Very bad gas mileage

[jfrank133]

I have a 1978 GMC pickup 1500 series, last Jan. of o4 I had a new motor (305) put in. An Eledbrock performer series manifold sits on top with a Eledbrock 600 with electric choke. For the past 3 months my gas mileage has been getting worse I have changed the plugs put new plug wire on and thats should have fixed it but it didnt, can you give me any ideas what I can try next, I am chasnging the jets this weekend so any help would be of great need.

[60-66 Nut]

Sounds like it could be a carburator problem. I'm not real familiar with the Edelbrock carbs, but I remember the older Holley and Motorcraft carbs had a vacuum oprerated power valve. When they went bad, the vehicle might still run OK, but the mileage went to crap. I don't know if Edelbrock has the same setup or not, but I'm sure a fresh overhaul wouldn't hurt anyway. Good Luck.

60-66 Nut

[gldevall]

What is bad gas mileage? Where were you and where are you now? 17 MPG before and now 14 MPG

[75stepside]

i've seen a drop in gas mileage in the last month or so in my daily driver... from 25mpg to 21 - in my 2000 2.2L s10... a friend of mine runs a F#&d F-150 w/ 4.8L and was getting 14, now getting around 11mpg... ouch.

[N2TRUX]

In the northern states you can expect a drop in mileage during the winter due to the cold weather formula fuel the gas companies sell. Beyond that you need to determine where the problem is before you start throwing parts at it.

If you were getting good mileage before, and it's dropped now it won't be your jets. These are the same jets that produced good mileage so it's not them.

Is the truck running smooth? Is the power about the same? Has the exhaust color changed? Let us help you diagnoise your problem before you start changing parts....

[chevydrummer76]

Originally Posted by gldevall

What is bad gas mileage? Where were you and where are you now? 17 MPG before and now 14 MPG

Hey what are you running under the hood...I saw you have a 14.5 time slip. I was just curious what all you have done to your taruck.

[TrainD5115]

I'm having similar problems with my 79 since in got cold. One of my problems was someone put in the wrong thermostat or it was one that was only letting the truck heat up to the first notch on the gauge. (Even in rush hour traffic in August!) I put in a factory 192 degree one and ran some Amsoil fuel system cleaner in the tank on Friday. We'll see what happens. I have a bigger problem and that is the butterflies are off their seat or so another member tells me. The short of it is the idle is way up and I can't get it down because backing out the screw does't effect idle anymore. Anyway, I understand your problem and wish you luck.

[nucklhead13]

This sounds just like the hassle I've had with my 72 Chevy 3/4 ton step side. After I got the engine rebuilt it seemed to burn too much gas.

That hit me hard because when I still had a 71 1/2 ton short bed it allways seemed to do fine. I'd still have it now accept while I was in a six month coma my wife had a head on colision with it. Good thing old pickups are so tough or she wouldn't have survived.

That worn out carb sounds right. It's just too bade I've sold that step side so I could get a 78 1/2 ton 4 wheeler to get accross our cattle guard into town.

I can see how carbs can get messed up through my six years hauling gas from several refineries, and how my 80 sportster allways runs better when the gas making the carb is still clean.

Now that winters allmost over I'm going to have to check milage on that 4 wheeler. I just hope that carb is fine so I can use it to get motorcycle parts any time.

[jimfulco]

Might also check the distributor advance mechanism.

[JonnyTechMech]

It would change out the in-line fuel filter, found on the inside frame, just behind the cab door location, on the passenger side, IF it had a TBI originally. Might just check for one anyhow.

JonnyTM