TOPIC: 76 Chev 1 ton 350 cid- 88% mileage increase

This is a story about a 1976 Chevy, 350 engine, automatic transmission 1 Ton Pickup Truck. It has a 1 ton rear end, i’m not sure what gear ratio is, it’s geared for towing. Rpm is high on the freeway.

For many years it was getting 8.5 miles per gallon with a quadrajet carb. It seems that the Quadrajet carb can’t be grooved, not enough room. With an Edelbrock 1406 and the Groove it got 9 mpg, i knew something was wrong. With help from the Edelbrock tech support it turns out that the fuel pressure is to high with standard fuel pump. With a gauge it tested at 7 psi. Edelbrock says it needs to be less, Between 1 & 6 psi.
With an adjustable pressure regulator installed and set to 5 psi the mileage per gallon increased to 13, WOW!
Even with the Groove the excess fuel just dumped through the system like pouring water through an open window. I bet there are a lot of cars out there dumping excess fuel through their carbs. because of excess fuel pressure!
After Adjusting the jets and metering rods the miles per gallon went to 15, a total increase of 88%.
The mileage test was done at regular surface street speeds. 35 mph stop & go.
From the Edelbrock Carb. owners manual available on line Model #1406 ROD/JET REFERENCE CHART REF# 7 was tried, too lean in cruise mode, ran rough at beginning of throttle. REF# 8 same thing. REF# 5 was tried, it was better but still a little lean at initial throttle.
REF# 21 did the trick,
.095 main jet (down from .098 stock)
.070 x .047 metering rods ( .075 x .047 stock)
Vacuum leaks are critical to fix, i probably have a leaky intake manifold, there is oil leaking around the base. After new gaskets and sealer (a little bit of a job, but not to hard with the help a shop manual) there should be a couple of extra miles per gallon possible.
Wider gapping of plugs still to come, and highway mpg!

Happy Grooving!

This is a GREAT write-up, Chris! I don't know how I missed it when you put it up, but there you go... I guess I AM human, after all!

Score +1 Karma!

Now, how about those highway reports?

Ron

Hi, since I do have the same engine and carb (not installed yet), I would like to know if you tried some other settings or if number 21 is THE one that makes it right for you. Please keep us informed as many othe guys may have that engine-card combination. Thanks. Lapprentis

I 'had' the same Edelbrock carb on the chevy 454 in the RV.

Engine ran great, exhaust emissions were very clean. I already had the metering rods at max and was working on getting the pump pressure down.

I say 'had' because it died not to long ago due to unrelated problems.

Alternator belt took out the power steering belt and ripped a big hole in the lower radiator hose... it wasn't pretty.

I'm pretty sure the combination that Chris came up with will work for any engine using that carb. Getting the fuel pressure down is an important part of the puzzle.

Karl Fortner
Tacoma, Washington

Hi Karl, thanks for replying. Ron already suggested me to go for the bigger mettering rod in order, I guess, to have lee fuel passing through but the 1406 already come stock with the largest so the is no other alternative I am aware of then get smaller jets and uses appropriate rod like Chris did. Here is what I plan to do, pleas your advices are welcome: I will get that combination on the carb as well as a fuel pressure regulator (adjustable) and enlarge the gapping of the titanium plugs I just bought. Since there do not seem to be any PVC or such on the old Chevy 350 (correct me if I am wrong), I will not have to mess with that. Winter is coming in overhere (deep into the worst part of the rust belt), wmy project will be postpone until next spring....Keep in touch Karl and much Thanks.
PS: I plan-suggest to regroup all the Edelbrock related emails on this forum within one common post so that anyone can easily find information.

Hello to all, Chris here. With the main goal to achieve better gas mileage i thought reducing the amount of gas going through the carb and still having the engine perform properly is the goal. The Edelbrock 1406 documentation says the stock jetting is a little on the lean side, already set up for fuel economy, I thought i would push it as far as i could without causing harm to the engine. Which is hard to know for shure, so one has to be careful.

The chart in the owners manual uses a generalized sweet spot graph with pre determined main jet & metering rod combinations to make things easer and safer to the engine.

By reducing the main jet size down one the engine ran poorly at slight throttle ie cruse mode, not enough gas. the engine ran great at idle once it is set properly, keeping in mind the idle is a different source of fuel flow. The engine would stall out with just a slight amount of throttle. slight throttle gas flow comes from the top fatter part of the metering rods in the main jets, as the rods are pulled up by the gas peddle linkage the skinnier part of the metering rods let more and more gas through the main jets, this is the power mode part of the page 22 chart.

After trying a few combinations( at least 5) mostly buying different metering rods and keeping the one down(leaner).095 from stock .098 main jet, combination #21 worked best. The engine is just a tad lean at slight throttle(cruise mode) but i am living with that since the alternative is going back to the stock main jet size .098

Choice #21 on page 22 of the carb owners manual makes the cruse mode(slight throttle) more rich and the power mode(more throttle) more lean. Since i wasn't having a problem with more throttle(power mode) i thought leaning it out would make for better overall gas mileage.
I also installed one step leaner secondary main jets to have even more fuel economy, secondary main jets flow fuel for a lot of peddle to full peddle
I probably have a little less overall power at moderate to full peddle but that still works for my needs and best gas mileage.

I hope this information is help full. It was trial and error, learning how the shape
(profile) of the metering rods moving up and down through the main jets supplies the engine gas at different levels of air intake ie. throttle.

so many variables.

Happy Grooving to all and to all a good night.
Chris

Ps. feel free to ask any questions.

i have another post on a 1999 dodge caravan also

Hi Chris,

I'm thinking that if one can get better control of the fuel pressure to the carburetor
there would be less need to make adjustments on the carburetor itself allowing it to operate under supply and demand as it was designed.

I know the mechanical fuel pump on the 454 operated at 7psi as well, I was able to get it down to 5psi without changing jets (which were stock). Mpg's came up slightly without inhibiting performance when needed. I do think 2-4psi would be better.

By the way, are you familiar with the idle air screw adjustments using the tach and vacuum guage?

Basically you adjust the idle up to 850rpms on the engine, watching the vacuum guage, which is monitoring the manifold vacuum you close one idle air screw down and adjust it back open until you reach maximum vacuum, then repeat for the other screw.

Then adjust your idle back down to normal idle rpm's.

If nothing else the exhaust cleans up really nice.

If there is a fuel return line back to the tank most regulators will work in that mode, My vehicle had two tanks so doing that was not an option and without it, controlling fuel pressure was limited.

Food for thought

Karl Fortner
Tacoma, Washington

Hat's off to you, Chris! That is a great concise explanation of how the main metering system on Edelbrock (and Carter) AFB's works. I can tell you learned alot in this exercise!

Here is a little tip for anybody trying to tune the primary side metering system on these Carbs:

simple math. The Edelbrock rod/jet charts are a BIG help. I have found that even those charts don't tell the full story, however. Just distill it down to a single number: Start with the jet size number, subtract the first two digits of the rod size number, that gives a single reference #. Then do the same for the second two digits of the rod size number.

This way you get a single target # for the cruise/idle (fatter) upper step of the rod.

And repeated for the second two digits of the power (thin) lower step of the rod.

So this gives a simple two number profile of the fuel map created with any rod/jet combo.

95 jet minus 70 rod diameter cruise step equals 25 that's the cruise #
95 jet minus 47 rod diameter power step equals 52 that's the power #

Point being: this allows you to think outside of those fuel maps in the book/website! One of those rod/jet kits Edelbrock sells for each model# Performer carb helps a lot. You can check on edelbrock's site and see the whole selection of jets rods and all else they make. A parts store that stocks a variety of edelbrock rods/jets is a BIG help.

BTW Edelbrock Performer and Carter AFB parts are Interchangeable. Still lots of Carter AFB's around, though Carter is gone. So if you wound up w/ a combo of Edel. and Carter rods/jets like I have it's all ok. Carter used slightly different rod sizes than Edel. You may be able to really fine-tune the Primary rod//jet combo depending on what rods/jets ya kin git yer mitts on!

Tracyg Gadgetman Reno

Hi Karl, thanks for the idle adjustment tip.
On my 76 Chevy i used a cheap adjustable low pressure regulator from Oreilly auto parts. it didn't need a return line to the gas tank. I also put in a small gauge so i could see the pressure level.Some day i will try to find a higher quality low pressure regulator( non return style) & gauge from an internet supplier like Jegs or Summit Racing for more accuracy and longevity.
My 76 Chevy had no improvement in gas mileage after the Groove until i dropped the fuel pressure to the carb down from around 7 psi to around 4 or 5 psi. Edelbrocks don't like more than 5 or 6. They pass excess gas through the carb. I bet other carbs do also.
Once you have reasonable fuel pressure within Edelbrocks requirements the jetting can be fine tuned for a safe leaned down condition with smooth transitions from idle to full open. Slight throttle being the most difficult to adjust for, i found.
Happy Grooving,
Chris

Hi Tracy, Thanks for the carb tuning data. when i have some time i will check into a metering rod from Carter that mite give me a little more gas (smaller diameter) at cruse mode (slight pedal). .069 or .067 x .047 would be worth a try. I wonder if it is available. Thanks again for your kind words.
Chris