Gas mileage/carb question??

[FatGuy-LttlCoat]

I've got an '86 chevy k10 stepside pickup, with a 350 with the quadrajet carb.
I'm only getting between 8.5 - 10 mpg. I know these trucks arent known for great gas mileage, but i would think it'd be higher than that. What do you guys think? (i haven't got any idea on what gearing i have, in case that question comes up.)

Everyone i know is saying that i should just buy a new carb. What would you suggest a good carb would be for my truck?

I'd appreciate any help you can give me.

Matt

[69BBsuburban]

You need a good tune-up......maybe a carb rebuild too.
Father-in-law has an '85 LB, 2wd, 350, Quadrajet, 3.73 and gets @ 14-17 mpg......

[FatGuy-LttlCoat]

I had a complete tune up just after i bought it last may, and my gas mileage has been like this ever since, so i dont think thats whats doing it. With the carb rebuild, what are the opinions of the members here: rebuild vs new carb?

thanks for the reply

[78chev]

I have the same setup in my 78. The motor was recently factory rebuilt and runs perfectly but last week I got an oil change and went to a different gas station than I normally go to and my mileage got drastically worse. I checked the oil before the oil change and it was low. (probably a result of break in) Do you think the extra/different oil could make that big of a difference?

[Pyrotechnic]

Make sure your timing is set correctly. Also, old distributors tend to wear out and produce a funky advance curve that messes up your performance and mileage. Also, if the carb is set too rich, this will waste gas.

You do have a 4x4 truck though. I would expect somewhere between 9-12 MPG, depending on how you drive.

[magnethead]

my friend has a 84 C10 with mudding tires on it and gets 12 mpg at best with a 305.

[crm318]

well if you decide to go with a new carb, you should stick with the quadrajet. it is the best carb for milage and performance. you cant beat the efficiency of the triple venturi design of the quadrajet.

[gchemist]

The best from my old 305 was 18 MPG at 70 MPH!! It takes some work though!! I've made a lot of modifications to my truck. Everything has to work together.

4X4 have extra weight. FI is probably the best way to go. Aftermarket TBI units are easy to bolt on. Consider the swap if you plan to keep the truck for a long time.
Here are some helpful links.
http://www.persh.org/pickup/tbi-1.htm
http://users.california.com/~eagle/howell.html
http://www.centuryperformance.com/vacuum.asp

[N2TRUX]

Lets start with a bit more information. You said it's a "'86 chevy k10 stepside pickup, with a 350 with the quadrajet carb" but you don't mention a few other important elements. What transmission, gears, and tires does it have? If you happen to have high gears like 3.08's and a tire that's taller than stock your mileage will be affected.

Give us more details before you blame the carb...

[Roper]

Originally Posted by FatGuy-LttlCoat

I had a complete tune up just after i bought it last may, and my gas mileage has been like this ever since, so i dont think thats whats doing it. With the carb rebuild, what are the opinions of the members here: rebuild vs new carb?

thanks for the reply

When I first started having carb troubles I asked my mechanic about replacing it with a new edelbrock. He said dont waste your money/ dont throw out that good carb. Let me rebuild it.
So I had my quadrajet professionally rebuilt. It still didnt run right. To rich/no power/floads. Took it back to him a few more times and dumped more money into it and it still didnt work right. Apparently old carbs can warp, get pin holes, etc in certain parts like the float.
Anyway I bought a new edelbrock 650cfm off road, thunder series carb and a performer intake. This stuff came with great instructions. Even came with a how to CD! Easy to install, easy to work at!
I also got rid of all the polution gear that was on the truck except for the pcb.
Truck works great. Way better than stock.Power/mileage is way better than what I had before. Course what I had before wasnt working right.
Also the parts guy told me that I cant buy a new quadrajet. Only a rebuilt one that might not be any better than my old rebuilt carb.
I cant tell you my mileage because my speedometer is off.

[piecesparts]

I take it, that your carb is the factory MESS and the computer in your truck is still involved in your system. The quadrajet is a good carb, but after a number of years is definitely due for a rebuild. If it is one that has computer controls, I never got along with them very well. The timing is very important, but if you still run the computer, that is hurting you. I took my 84 and my Son's 86 to HEI distributors and got better performance and tuning capabilities from that change. I eventually ditched the computers, air pumps, and anything else that would hinder performance in my engines---In Kansas the smopggies are not alive and well.

The discussion on tire size and gearing is important here. My Son's truck had 3.08 gears in the rear and with the 700R4 tranny that we installed, his gas m ileage was around 12 MPG. We changed to 3.73 gears and GAINED 2 MPG just because he was running outside the power curve of the motor. I get 15 MPG with my 84 stroker motor. We are NOT using the factory carbs anymore.

[Roper]

Also my truck gets better mileage now with 4.10 gears and 33" tires than it ever did with 3.08 gears and 31" tires with my 3 speed automatic.

[FatGuy-LttlCoat]

thanks for all the advice, i really appreciate it.

I knew i wouldn't have enough info in the first post, i always seem to leave something out. Ok, I think i've got a 700r4 trans. I've got 33" tires right now, but was running 31's during the winter, and i haven't got the foggiest idea what my gearing is.(I'm not exactly mechanically inclined, just the basics pretty much)

I guess i'll take the truck down to my mechanic and see what he thinks he can do with the carb. Thanks again for the help.


matt

[Ivan D.]

Matt, I know the feeling very well - my 2wd C10 with a relatively fresh engine (with ported heads too) never gets more than 10mpg, no matter what I do or how I drive. 350 engine, 350 tranny, 3.08 rear gears, 33" tires, and a double-pumper Holley with no vacuum leaks. I have done pretty much everything possible to that thing - jets are so small you can't even buy them aftermarket, and the secondaries delayed so much that to have them open when passing someone I should have jumped on the gas yesterday at the same time. I have tried three different primary power valves, and the only thing that happens is loss of low-end power. I have had the base timing anywhere between 8 and 12*BTDC, and the distributor hooked up to both ported and manifold vacuum - mileage don't care at all, however running at 10* base and manifold advance seems to remove the idle-1000rpms hesitation and improve low-end power. There's just no winning over that truck, only thing I can do is floor it more often so at least I get some fun for all the gas I put into it.

[crm318]

Ivan D, i guarantee that if you were to switch to a quadrajet you would get better milage and performance.

[crm318]

the primaries on a holly are still larger than those on a qjet an arent as efficient. i see alot of guys ditching the factory carb on their factory motors and putting on an edelbrock or holly to fix milage or performance problems. a simple rebuild will do the trick. these carbs came from the factory for a reason from 305s to 365 hp 454s. when the secondaries kick in, they will only suck in what the engine pulls out of them. i have a 350/th350/3.08 in my 72 cheyenne lwb and i get far better milage, roast the tires until i let off the gas, and pull till 5600 rpm. i know these are bold statements, but its just something to ponder.....

[Ivan D.]

I got #60 jets in my primaries, can't even find these unless order directly from Holley, and even the Holley kits start at #64. Secondaries get #62 jets, and are delayed all the way to 3/4 throttle, so unless I really floor it they don't even crack. Really, the secondary opening motion is a pretty much 1:1 copy of my Q-jet, somehow i wasn't too happy with the secondaries opening at 1/4 throttle. My power valves are 8.5 primary (half the idle vacuum, which is 17"Hg) and 6.5 secondary, may go down to 6.5 primary and 4.5 secondary tho, test driving will show which is better combo. It's definitely not yer typical double pumper, and I'm firmly determined to make it hit the 15mpg mark

By the way, that may be a real dumb question, but if the engine runs lean all the time will it still be a gas hog? I never see black smoke coming outta the exhaust, so I can't be running too rich, yet again the fumes have a pretty sweet and heavy smell to them...

[piecesparts]

Originally Posted by Ivan D.

I got #60 jets in my primaries, can't even find these unless order directly from Holley, and even the Holley kits start at #64. Secondaries get #62 jets, and are delayed all the way to 3/4 throttle, so unless I really floor it they don't even crack. Really, the secondary opening motion is a pretty much 1:1 copy of my Q-jet, somehow i wasn't too happy with the secondaries opening at 1/4 throttle. My power valves are 8.5 primary (half the idle vacuum, which is 17"Hg) and 6.5 secondary, may go down to 6.5 primary and 4.5 secondary tho, test driving will show which is better combo. It's definitely not yer typical double pumper, and I'm firmly determined to make it hit the 15mpg mark

By the way, that may be a real dumb question, but if the engine runs lean all the time will it still be a gas hog? I never see black smoke coming outta the exhaust, so I can't be running too rich, yet again the fumes have a pretty sweet and heavy smell to them...

Ivan D, what are you running for a cam in that recently "refreshed" engine? If you are running a rather lumpy cam, then you are trying to make up for it with adjustments to your idle circuit on yur carb, to keep it at an even idle when stopped. This is especially hard to do when the truck is in gear, with an automatic tranny. This process takes the idle mixture out of the idle circuit and puts it in the "transition" area towards the primaries coming open, because you are trying to make up for the idle by making the engine idle faster. I solved some of that when I put in a different stall convertor in my tranny and got the truck to sit at a stop light without trying to pull the truck through the light on a high idle. Along with a bigger cam, comes that sweet heavy smell of gas fumes that you are talking about. I managed to get my 383 stroker to a 15 MPG with the lopey cam and an auto tranny.

Don't take the engine too lean or it will hurt your internal parts.

[Walts_restoration]

Haha, anyone think of a ls7+t56 swap? i did. These trucks are reasons why oil companys can charge so much!

I dont really care about gas mileage, i just try to light foot it, and i hope my camaro can get third gear (2150rpm on the way home) opposed to second (3000rpm on the ride home).

Dang automatic trannys, first one ive ever had that has dropped a gear, and it had to be on my daily driver.

[piecesparts]

Hell the LS-7 should get decent mileage, don't you think?

[Ivan D.]

Originally Posted by piecesparts

Ivan D, what are you running for a cam in that recently "refreshed" engine? If you are running a rather lumpy cam, then you are trying to make up for it with adjustments to your idle circuit on yur carb, to keep it at an even idle when stopped. This is especially hard to do when the truck is in gear, with an automatic tranny. This process takes the idle mixture out of the idle circuit and puts it in the "transition" area towards the primaries coming open, because you are trying to make up for the idle by making the engine idle faster. I solved some of that when I put in a different stall convertor in my tranny and got the truck to sit at a stop light without trying to pull the truck through the light on a high idle. Along with a bigger cam, comes that sweet heavy smell of gas fumes that you are talking about. I managed to get my 383 stroker to a 15 MPG with the lopey cam and an auto tranny.

Don't take the engine too lean or it will hurt your internal parts.

Howdy, my cam ain't nothing special, just a Melling towing/RV cam, MTC1:

- duration SAE - 278/288
- duration 0.050 - 204/214
- lift cam - .280/.295
- lift valve - .420/.443
- lobe separation - 107/117

As you see, definitely not a lumpy cam by any means. With the vacuum advance hooked up to ported vacuum I can go as low as 800rpms free idle, 500 in gear at the stop light. Now with the advance hooked up to manifold vacuum it idles beautifully at 600rpms free and 400rpms in gear at the light. idle screws are backed up 1 3/4 turns out from closed position, so nothing crazy rich there either...

Edit: I also tested my primary floats for pinholes - two hours underwater in an upright position, when weight was removed float popped right up, so I believe it's good. Still need to check the secondary float tho...