I've done everything I can with ignition and gearing to try and improve the millage in my 360 duster but I still only get about 13mpg. Here's the combo 360+0.030" "340" cam 2.02-1.625 "j"heads
4150-650dp holley 4sp w/2.76 gear 27" tall tires.
Everything I read about Holley carbs are that they are great for performance but poor for millage. I have done the small jets-6.5pv loosen up the acc pump thing but it really hasen't made much diffrence.
I would like to know what carb would be recomended to try and get some better millage out of this thing. I don't have room for a adaptor so it needs to be a square bore"flange" carb. Do the edelbrock's typically give better fuel economy?

1st choice(by far) 600 eddie 2nd choice 600 vac holley but go with either a reworked one, one of the aftermarket clones of it, a complete carb or maybe a Proform main body which gets good reviews maybe aftermarket metering blocks.

Thermoquad best of both worlds for the street.

Very simple ... Performer or SP2P intake with a ThermoQuad carb. NO square-bore flange carb is even in the ball-park.

Well the thing is I put this togather with parts I had stashed away over the years. I'd rather not put to much into this as I am trying to save up for a magnum based 360 or 408. I hope to do that in aa couple of years so I wanted to just try a carb prefeably used that had a history of reasonnably good millage.

I'd find a used 600 eddie put a kit in it & I think you'd be happy with that & not out too much money.

I disagree that a square bore carb can't do as well as a spread.

I've tuned a 625 road demon on my 440 winni. The 440 is original, well worn, and burns some oil.
I get 8 mpg (us gallon) and 10mpg Cdn gallon towing my car over some of the highest mountain passes in the world.

Car's 3800pounds, plus the 18 ft trailer, the tools, and of course the winni with the gear and the family in it, which I'm scared to even guess at the weight on. It get's better milleage than any gas tow rig I've ever owned. TQ or otherwise.

But back to the build....

I agree that the dp is going to hurt mileage, BUT it should only be a factor when your romping on it. In the primary's it shouldn't matter.

I notice no cam specs. A 340 was a performance motor, so it wants to perform. Not economize.

IMHO I think the 13 mpusg you are getting isn't too far off of anything you can hope for.

If there is something wrong with the holley then you may see some improvement.

A smaller carb (not smaller jets) may improve velocity, and fuel atomization, so if you can find something smaller (regardless of brand) you may see an improvement there also.

Put a vacuum gauge on it, and watch it while you drive. Your driving habits may adjust a bit and help you too.

It's very difficult to build an old motor to perform, and then get mileage out of it.

Grab an Eddy, They are very tunable on the slow speed circuits. Sometimes advancing your cam timing will net you a couple miles per gallon also.

The reason Double Pumpers are not MPG friendly isn't the additional accel pump, because if you are driving for mileage, it should never operate.

Double pumpers have smaller power valve channel restrictors so if you tune for power, cruise will be rich. If you tune for economy, power will be lean.

Opening up the PVCR's will change it but I wouldn't go fooling with it without a good way to check air fuel ratio.

back in the day i found that the factory avs would mileage better than a holley counterpart. the 3 step metering rods may have been a big factor. the edelbrock 650 avs is nearly identical in size to a 340 avs.

For mileage I like the TQ best followed by the Carter AVS.

Sheldon

It is not a carb
but if what you desire is better fuel economy
and also the ability to get best power
a TBI or MPSFI setup
could do up to 30% better.

If you are on a limited budget
factory EFI units pulled
from a junkyard
(Chrysler, GM or Ford units all could work)
would give acceptable results

I would also urge not neglecting
the easy Highway MPG gains
that can come from aerodynamics
or easier rolling resistance tires

It is hard to improve engine efficiency
but this article does show
that improvements have been made
with operating near the limit of lean air/fuel
including much research by Chrysler:

http://www.designnews.com/article/9710-Thirty_Year_Quest_for_Lean_Burn.php

it is doable
to get your Duster over 20 MPG
without breaking the bank

The reason for no cam specs is I don't know what it is for sure. I pulled it from a 318 I had and mesured the lobe lift and the best I could do is somewere in the .45" range with 1.5 rockers so I'm guessing a 340 size cam. I know my build is just wack but its what I had.
Above cam
.03 over 360
Sp 405 pistons (mybe 8.8-1)
587 heads stage II hughes port w/11/32 valves.
wieand dualplane intake#7507
blackjack headers--2 1/2" exh
8 3/4 w 2.76 gear Have 3.23 and 3.91
standard 4 sp working on a O/D 4sp
I drive to work regularly and its about 26 miles each way highway but sometimes heavy traffic. usually cruise between 60-70mph 2100-2500rpm.
It just seems that no matter what I do my millage stays withing 1mpg.So I'm thinking that the "cruise" fuel mixture on these holleys is just really rich?

the double pumpers are biased toward larger main jets. some of them, or maybe all (?) have smaller power valve resriction orifices than some of the vacuum carbs. this means you need larger jets for wide open throttle, which effects cruise. if you have a square bore manifold try a carter or edelbrock with the small 1 3/16" primarys.

Hi Mod5v:

As CKJ440 hinted - have you considered purchasing a wideband O2 meter and watching what's going on with your AFR as you drive? It can be a real eye opener.

Every performance oriented Holley I've ever seen was rich at light cruise. It can be tuned out, but it's very hard to do when you can't measure what's happening.

In the summer months, I run an older 950 HP series, and when it gets cold, I switch to an ancient 3310-1 (b/c it has a choke). After some work, both cruise in the 14 - 15:0 AFR neighborhood.

I can tell you what typically has to be done if you're interested. I know you were asking about other carbs, I just thought I'd throw another option your way.

Jim

Put on a 600 Holley vac secondary.It will work fine.I ran a 318 with 340 cam and same gearing as your car and got 22mpg.Mine was jetted at 64pri,left the stock metering plate in the back.

Get a LM-1 meter. Properly setup and tuned a thermoquad would be real tough to beat. Easier to find and tune would be the Edelbrock.
I had a set of 750 eddies that I modified and tuned with my LM-1 on my tunnelrammed 528 hemi. It got an honest 10mpg driving it nice. The same motor and intake with a set of tuned 850 thermoquads netted me 13.18mpg, (driving it real nice). Dave

As some of you may have deduced I am on a strict budget and I don't think a wideband 02 is in it. I also recently moved from Michigan to fortworth TX and was not able to bring any of my gerage goodies with me. I don't have the room to put the tools nor do I have anywhere to park a beater car while I work on the duster so any work I do has to be done on weekends and the car has to be ready to get me to work on Monday.I can pick up a used eddy from e-bay or a swapmeet for 100.00 or so and just wondered if it would be worth it. If all it gets me is a mile or 2 per gallon it's probably not worth it as it would take a couple of years to pay back and I hope to have a place to keep the car by then so I don't have to drive it every day. When that happens I hope to build a more performance oriented engine and have some fun with the car.

If you get an Ebay carb, any carb, you had best budget for a rebuild kit too. This is yet another area the AFB style cab wins out over the Holley, extremely esy to rebuild and Edelbrock has the owner's manual online, so you can verify settings, jets and rods.

If you were closer to me, I live in San Angelo, I could help, but I am sure there is a Mopaters member around you.