fuel usage

[vzfox]

I am in the process of rebuilding this truck I found from someone who had it sitting for quite a while. I have it running pretty good and drives fairly well also. My only real problem right now is fuel usage. I have a 350 tranny and motor. I put an adapter plate on the intake and went from 4bbl to 2bbl carb. It is a brand new carb and doesnt leak fuel or have a vacuum leak. It doesnt smoke, idle rough or run rough....but it sucks the gas down quicker than I can put it in. I adjusted the carb according to the book and checked the timing...all seems to be good.

 

Right now, it is seriously getting approx. 2 miles per gallon.....REALLY.

 

What do you think could be the issue with it sucking down the gasoline so quickly? There is no fuel odor either....under the hood or outside the truck. Any and all help to save me fuel is greatly appriciated.

[clm7214]

What carb is it? Brand...CFM? If your numbers are right, 2MPG means somethings very wrong. I had a 383 stroker with a tuned 650cfm Edelbrock in my 79 Jimmy and i got 8mpg. Have you tried the hand over the exhaust tip thing yet. Put your hand over it and then smell for excess fuel on your hand. Do you have cats on it?

[vzfox]

I am sure the brand of carb is cheap. I bought it from O'rielly Auto Parts, which is similar to Auto Zone or Advance Auto Parts. Brand new it cost me $105. After reading your reply here, I went out to my garage and did the exhaust thing. It runs nice and smooth and doesnt have a fuel odor from the exhaust. I did try adjusting the idle adjustment screws again just to be sure I had it at the best idle, and it seems to be set fairly decent. I looked all around the carb as it was running and did not smell anything nor did I see any leaks. I checked the vacuum lines again, and they are good. I replaced most of them when I bought the truck. I have also replaced all the spark plugs, wires, dist cap and rotor.

 

Like I said, it runs and drives good......well, as good as it can with Interco mudders on it. The tires alone make for a "hassleing" ride. I agree with you though, something is definately not right with the fuel usage. I have gone through over a quarter tank of gas in only 10 miles....UGHHHHHH!!

[vzfox]

Oh yea.....no catalytic converters. Only headers with glass packs.

[clm7214]

Its hard to believe it runs good sucking that much fuel down. I dont see how that 350in is burning it all. When i would play with diffrent jets and rods i could instantly tell if i went too rich. Sluggish at WOT, delayed rev when punching it in neutral. Your boggers cant be the reason for the mpg. I ran Baja Claws on my jimmy and still had 8mpg. Since you said you got it at o'rilley im bettin its a remanufactured quadrajet. 2 big holes and two smaller holes on the bottom of your carb, for the primaries and secondaries. Your idle screws just set A/F mixture at idle, not too much to do with MPG once you get away from idle. To lean it out you have to change the rods, or the primary and secondary jets in the bottom of the carb. Did you have it running before the new carb? How did it run then?

[vzfox]

It is not a quadrajet....in my original post I stated that I had replaced that with a 2bbl carb. It had a 2bbl carb on it when I bought it and decided to go back with the same. It still ran OK, but not as good as it does now with the new carb. It doesnt have canisters nor egr valve. The new carb only has 3 vacuum line holes and none at the top of the carb, all small ones at the bottom (besides the brake booster line).

[clm7214]

AH..Sorry, i thought you said you went from 2barrel to 4. My mistake. If its a 2barrel and your getting 2mpg, thats rediculous. 1mpg per barrel, haha. Ill keep thinking, but the 2barrel is not my cup of tea, wouldve gone with a reman. quadrajet. Dont lose hope.

[vzfox]

thanx bud.....I do appriciate your assistance. The truck had been sitting for quite a while before I got it, someone else's project that never got started, now its my project and I am almost complete. When I first got it, I did run a motor flush through it to clean it up. I have done all I know to do to clean this truck up, and like I said, it does run decently and dont smoke. I am going to get it out on the road some today, maybe the highway, and see if the mileage gets any better. Maybe it was just sitting for so long that it needs to be ran for a bit.

 

Hope I dont get out and run smooth out of gas....now that would totally suck, haha. wish me luck and I'll let you know what I find out.

[2500HD4X]

This might be a stupid suggestion/question....But how are you measuring your fuel usage? The gauge could be way off....Are you refilling the tank to the top after every trip? Or going by what you see on a 27-28 year old fuel gauge? It might not be as bad as you think....Just a thought....Good luck with it.

[CapitalTruck]

I agree with the last post. You should measure your mileage in the old school way. In fact, make a measured road course with a vehicle with a known good odometer and then run the same one with your truck, just to make sure the odo. isn't playing with you. If that turns out to be okay, calculate mileage the old school way and see what you're getting. I had a 1985 C10 with the 5.0 liter, got about 12 mpg. However, you have to remember, the tanks on those trucks are 16 gallon or so. 10-12 miles per each of those 16 gallons doesn't give you a hell of a lot of cruising range. There were times when I drove that thing that I thought I could actually see the gauge going down. Compared to that truck, my '04 4.8 Liter is an economy car. Take the real mileage and let us know whats going on.

[vzfox]

Yea, you guys are right....I have been relying on the factory gauge, but I think it is pretty close to correct. When the gauge said almost empty, I put 15 gallons of gas in it and it went up to full. In the other tank (saddle bag tanks), when it was empty I put 5 gallons in it and the gauge read 1/4 tank. As for the odometer, well the speedo dont work, I cant get the cable in the back of it to stay in. I was going to take it out on the highway today, but never got around to it. Just stayed around town. Over the next couple days, I am going to have my truck in the tranny shop to have it overhauled.

 

A couple other things I wanted to ask you guys....first, my title says it is a half ton but the weight of the truck is 4600 lbs. Sounds kinda heavy to be just a half ton, and yes the vin does match. The other thing is the accelerator cable....when I went to purchase one at any parts place, including the dealer, they gave me a cable that was 8 inches too long. I found a cable in a wrecking yard that came off a '74 2500 that is almost the correct length. The only problem this one is giving me is when the truck is running, the intial push of the gas pedal is sticky. If you try to push the pedal while at a stop sign or such to go, it is hard to press and when you apply enough pressure to move it, it moves way too far and the truck takes off. It moves freely when the truck isnt running. I have tried greasing the cable, spraying lube into the cable housing and adjusting the throttle spring to where it is just tight/loose enough to bring the throttle back to idle. Do you guys know of a place that can make an accelorator cable to fit, instead of buying one that is suppose to fit the year and engine size?

[clm7214]

Its a 1/2 ton. Remember, in 79' everything was made of steel and iron. My Jimmy weighs even more than your truck does. These trucks can haul alot more than they say too. As far as the throttle cable, mine did the same thing. Got a new one from LMC and put a spring from the junk pile on it.

[Ape0r]

Shoot, my '04 halfy has a 'curb' weight around 4600 lb. The Ex/crew cabs are even heavier.

[Daubs]

If it were me, I would ditch the 2 barrel and put a quadrajet 4 bbl on. The quadrajet is very good on mileage and will only open the secondaries when needed. 2 bbl will be blasting gas in there all the time to make up for lack of secondaries. I've heard this problem before with 2 bbl's.

 

Remember, that truck was probably designed for the 4 bbl carb. You arn't going to get that good of MPG...DAd had a 76 K-20HD 4x4 with 350 and 4 speed manual tranny. And he got...are you ready...a whopping 6 MPG! City or highway. But that truck has massive balls!

 

Ditch the 2 bbl, put on a good quadrajet and let her run and see what happens.

[I-H8-4Ds]

I have to agree with the last post. In my humble experience, I have found a 2 bbl is too much of a compromise between high and low speed, and doesn't do either very well, especially the Rochester 2 bbl. The quadrajet has small, triple venturi primaries which are great for economy and low speed operation. The secondary mixture is easily adjusted by the amount of spring tension on the air valve, which raises and lowers the fuel metering rods. I have always gotten better mpg with the quad (as long as I kept my foot out of the secondaries).

 

What do your spark plugs look like? If they are black or dark brown, you are running too rich and need to lean it out quite a bit.