Terrible gas mileage,

[jagarra]

I have been tracking the MPG on my truck since i have had it, it has finally settled on about 6.2 MPG, OUCH!!!!

I have a 500 CFM Holley on it, recent tune, valve adjustment, runs like a top just horrid mileage. I have the stock 3.54 gearing and a 420 tranny. The entire fuel system was replaced after I bought it

I did try jetting it smaller than what came with carb, mileage went down to 6.1, so I put the stock jets back in, went up to 6.27. I can go smaller as I bought 2 sets, each going down one notch from stock. I do have a big air cleaner with a paper filter so it is getting a lot of air.

the elevation here is 4450

[Ed Snyder]

Originally Posted by jagarra

I have been tracking the MPG on my truck since i have had it, it has finally settled on about 6.2 MPG, OUCH!!!!

I have a 500 CFM Holley on it, recent tune, valve adjustment, runs like a top just horrid mileage. I have the stock 3.54 gearing and a 420 tranny. The entire fuel system was replaced after I bought it

I did try jetting it smaller than what came with carb, mileage went down to 6.1, so I put the stock jets back in, went up to 6.27. I can go smaller as I bought 2 sets, each going down one notch from stock. I do have a big air cleaner with a paper filter so it is getting a lot of air.

the elevation here is 4450

Lots of things can affect mileage. Some things to check are ignition timing, tire pressure, brakes dragging, and front end alignment. I'm sure others here can suggest more to check. Are you 100% sure the rear end is a 3.54? You should be getting closer to 10 MPG if not better. If it sits for a long time, you could be losing some fuel to evaporation. Lots of hilly driving obviously affects mileage too.

[AZKen]

As said, we don't know your usage, loading, terrain and possible Holley carb internals. Even with a 305, that's pretty low MPG. Unknown history from PO. If you 100% positive about ratio, great, but could have 3.92 real easily. You can't do accurate ratio calculation by tire/shaft rotation.
You may be falling into the category of "playing with it too much", "modifying" too much. Original factory motor Synergy gets out of whack. Go back and try a 350 CFM original carb. You could be wasting gas.

[jagarra]

I am pretty sure I have a 3.54 rear end, I found a tag on the 3rd member cover indicating the ratio and that it was a posi rear end. All the hydraulics have been rebuilt, the brakes don't feel that they are dragging when I put the truck in neutral and coast to a stop light.
I do have the tuning book for the 2300 series Holley, I remember reading that if you lean a mixture too much that more fuel will be used to do the same work. I have a Summit warehouse in town so getting jets a just a short ride away, maybe I am going in the wrong direction with jets.
Since tomorrow is gonna be nice, before another storm, I will check the timing and pull a plug to get an idea on how they are burning.
As far as driving, it gets used just around town, going to dump transfer station, errands, and no major driving in the Sierras.
Fuel could be evaporating or my speedo could be off, too, like Ed said. I did notice that the needle is jumping a bit, indicating that cable needs lubrication. I still have the original carburetor and air cleaner, so I could put it back on, but it sure runs better with the Holley, the only thing I had to reroute was the choke cable since it comes in on the right side on the Holley.
Today when I was driving around I smelled gas, found a leak on the fuel line going to carburetor, fixed that so that may have contributed to poor mileage too, but it hasn't been driven much in the last 6 months, that only would leak when the motor was running anyway.

[LEWISMATKIN]

jagarra, what size tires are on the truck? If you have 15" tires, your engine will sound like a screaming dive bomber. That will kill fuel mileage. Also, what type of gas are you using? If you are using 87 octane gas, you are burning 10% corn squeezings and that will affect fuel mileage as well. Driving habits also have a great effect on fuel mileage, too.

[jagarra]

Lewis,

I am running 15" tires, 7.00x15 so the OD is about 30". I run regular, I have been using pure gas, no corn when I can get it, the last 2 tanks have been this fuel.


A bit more information. I have been running this carburetor since May of 2015, problem is that I never dated my mileage figures. I believe I got it running and licensed it around January of that year, so the mileage has been steadily declining over the past few years.

[James]

I would check the float level. It might be too high causing it to be rich. Remove the plug on the side of the carb. With the engine running it should barely run out.

My truck is still running the stock carburetor and last weekend it got 15.1 mpg (been averaging 14.5 mpg). I have a 3.54 rear axle and 16" rims with LTX M/S2 LT235/85R16E tires.

[jagarra]

I am going to check a lot of stuff today. Ran some figures to see how things were changing.

original carb 337.3 miles used 41.4 gal average over that period 9.11 MPG
Best mileage 11.18, worse was 7.7 MPG

Holley 844 miles used 102.2 gals for an average of 8.25 MPG
Best was 11.4 on the first tank with the Holley
worse was 6.1 MPG as of the fill before the one yesterday.

looks like i have a lot of items to check to see what has changed.

One thing that pops out is the truck doesn't like pure gas, will see how is goes on this tank. Does pure gas evaporate faster? Truck sat unused for quite awhile before the bad MPG figures.

[James]

Gas can go bad in just a couple of months. With our truck, the tank is vented at the cap. This allow the tank to breath the outside air. This air has moisture and oxygen in it. The moisture will condense into water and lay in the bottom creating rust (or mixed with the alcohol in the fuel). Then the oxygen will blend with the fuel making it darker and smelling foul. All of with it will make the engine run poorly (poor mileage too). If the fuel cap was like the new truck the entire fuel system would last for several months without problems.

What I would do if the truck is running badly, topped off the tank the drive it until it is low on fuel then topped it off again and the problem will go away (providing nothing else is wrong with the engine).

[jagarra]

Well I think I have solved part of the problem. I seem to go through the plugs on the opening that are used on the carburetor to block off vacuum ports, this is the 2nd time I have had to replace some of them. I found cap that had disintegrated enough to allow air to come into the carb base. Checked plugs and they were a little rich, still brown but not tan. Dwell was right on, also checked vacuum at idle, it was 12.

Went for a drive including a trek up the mountain to Virginia City, down into the valley and to home, filled up again, got 11.19 MPG.