Horrendous gas mileage issue.

[Geizhals]

So I have a 1985 gmc k2500 with 4x4, 350 and an edelbrock 1406 on it currently. I had a 1906 till they kept failing. Anyways this truck gets about 5 mpg, which I know is gonna be common, but I feel its still excessive.

Any thoughts how to fix it?

Note 1. I got this truck when my dad passed, his uncle bought it new and had the service center put different axles under it. It's slower than molasses in winter. So I think it's geared for hauling, which I'm wondering is why its so bad on gas mileage.
Note 2. I've resealed from the heads up, headers, new plugs, wires, distributor cap and coil. New timing chain, old one i could move about an inch in and out from the center.
Note 3. I bought some parts off a friend im gonna put on it and see if it helps. Edelbrock performer intake, edelbrock high rise valve covers. Msd ignition box(6al without the rev limiter), msd distributor without vacuum advance, and streetfire wires.


Any thoughts are well appreciated.

 

[potent rodent]

first off u need a dist that has vacume advance there is some of your gas milage problems , the edelbrock1406 is a good carb but not very good for gas milage , the qjet is your best bet for milage if that what u are wanting, i have edelbrock 1904 q jets on 2 of m trucks and getting 15 from one and 12 from the other, the 12 mpg has cam and headers , what gears are u running ?

 

[Frankenchevy]

You definitely have an issue. Check for vacuum leaks, check timing, valve adjustment, tune the carb with a vacuum gauge, etc.

When you did the timing chain are you sure you aligned the cam to the crank properly?

Also, gears for hauling (numerically higher) should make it accelerate quicker.

 

[mcarlo86]

If the pickup was re-geared at some point, is the speedometer accurate? How are you figuring your gas mileage? I would agree with @potent rodent that the Qjet is the best carb for economy while still also a good performance carb. How hard do you drive it? If you foot is in the carb all the time, you won't get very good mileage either. The MSD ignition is probably overkill and I agree you should have vacuum advance. A stock GM HEI in good condition will perform just fine for a 4X4 3/4 ton in most applications unless you are trying to rev it to the moon. Another thing to check is for fuel leaks. My dad had a '69 C10 that he sold back in the day because it was getting awful gas mileage and the guy who bought it from him found the fuel pump was leaking, but only when it was running.

 

[Ricko1966]

Make sure have no vacuum leaks, make sure your plug wires are on correctly, those things should be evident if you are settings carb with a vacuum gauge.Okay make sure you are timed correctly and your vacuum advance is hooked up and working and and centrifugal weights are moving.On the cheap go to dollar tree buy a marinade injector it looks like a oversize doctor syringe hook it to the line going to the distributor watch your timing marks with a light rev the engine marks should flutctuate with engine rpm.Let the engine idle,pull the syringe marks should move and stay in 1 spot until you push the syringe back in.None of the parts you bought from your friend are going to fix your mileage issue,and some will make it worse, don't change anything until you get the milage issue figured out, other wise there will be new pieces of the puzzle to figure out.Seriously don't change anything until you figure out your mileage issue, so you think changing valve cover can't hurt, unless of course you put the plug wires back on wrong and don't notice it or accidentally knock a vacuum line loose somewhere else, etc. Seen it too many times someone trying to fix 1 problem and unknowingly creating 3 more.

 

[Matt69olds]

If the truck really only gets 5 mpg, I would think the cause should be pretty easy to find!

As stated before, make sure the speedo is correct. There is no way to a accurately calculate mpg if you don’t know the distance driven.

Make sure the tune is good. Is the timing correct? Is the mechanical and vacuum advance functional? How is the carb tune? Choke opening all the way?

Fuel leak?

While you will never get good mileage in a old square body, unless your dragging a heavy trailer with your foot to the floor all the time, it should be pretty easy to improve on 5mpg.