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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: 1980cdeluxe on October 20, 2006, 11:30:00 pm

Title: Possible reasons for burning SO much gas.
Post by: 1980cdeluxe on October 20, 2006, 11:30:00 pm: Well I recently got my 80cdeluxe 4x4 back on the road, and man o man how I forgot how unreal the gas milage on this truck is. It had a full tune up before I parked it, and it doesn't leak gas obviously, but what could be some other reasons making this truck get 5mpg (ROUGHLY). It's basically pouring out at that speed, and it's only got a smallblock 350 from a 80 series ton van in it. 3.08 gearing, th400 transmission... I believe the timing is on, idle is 600-800rpm, and the carburetor is a 4brl holly 650cfm electric choke.. Doesn't matter if i'm on it all the time, or babing it... Gas milage doesn't change whatsoever. The stock quadrajet was just as bad. The motor has around 40k from when I got it.
Edited by: 1980cdeluxe at: 10/20/06 10:57 pm
Title: Re: Possible reasons for burning SO much gas.
Post by: Lt.Del on October 21, 2006, 12:34:00 am: vacuum leak would be first guess. Do the ole wd40 trick to see if rpms go up. then adjust carb.
SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4
Title: Re: Possible reasons for burning SO much gas.
Post by: VileZambonie on October 21, 2006, 05:45:00 pm: If everything is tuned properly as far as the ignition system is concerned then I would suggest recalibrating the carburetor.
Title: Re: Possible reasons for burning SO much gas.
Post by: 1980cdeluxe on October 23, 2006, 01:12:00 am: What do you mean by recalibrating the carburetor, sorry for the vague question.

Multiple things i've noticed: Engine runs HORRIBLE when cold, miss, black smoke, will backfire if I hit the gas more then a little etc. The engine runs great when at operating temperature at idle and AFTER about 1k rpm (smoking stops completely once it's warm). It has a slight hesitation when I inch on the throttle. I've also noticed if I punch it going down the road or at idle it has a dead spot in the carburetor before it "kicks" in and takes off. If I ease down onto so far then open it up it's fine. I also often smell gas once I park and get out of the truck, but I haven't found any leaks.

I suspect a carburetor problem, but yeah...

Edited by: 1980cdeluxe at: 10/23/06 12:13 am
Title: Re: Possible reasons for burning SO much gas.
Post by: Lt.Del on October 23, 2006, 11:01:00 am: Quote:
I believe the timing is on,

sounds like a timing issue or carb is way off. Perhaps choke is off.

double check timing.
SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4
Title: Sounds like
Post by: Redneckchevy on October 23, 2006, 09:32:00 pm: Your choke is sticking..
Title: Re: Sounds like
Post by: DnStClr on October 27, 2006, 11:08:00 am: And one other thing is the tranny. I have a TH400 too- and am considering getting an overdrive (700R4) to get better gas mileage.
Don St.Clair
Culleoka Tn
Title: Re: Sounds like
Post by: 1980cdeluxe on October 28, 2006, 01:45:00 pm: I believe it has high gears, low rpms at 65mph so... My brother in-law agreed to let me try his quadrajet (originally came off my motor, I let him use it when I got the holly). He told me he recalibrated it to burn less gas. Going to see if It actually helps any, thanks for the tips so far.
Title: bad mpg
Post by: presleyhome on November 26, 2006, 08:43:00 am: Curious if you had anything new with this. I have an 81 GMC C25 with a 350M, Quadrajet and 400 trans. Mileage just stinks from idle to 40 Mph. You can smell the rich gas. On the freeway it gets 15mpg at 65+. Timing is on, carb is brand new and all hoses are new. Did the same thing with the original carb, so I guessing it was not the issue. I gather there is now easy way to test each of those sensor switches that the vacuum hoses connect to. Must be 4 or five of them. Maybe the HEI unit? Motor runs great, but man....4 or five trips to the local grocery and I'm through a tank of gas!

Jim P
Title: Re: bad mpg
Post by: VileZambonie on November 26, 2006, 12:51:00 pm: If you are getting poor fuel economy again the 1st thing to do is make sure all of your ignition components are healthy (Cap & rotor, ignition wires, spark plugs -make sure they are correctly gapped etc) Check your base timing and your timing advance. If your trucks runs poorly after that and gets bad fuel mileage check your fuel metering. Carburetors are very basic. It's sort of like drinking soda through a straw. Odds are if you haven't toyed with the carb it's probably set up optimally for your engine however could be leaking internally. Q-Jets are notorious for internal leakage. If you rebuild the carb make sure you use some jb weld or epoxy to seal up the plugs at the bottom of the venturis. Other than that if you are still using that much fuel check your tank and lines and check your oil level to make sure it isn't leaking into the crankcase.
Title: Re: bad mpg
Post by: xtreme80 on December 04, 2006, 12:00:00 pm: I know this is an older post, but I thought I would give somewhat of an update to your problem since mine just did the same thing.

I had a 350 with a factory QJ carb on it. I was getting 10.2 mpg after I did the mileage test. Had all the same problems as you when cold, and at low RPMs. Yesterday it started getting worse and worse. Took the intake off the top of the carb and looked into the butterfly valve with a flashlight. I could see gas pouring into the carb where it shouldn't be. The culprit: a leaky carb causing the engine to run like crap at low RPMs.

My guess for you would be to buy a 20 dollar carb rebuild kit, and a new float, and rebuild the carb. It's only about a 2 hour project, and a Haynes manual will help out alot.
Title: Re: Possible reasons for burning SO much gas.
Post by: ssgoodman on December 30, 2007, 06:23:59 pm: sorry for bumping an old post but i am having some of the same problems. i am definately not an engine person, i know the basics and can do lot of the work if i can read a manual or if someone tells me what to do its just i cant really figure alot of it out on my own. i know the truck does need new spark plugs and probably new wires since i believe its all original under the hood.

my question is when do you know its time to rebuild a carb? and how tough would it be to rebuild a carter quadrajet if i have never done it before? i want to wait until this spring to keep my truck on the road for the winter but if it didnt take long i would consider doing it here soon.

Sutton
Title: Re: Possible reasons for burning SO much gas.
Post by: VileZambonie on December 30, 2007, 07:05:19 pm: Carburetors get dirty, gaskets crack, floats get burned, linkages get sticky, choke coils wear out, deposits build up, shafts wear out etc. If you identify a specific problem with your carburetor that would require taking it apart then it's time for an overhaul. If you're just doing a tune up get some carb clean and just clean it and change the filter.
Title: Re: Possible reasons for burning SO much gas.
Post by: werewolfx13 on December 30, 2007, 08:42:52 pm: Vile covered the reasons to rebuild the carb thoroughly..I won't rebuild a qjet, I replace it with somthing I'm comfortable tearing into, so how hard it would be for you, I don't know. Replace the plugs and wires and check the timing..then *SLOWLY* pour some seafoam through the carb with the engine running if you want, or get some of that spray cleaner.

The OP's problem sounded like a blown powervalve or accelerator pump diaphram to me..my '84 with a holley had the EXACT same symptoms, and it turned out to be three things: the accelerator pump diaphram was almost entirely gone, powervalve was bad, and the secondaries were screwed all the way shut.
Title: Re: Possible reasons for burning SO much gas.
Post by: Bowtie Bomber on December 31, 2007, 05:50:03 pm: My Holley did things like that also. Rebuilding it or if you are VERY lazy, leave the carb on the intake and just take the front bowl off the carb and rebuild it from there if you are having idle problems, since the front bowl controls the primary's and supplies the most amount of fuel the engine consumes. I personally take the whole thing off, but if it's a power valve then that works just fine.

Also gunked up needle and seats can make your carb act funny. Be sure to check those before anything in my opinion.
Title: Re: Possible reasons for burning SO much gas.
Post by: Dragon on January 02, 2008, 04:37:00 pm: Quote from: ssgoodman on December 30, 2007, 06:23:59 pm
sorry for bumping an old post but i am having some of the same problems. i am definately not an engine person, i know the basics and can do lot of the work if i can read a manual or if someone tells me what to do its just i cant really figure alot of it out on my own. i know the truck does need new spark plugs and probably new wires since i believe its all original under the hood.

my question is when do you know its time to rebuild a carb? and how tough would it be to rebuild a carter quadrajet if i have never done it before? i want to wait until this spring to keep my truck on the road for the winter but if it didnt take long i would consider doing it here soon.

Sutton
Not to be mean or anything, but could you please post what year Truck or engine it is you are trying to bring back up to par??? The reason why I say that, is cause there are other things for the various years that could also be causing problems. The above 1980 in the first post, I would also ask as to weither or not the Oxygen Sensor has been replaced, as one that is malfunctioning will also cause a problem with the engines fuel economy.

If your truck is older than 1980, then you might not have one (Some Cali cars and Trucks got them as early as late 78/early 79, depending on the vehicles use), but I would check to make sure.

The Charcoal Evap Canister has also been known to give difficulties, when they replaceable bottom filter is dirty enough, again depends on weither your truck came with one or not....

As for your Carb, is it a Rochester Quadra-jet or a Carter 4 barrel??? A Carter is Basically the Same as an Edelbrock Carb, and be a bit tricky to rebuild the first time out, but isn't too difficult. If it is a Rochester Quadra-Jet, they are much easier to rebuild, after the first time you have done it. Personally I can rebuild a Quadra-Jet in my sleep, and am more than willing to tear it apart again when need be.. QJ's can be a bit more trickier to get them to adjust properly, but once you have, they can be one of the most Economical, and Performance Oriented Carb available. When is it a good time to rebuild your carb??? For me, it is when I get a new car/truck, and I don't know if it has been done before(Especially if there is a good build up of dirt,grease, and etc present). Someone else, I would suggest that they do a rebuild, after they have preformed the basics(Ie replace Wires,Cap,Rotor,Module(if an HEI)/Points & Breakers(Also properly gapped/adjusted),Dist Springs(If Needed),Coil, and Plugs(ofcourse properly gapped). Once you have done all of that, then I would suggest to rebuild your carb...
Title: Re: Possible reasons for burning SO much gas.
Post by: werewolfx13 on January 02, 2008, 10:54:07 pm: Not all 80 up vehicles had O2 sensors..AFAIK, none of these trucks got them til '87. I've never seen an 86 or earlier with one, including a 100% original '85 that my barber bought brand new..Only things that have ever been changed are fluids, filters, battery, and tires. Its still on the original plugs and wires.
Title: Re: Possible reasons for burning SO much gas.
Post by: Dragon on January 03, 2008, 02:12:31 am: Quote from: werewolfx13 on January 02, 2008, 10:54:07 pm
Not all 80 up vehicles had O2 sensors..AFAIK, none of these trucks got them til '87. I've never seen an 86 or earlier with one, including a 100% original '85 that my barber bought brand new..Only things that have ever been changed are fluids, filters, battery, and tires. Its still on the original plugs and wires.
In my Post I never stated that All '80 and up Vehicles got O2 Sensors, alot of them did, especially Cars/Trucks and Etc that were sold in California. Personally I have seen Late '78- Early '80 Pickups/Suburbans/Jimmys/Blazers/Vans/RV's/Etc that were Originally order in California, Reside in near by States(Also some that were Shipped to Canada), and All of them Had O2's(mounted on a manifold) with the Cats(with Ebay, these trucks can migrate pretty far!).

I'm sure you have seen a 100% Original '85 (& even later on some models) without an O2 and possibly a Cat, O2's weren't mandatory till around '93 or so on all Vehicles. As I stated in my previous post, neither SSgoodman, or 1980cdeluxe have their locations(State/Origin State of Said Vehicle) posted( In SSgoodman's case, he does give the year of the engine or truck he is working on), so I thought it would be a good idea to cover what hadn't been before.....
Title: Re: Possible reasons for burning SO much gas.
Post by: JJSZABO on January 03, 2008, 08:03:33 am: Dragon, I read it as all 1980 and above also. :) My 1986 c-10 did not have an o2 sensor, but it had (has)cats.

With my limited knowledge, o2 sensors are used in computer controlled vehicles to help the computer regulate the air/fuel mix and to switch from "open loop" (when vehicle is cold) so it uses predetermined air/fuel tables to closed loop (when vehicle warms up) so the sensors regulate a/f mix. The down stream o2 sensors only check to see if the catylatic convertors are there.

I don't see a need for o2 sensors on a carburator vehicle, unless the o2 helps the carb regulate air/fuel electronically.

As I said my knowledge is limited. I am sure someone with alot more knowledge will pipe in.
Title: Re: Possible reasons for burning SO much gas.
Post by: VileZambonie on January 03, 2008, 10:07:41 am: O2's are only used in feedback systems so either TBI or a carb that has the MC solenoid. Post Catalyst O2's were never used in these trucks and are pretty much OBDII