Gas milage help

[demian5]

Originally Posted by Problems

Me too. A 2017 Silverado with a 5.3 and a 6l80 gets 16/22. A bigger motor with half the gears and a carb will never live up to that.

I have a 2015 with the 6-speed. 21 is best on long drives. pulling the toy hauler... 6-10

[Grumpy old man]

If you can't afford the gas you can't afford the truck ,Buy a kia or ride passenger with someone getting 18-20 ...

[70 shorty]

I've heard people say the Quadrajet will actually do better on gas than a 2 barrel because the primaries on the Quadrajet are smaller. This of course only works IF you keep your foot out of it, and therefore out of the secondaries. I"ve heard this from more than one source, so I think it definitely has merit although I have no first hand experience.

[Yooper truck]

Is the odometer reading distance correctly? I had an odometer on my 71 C-10 that was off by 12% and it skewed my mileage. I checked it over 100 miles using GPS and highway mileage markers to confirm how far off it was. The weird thing was it was registering speed correctly, but the odometer was way off.

[Steeveedee]

It also occurs to me that there may be a mismatch of parts, where the manifold and carburetor don't match the engine's design. But a tiny 2 bbl on a factory manifold should act like a big four barrel that just isn't opened big. My son put a single-plane manifold on his Dart with a 318 and it got $**t for mileage. A dual plane E-brock manifold yielded a significant improvement, even with the same carburetor- like 3 MPG worth.

OP, stick with us. We'll help you get it sorted.

[Awann99]

Ok so I bought a used quad jet carb. Late 80’s and a new manifold and once I put that on I will let everyone know how it improves.
Also. I’m having another problem. If I’m driving some times when I let of the gas and push in the brakes the truck wants to stall. So I have to drive it like a weird Manuel pushing the gas and brakes.
Thanks for the help so far.
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[ericfallon]

Originally Posted by Awann99

Ok so I bought a used quad jet carb. Late 80’s and a new manifold and once I put that on I will let everyone know how it improves.
Also. I’m having another problem. If I’m driving some times when I let of the gas and push in the brakes the truck wants to stall. So I have to drive it like a weird Manuel pushing the gas and brakes.
Thanks for the help so far.
Posted via Mobile Device

Late 80's q-jet is a CCC(computer controlled carb) and will not work without an ecu (computer.)

[muley]

Anyone notice the open vacuum port lower right front of carb?

[67ChevyRedneck]

Is this connected? If not, it's causing a huge vacuum leak and would explain why it wants to stall when slowing down.

Is the vacuum advance on the HEI plugged in?

[Awann99]

Originally Posted by 67ChevyRedneck

Is this connected? If not, it's causing a huge vacuum leak and would explain why it wants to stall when slowing down.

Is the vacuum advance on the HEI plugged in?

It is plugged in now. The top is going to the transmission and the other incoming from my EGR and vacuum advance off the distrbuter
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[jocko]

Originally Posted by demian5

Dude, gas mileage is my last thought when daily driving my old truck.

I couldn't agree more, stopping more often at the pump is a fun tax I'm more than willing to pay. I DO like lowering the highway rpm for other reasons, but better mpg is a nice side effect.

[Awann99]

This is the carb and manifold I got for the GMC
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[El Dorado Jim]

Originally Posted by engineer_gregh

Dave I just spit coffee out my nose laughing at 18-20 mpg with a long bed 69 GMC truck. No disrespect, so please don't get mad at me.

x2

[jeffahart]

I'm gonna go ahead and stick up for Dave here. His truck is so clean and polished, that there is no coefficient of drag. The rest of us all have rust and crud anchoring us the those pesky molecules.

A plug for Dave... He has one of the nicest trucks on this forum. Now that I know it get's that kind of mileage... I really want that truck!


j

[rjs53]

Don't know how long the carbs been sitting that you bought but make sure its clean inside fuel bowl, jets and vacuum passages. Use new base gasket and don't over tighten the carb to intake, you'll warp it or strip the threads. Check your timing, a good starting point would be 8dgs btdc. What motor from jegs? Part #. What rear gear ratio, if you don't know, spin a tire one full revolution and count how many times the driveline goes around. The new motor will use a little more fuel until you get some miles on it.

[dmjlambert]

Originally Posted by Yooper truck

Is the odometer reading distance correctly? ...... it was registering speed correctly, but the odometer was way off.

+1 from me. I recommend checking this in order to get an accurate take on what's going on, before you make changes. My odometer does not work well, speedometer is OK. It is very easy to check, just drive a known distance according to mile markers or along the highway and compare to your favorite navigation app.

[ls1nova71]

Originally Posted by Grumpy old man

350/auto/long bed should get about 10mpg so it's either your tune ,your rear gear ratio or your right foot that's bringing down to 6.8 . spending $800.00 on a different set up ...Well $800 buy's a lot of gas

I always have to laugh bat these kind of statements. Although they may have some truth to them, simply doing some math may surprise you. So, lets just try it! Going to say gas on average is $2.50 per gallon, so $800 will buy you 320 gallons of gas. Now, the OP says he's getting 6.8 mpg, so with his 320 gallons of fuel, he can go 2176 miles. So since he was planning on driving it to school 60 miles three days a week, just that driving alone will use that $800 in just a touch over 12 weeks, so the fact is $800 does not buy you a lot of gas! Its kind of shocking when you actually start looking at it....

My 5.3 swap does a consistent 17mpg although I had hoped for a little better, its still better than the 350/350 quadrajet I pulled out of it that only ever got 10mpg no matter how grandpa like I drove it.

[ZEKE68]

When I got my truck it had a six with the single barrel, powerglide and 3.73 gears and got around 10 mpg. I put a Edelbrock 500 cfm four barrel on the six and got around 9.5 mpg. I am now putting a 454 in it with two four barrels and an overdrive auto. Will likely get around 9.5 mpg!

[Don Quixote]

Originally Posted by ZEKE68

When I got my truck it had a six with the single barrel, powerglide and 3.73 gears and got around 10 mpg. I put a Edelbrock 500 cfm four barrel on the six and got around 9.5 mpg. I am now putting a 454 in it with two four barrels and an overdrive auto. Will likely get around 9.5 mpg!

Thanks for crushing my hopes. I have a 292, 500cfm Edelbrock on an Offenhauser, 4 speed and 3.73. I get about 9.5mpg. I was hoping to tune in better mileage but seeing you are hitting exactly what I get, I think my hopes are dashed.

[davepl]

I know it's an outrageous claim on it's face (which is why I made it!) but before you dismiss it, consider this. With a small carb on a small 283 or 307 V8, these trucks they can run just as efficiently, excluding the combustion chamber design, as the most modern vehicle, but only at one rpm and load. A modern engine can do it across the whole spectrum. But even an old engine has a sweet spot where the AFR is just right.

The problem is that the 1969 can only do that at one rpm (peak torque, ideally) and without every varying the load, lest the accelerator pump kick in.

And because these things are bricks and lack overdrive you're at a painfully slow speed, like 50 mph.

Make sure you also take notice that I said FLAT like flat to within 1 foot over 50 miles. The truck has to climb nothing, so the accelerator pump never strokes, the power valves never open, nothing. Essentially on cruise control.

So all I'm claiming is that the old 307 can cruise at an unchanged speed like 50mph very efficiently I think. I think it can approach modern truck efficiency. But if you drive it Oregon instead of Saskatchewan, all bets are off. Climb a even slight hill and you invoke the acclerator pump maybe, or come off the cruise circuit, and it changes a lot.

I sure can't think of any reason a properly tuned truck couldn't get 18mpg at 50mph but I can think of lots of reasons it couldn't get anywhere near that in most situations.

So keep in mind I'm basically talking about getting 18mpg driving slowly on a pool table.

[Steeveedee]

I'd personally be astonished if a V-8 truck back in the day got 18 MPG. The amount of wind it has to push at speed is going to be against that, at least with the technology available back in the day. These things are like slightly rounded bricks, as far as wind resistance is concerned. The computer controls these days augment the curved shapes in order to get even a small improvement. We buy these trucks to do work. Work takes energy; that comes out of the gasoline (or diesel fuel).

[Don Quixote]

I think of it this way, I ran a small block in my truck for years and got decent mileage empty, around 12mpg but as soon as I hooked up my trailer I was fortunate to get 6mpg and I was lucky to get 1 or 2 cross country trips before I had to take it apart. I built and put in the truck's original engine, a 292 and now my mileage is crap all the time, about 9 to 9-1/2mpg but that's regardless of where I am or what I'm pulling and that engine can slowly pull the Rock of Gibraltar across the country without faltering. The old truck has been paid off for 21 years so I see no issue donating to the gas tank to have such reliability as I do.

At 6.5mpg, it's likely you just need a good tune up and timing adjustment. I've experienced that with my small block but it didn't take much for it to come up to about 12mpg-ish.

[Awann99]

Ok guys. I jacked up the back of the GMC and spun the tire twice and the drive shaft rotated just a little over three times. So I’m guessing I have a 3.08 rear end. I checked and I do have a TH350 tranny. Another thing I discovered is the brakes are stiff in the back so the tires won’t freely spin. So that was one of the reasons my gas milage was terrible. But the bolts for my quad jet came in today so hopefully I can get it all hooked of tomorrow.
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[bbrriiaann1973]

My 70 C10 with a 383/700R4 gets about 16 or so and I wish it got 10! Because then that would mean that I was holding it to the floor more often.

[Ironangel]

Thanks for checking back in Awann, dont get discouraged by some of the comments. A 350 with a 350 Turbo-Hydramatic and 3:08 gears can easily get 12 mpg, with a well tuned 4MV Quadrajet. I have two 72 Chevy C/10's that started life with the same 350/350THM and 3:08 gears, one was a 2bbl the other is a Quadrajet sitting on an Edelbrock Performer intake like the one shown in your pic. Buying a used carb (like the one in your pic) will almost certainly require a cleaning and a rebuild kit. To do that you'll need to ID that carb and find out what it came off of because the choke mechanism appears to be a hot air operated choke unit. Thats ok because they can easily be converted to electric or even manually operated. My 2bbl Rochester never came close to getting the mpg's the Quadrajet did and when you look at the diameter differences between the throttle butterfly's you'll see why the Quadrajet is renowned for it's mileage capabilities. Here's a good read to get you started in ID-ing your carb. http://www.chevyhardcore.com/tech-st...ication-guide/ But before you spend another dime educating yourself and buying carbs and carb parts you should really consider what S10MK said in post #4 because by the time you rebuild or replace that Quadrajet and consider the costs involved a self programming fuel injection unit can be had and installed for little more than a properly set up Quadrajet. The FiTech and the Holly Sniper are getting good reviews, something worth considering. If you haven't already, converting over to an HEI ignition will help with mileage as well. There are a few of us die hard Quadrajet guys still kickin it and willing to help on this forum...