Gas milage help

[Awann99]

I have a 1969 gmc longbed that I just put a new crate 350 in from jegs. I reused the original manifold and it has a late 70’s Rochester 2 Barrel carb on it. I believe it’s a 2vg. I’m not sure what the gear ratio is but it has been changed from the stock. Th 400 tranny. After taking for a few drives I calculated it getting 6.8 mpg. How in the world could I pull this up. I was wanting to drive it 60 miles three days a week to school. But it’s imposslibe with that kind of MPG.
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[vics stuff]

I would check your ignition timing. Then check your carburetor adjustments.
These never did any good on fuel even when new.
Vic

[Awann99]

Originally Posted by vics stuff

I would check your ignition timing. Then check your carburetor adjustments.
These never did any good on fuel even when new.
Vic

Ok I will do that. What would be the best replacement carb to get better gas milage.
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[s10mk]

Originally Posted by Awann99

Ok I will do that. What would be the best replacement carb to get better gas milage.
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Hi there!
If you are looking to buy a new carb for the purpose of improved mpg, I would first consider one of these.
https://www.summitracing.com/parts/fif-30003
Its a self learning fuel injection system, that will get you the best fuel mileage because when cruising, its constantly trimming fuel to keep the engine running at stoich air fuel ratio. You mentioned that your truck is a long bed, is it a 1/2 or 3/4 ton? What rear axle ratio does it have?

[engineer_gregh]

Something is off if your gas mileage is that bad. Even my hopped up big block 402 does better than that. It would help if you let everyone know some of the following; HEI or points, ignition timing (initial and full), plug gap and even what plugs your running, rear end ratio, manifold vacuum, and even pictures of your carb and engine. There are some very good mechanics on this site that can help. A 69 2 wheel drive pickup with a new crate engine should do at least 10 MPG, if not better. Just curious, why did you put a 2 barrel carb on a new crate motor?

[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

[KQQL IT]

My cap and rotor where corroded terribly and seemed like the mileage went up a bunch after swapping those two parts

[Kudzupatch]

After making sure that all the plugs, wires, timing etc is right, you may have the wrong jets in the carb causing it to run rich. Burning more fuel costing you milage.

[ericfallon]

You sure you don't have a fuel leak.. my 540 cid big block gets better than that..

[Barnfind46]

My 68 C-20 got 8 mpg when I got it. 350 v-8 with auto and someone had changed the gears to 4.88's.It also ran hot in the summer. Found out it had a radiator from an inline six. Installed a new V-8 radiator,changed the rear end to 3.73's and did a good tune up and now I get 13-14 mpg. There is hope!

[demian5]

Originally Posted by Awann99

I have a 1969 gmc longbed ... getting 6.8 mpg. How in the world could I pull this up. I was wanting to drive it 60 miles three days a week to school. But it’s imposslibe with that kind of MPG.
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Get a commuter car. My son is in high school with a 86 Scotttsdale C20 with TH400 and 4:10 and gets about the same with a 350 and 4BBL.

Hence the reason we don't see these things all over the roads anymore.

[Steeveedee]

There is something wrong with your setup. I'd bet most of it is going to be the timing. I have a 400/400/3.54 axle setup and I get 7MPG when towing a 26 foot long travel trailer. Did the engine come with a harmonic balancer and a pointer for the timing already installed, or did you have to add a pointer? I've seen those not be in the right place, and the timing ended either retarded or over advanced. How does your exhaust look? Getting a lot of blue or black smoke? That is an indication of loading fuel in the carburetor. You can look down the inside with the engine running and see if fuel is dripping. No need to blip the throttle when doing so.

[davepl]

I like those original 2bbl carbs on those, they run well and are economical. I would guess yours is wildly out of tune.

Properly set up I imagine you should get 12-14mpg in the city and maybe 18-20 on the highway. There will always be heroic tales of people getting 30mpg and so on but I'm talking about normal driving.

You must have something fundamentally wrong with the carb. That's a good thing, because it can be fixed!

Also the easiest way to calculate mileage is to fill it, drive 100 miles, and then refill. Don't try to measure consumption (as opposed to refilling), it's too inaccurate.

[jeffahart]

Originally Posted by Awann99

I just put a new crate 350 in from jegs.
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I feel like that explains it. What 350? You need to go back to your engine builder and get your engine set up for the design. Hey, you could throw money good after bad. I would not create a finicky setup that you may be forever dickering with. Establish a base line to work from; get your setup for that build and then upgrade from there. This all should be worked out before the trigger is pulled on the long block purchase!

Another two cents... worth the price paid!

J

[trac209]

Also since you have a new engine be sure you’re not running so rich that you’re flooding the cylinders with fuel. That won’t do your oil or your new bearings any good

[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.

[toolboxchev]

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.

10 mpg seems to be the average. Now if I was going down Cabbage Patch hill, with the wind behind me, a 2 barrel carb, better rear gearing, prayers answered from God, some high quality fuel, a perfect tune, ect...

I WOULD KNOW FOR FACT, I would be getting 30 MPG.

Am i dreaming

[jocko]

Was your truck originally a 6 or 8 and was it originally a manual or auto? You need to determine your actual rear end gearing. If it’s 4.10 or even deeper, well, there you go, you’re probably close to tuned up. If you have a 3.07, then you have an issue. I’m assuming you don’t have a 3.07. If you have a tach and can tell us your tire diameter and what rpm you’re turning at a given highway speed (measured with a GPS, not your speedometer), we can tell you your rear gearing.

Regardless, once you wring that extra 2-3 mpg out of your setup with the best tuning you can muster, you might want to consider an overdrive trans if you’re hoping to drive 60 mile trips 3x per week. That will be your biggest bang for the buck in the fuel economy department. If dead stop acceleration matters absolutely zero to you, you could also keep your trans and look for a higher gear rear, like a 3.07 etc. But, $ dependent, the trans swap is the best all around solution. Once we know your rear gearing, we can be a lot more help on optimizing your mpg.

[davepl]

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.

Not in town, of course, and not normal driving, but trying hard I achieved that regularly with the following setup, and in both directions. It was on a 45 minute drive between Regina and Moose Jaw, totally flat and level. I'd fill up in Regina, drive to Moose Jaw, re-fill up, and do the math.

It was a 1969 C10 with a 307 four speed, 2bbl (maybe monojet).

Putz along at 90km (or 55mph).

You really don't think a well-tuned truck can manage 18mpg on flat ground at moderate speeds? Once you start speeding up and slowing down you're looking at 14, in town 12, and driving hard 10. But highway cruise is pretty efficient.

[Edit: I just looked it up, Regina is 1892 feet of elevation and Moose Jaw is 1893. It literally changes one foot in 50 miles... that's what you call FLAT]

One semi-related thing I noticed was that my Range Rover Supercharged gets about 12-13 in town. But in mountain driving through Rainier, when keeping the speed down to 40, it was getting mid-20s. Aerodynamic brick and changes in speed, I guess.

[Awann99]

Originally Posted by engineer_gregh

Something is off if your gas mileage is that bad. Even my hopped up big block 402 does better than that. It would help if you let everyone know some of the following; HEI or points, ignition timing (initial and full), plug gap and even what plugs your running, rear end ratio, manifold vacuum, and even pictures of your carb and engine. There are some very good mechanics on this site that can help. A 69 2 wheel drive pickup with a new crate engine should do at least 10 MPG, if not better. Just curious, why did you put a 2 barrel carb on a new crate motor?

I used that carb because it was one that was on the trucks original 350. That engine was old and wouldn’t get compression so I had a ship put a late 80,s rebuilt 305 in it. It got 14-16 mpg. That engine threw a rod in 3 months so I’ve now got the 350 from jegs. It’s made for 73-87 models. But they said it would work fine in mine. I will be home this weekend to get you more details on the truck that I don’t know off the top only my head. I’m not a mechanic but I’m trying to learn how to do this stuff. Here are a few pictures of the engine. I honestly am not sure if I even have hoses hooked up right (they have been changed since these pictures were taken)
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[Awann99]

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[demian5]

Anyone asked what size shoe he has and weight of his right foot?

[jocko]

No. We’re assuming he’s a perfect driver like the rest of us

[Problems]

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.

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.

[demian5]

Originally Posted by jocko

No. We’re assuming he’s a perfect driver like the rest of us

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