a PCV valve! You have to valve cover air breathers which is bad!

On the left valve cover a pcv valve will have a vacuum connection which is usually fed off the carb somewhere.

That throttle spring is SO wrong.

Help the guy out then, give him a good picture or something to know WHAT is wrong about it.

enaberif · 03-31-2009, 01:22 PM

Quote:

Originally Posted by Driveway Dreams

Help the guy out then, give him a good picture or something to know WHAT is wrong about it.

I'm not an expert on the throttle spring or that holley carb.

PCV valve.

Rain City · 03-31-2009, 01:31 PM

Quote:

Originally Posted by Driveway Dreams

Help the guy out then, give him a good picture or something to know WHAT is wrong about it.

Exactly, we can make fun of my truck all day... but at the end of the day I'm looking for some help here.

enaberif · 03-31-2009, 01:34 PM

Quote:

Originally Posted by Rain City

Exactly, we can make fun of my truck all day... but at the end of the day I'm looking for some help here.

Nobody is making fun of your truck. I laugh because your in the same situation that so many of us others have been in.

You buy a truck from a PO and you look at what they have done and you shake your head and just have to laugh and then spend the rest of your life fixing all the small mistakes :P

Rain City · 03-31-2009, 01:36 PM

So what should I do about the PCV valve thing, and what about that random hose that isn't hooked up to anything?

*78 Chevyrado* · 03-31-2009, 01:52 PM

You can't go by the guage as said above, you have to compare mileage to how much fuel it takes to fill it up. for example, I have a 20 gallon tank and when I get to E on the guage I still have 6 gallons left. Also when full it goes past the F on the guage but it only takes 4 gallons to get to the 3/4 full mark.

The point of having a dual spring is that when the spring you have fails eventually, the truck will be floored possibly causing an accident until you get a new spring. with dual springs when one breaks, the throttle will still go back closed and you can get it home.

enaberif · 03-31-2009, 01:54 PM

Quote:

Originally Posted by 78 Chevyrado

The point of having a dual spring is that when the spring you have fails eventually, the truck will be floored possibly causing an accident until you get a new spring. with dual springs when one breaks, the throttle will still go back closed and you can get it home.

Makes sense.

Tho looking at pictures am I the only one thinking this motor has had some work done to it?

That isn't a stock intake manifold nor are those stock heads.

03-30-2009, 07:03 PM	#1
Rain City Registered User Join Date: Mar 2009 Location: Seattle, WA Posts: 32	Horrible gas mileage I've been getting very crappy gas mileage in my 1981 350/th350/holley carb. I just drove to work and back and used about a little under a quarter of a tank (24 mile round trip) Is this what you guys experience with similar trucks? I put air in all the tires this weekend, and took my tailgate off this morning. Pic of my engine... Things I am considering.... PCV Valve New Spark Plugs Spark plug wires (??) Any thoughts? Is this normal should I get used to it?? Rain

03-30-2009, 07:32 PM	#2
flamingbig10 6.0 powered square Join Date: Feb 2008 Location: Mesquite texas Posts: 3,459	Re: Horrible gas mileage You should be getting a little better gas mileage, are they high way miles or city miles, I check/ replace what you listed and also the air filter, fuel filter, also cap and rotor __________________ Aaron 1980 c-10 short bed lowered, cammed 6.0, 4l80e 3600 stall, smp tune, wilwoods, and flames best time so far best et 7.86 @87 on motor 7.57 90 on a 75 shot Build http://67-72chevytrucks.com/vboard/s...d.php?t=283326

03-30-2009, 07:50 PM	#3
lurch115 Registered User Join Date: Oct 2006 Location: iowa Posts: 722	Re: Horrible gas mileage i'd be checking to make sure the needle and seats aren't bad and causing a flooding condition, but not enough to stall the motor...what speeds are you driving at to and from work??? and what size tank?? __________________ "Horsepower is how fast you hit the wall, torque is how far you carry the wall with you." '84 chevy half ton short box 2wd, 67,000 actual miles '03 chevy 1500 HD crew cab 4x4 standard box, 6.0L

03-30-2009, 08:29 PM	#4
pokethesmot profeshinaly unimployed Join Date: Dec 2008 Location: hazlehurst, GA Posts: 1,232	Re: Horrible gas mileage on myth busters it said you get better gas millage with the tail gate on and up __________________ my ride click here my myspace "having mud flaps is like asking for a smaller trophy"

03-30-2009, 08:46 PM	#5
factory2wdblazer's Registered User Join Date: Sep 2008 Location: northern, califorina Posts: 199	Re: Horrible gas mileage how big is the carb?. carb maynot be set right i know alot of people like to just open them up, or could need a tune up, or maybe timing,.......with a overdrive transmission you would have better gas mileage......

03-30-2009, 08:59 PM	#6
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: Horrible gas mileage Is this verified with a gas fillup? Don't go by what your gas gauge says. This era of gauges seems to go down slowly between half and full and then go down very quickly between half and empty. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

03-30-2009, 09:33 PM	#8
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: Horrible gas mileage What is your timing set at, what is your plug gap, what gear ratio do you have, what size tires are you running, do you have a 4X4, and have you set your carb by using a vacuum gauge, are you running a custom cam profile? These are all things that can affect your approach to economy. If the carb is too large of a CFM rating, it will flood the engine and give poor performance in daily driving and suck your billfold dry. You should be running a carb that is around 600 to 650 CFM for you size of engine with a truck for weight considerations and if you are running it hard,then look for a bigger unit. Last edited by piecesparts; 03-30-2009 at 09:33 PM.

03-30-2009, 09:45 PM	#10
UltraBlue71 Registered User Join Date: Feb 2009 Location: OKC OK Posts: 489	Re: Horrible gas mileage buy a honda or a toyota! j/k tune er up, check the timing, is a choke sticking open? Pull those plugs out and look at em, clean em up and check the gap, if ya have an ohmmeter you can check the resistance on your wires, look the cap and rotor over, spray some carb cleaner down your carb, If you give up on her and part her out, I got dibs on the valve covers and aircleaner, they are "tight" as my eleven year old says! Keep your foot out of it, opening up the secondaries with that hi-rise intake isnt going to help much either. Thats one ole poor boys way to get you started anyways! __________________ "You can put lipstick on a pig, sure it's pretty pig, but it's still just a PIG... 71 LWB

03-30-2009, 10:18 PM	#11
gchemist BAD BOW-Silverado XST Join Date: May 2000 Location: Senior Member from Austin, TX Posts: 6,431	Re: Horrible gas mileage Ditch the throttle return spring. Install a dual spring setup. I've seen springs break at the worst time. I've had it happen to me!! Install a taller air filter. A SB should have at a minimum 4". A 4.5" is great!! __________________ Gerardo a.k.a. Mad Chemist Silverado XST videos Last edited by gchemist; 03-30-2009 at 10:19 PM.

03-31-2009, 09:24 AM	#12
OrangeAnimal Registered User Join Date: Jan 2009 Location: New Orleans, LA Posts: 702	Re: Horrible gas mileage To me it sounds ok. I am getting about 10mpg in town. 350/TH350 with a 600cfm edelbrock and shorty headers with "RV" cam. stock gearing. I am sure it would get better as I have some trans issues. I would suspect somewhere in the area 12-14 in town would be about right. Probably about 17-18 on the highway provided you are not going 70 running over 2000rpms. __________________ 82 Chevy SWB "OrangeAnimal" Lot of Goodies! Sold 93 GMC 1500 "Angry Red" 383 TBI Powered Fun! 300hp/410ft.lbs. 09 Yaris wife's daily.

03-31-2009, 12:29 PM	#13
enaberif Registered User Join Date: Jan 2009 Location: Calgary, AB Posts: 1,181	Re: Horrible gas mileage I'm not sure what the point of the dual spring is but I'm running a single spring no problems. Tailgate put back on its a long time myth that helps and it doesn't. PCV valve? Shake it. If it rattles its fine. I notice an extremely long spring stretched on the left side. What is this for? As recommended purchase a 4-4.5" air filter When was the last time you checked your distributors cap and rotor? Or the springs? I notice your vac line to the vac advance is running off the carb I suggest tapping into the manifold source to give it a constant source of vacuum. Definitely change the plugs. Wires shouldn't be needed to be done.

03-31-2009, 12:40 PM	#14
impalamark Registered User Join Date: Feb 2008 Location: cocoa florida Posts: 183	Re: Horrible gas mileage wondering about that long over stretched spring hooked near the front of the intake . I'm hoping that's not your throttle return spring . if so I'd recommend getting one that fits properly for safety reasons because it is not safe . And I'm told can hurt the gas mileage by allowing the throttle to stay open slightly mark __________________ the avatar was my 62 I owned for 27 years 84 chevy short short bed black 84 gmc long bed copper/ buckskin 88 chevy k5 blazer white Life's short make the most of it Mark aka impalmark

03-31-2009, 01:16 PM	#17
Driveway Dreams I have car A.D.D. Join Date: Feb 2009 Location: Greeley, Northern Colorado Posts: 4,147	Re: Horrible gas mileage My guess is to check the Fuel Gauge. Like was said before, they are not known to be 100% acurate. Fill it up, put a 2 gallon fuel can in the bed, and run the truck till its EMPTY. Then you know 2 things. If your gauge is pretty close to acurate and you can find a true fuel milege for your truck. By the way, your pics from the left are actually from the right. Right and left are as determined from sitting in driver's seat. __________________ -Gerald Building vehicles in memory of the first "car guy" I ever knew, my Grandpa. Bob Dunham 1935-2009. Rest in peace. 61 GMC Suburban SOLD The 86 build thread SOLD Are you a Colorado member?Click here. Last edited by Driveway Dreams; 03-31-2009 at 01:17 PM.

03-31-2009, 01:17 PM	#18
enaberif Registered User Join Date: Jan 2009 Location: Calgary, AB Posts: 1,181	Re: Horrible gas mileage hahaha you don't have a PCV valve! You have to valve cover air breathers which is bad! On the left valve cover a pcv valve will have a vacuum connection which is usually fed off the carb somewhere. I have never seen a throttle spring like that ever! Last edited by enaberif; 03-31-2009 at 01:19 PM.

03-31-2009, 01:36 PM	#23
Rain City Registered User Join Date: Mar 2009 Location: Seattle, WA Posts: 32	Re: Horrible gas mileage So what should I do about the PCV valve thing, and what about that random hose that isn't hooked up to anything?



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03-31-2009, 01:52 PM	#24
78 Chevyrado Account Suspended Join Date: Dec 2007 Location: Unnapreciative, Trollville Posts: 2,079	Re: Horrible gas mileage You can't go by the guage as said above, you have to compare mileage to how much fuel it takes to fill it up. for example, I have a 20 gallon tank and when I get to E on the guage I still have 6 gallons left. Also when full it goes past the F on the guage but it only takes 4 gallons to get to the 3/4 full mark. The point of having a dual spring is that when the spring you have fails eventually, the truck will be floored possibly causing an accident until you get a new spring. with dual springs when one breaks, the throttle will still go back closed and you can get it home. Last edited by 78 Chevyrado; 03-31-2009 at 01:55 PM.

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