Edelbrock carburetor tuning guide

muffinsmuffins Regular
edited July 2010 in Man Cave
Well for those of you who like the edlebrock carter style carbs here ya go.

Idle srews: control only the fuel mixture at Idle and have little or
nothing directly related to the cruising or WOT position of your foot.

Metering rods: there is two diameters on the rods and if you look at
them close you'll be able to see the two steps on the rod.
Jets: for simple tuning the jet is just a variable to give you a more
finite adjustment of the metering rods.

Lesson 2.....

"You need Bigger Metering Rods to get more fuel" if someone says this
to you, RUN! as far away and as quickley as possible, never let this
person near your carb with anything other than a polishing cloth for the
air cleaner lid.

"Wire those wieghts open"...just pull out your gun and shoot this
stupid SOB, he's too stupid to own a car.

"Try putting that vacuum hose for the distributor on the bottom
nipple"...send him somwhere to a farm to look at real nipples he has
no Idea what he's talking about...if he picks up a screwdriver get the
sawzall out and cut his hand off.
Lesson 3....

Never let anyone touch your carb, learn how it works and what to
adjust and when, to get the result your looking for...unless... this big,
fat, bald headed guy with a dirty old Dodge hat on walks up and
asks..."Need some help..." don't even answer just hand him the tool
box, listen and watch....and he likes that green Poweraid stuff during
the day...at night it's ET (Early Times) and Pepsi..no Coke, Pepsi

Ok so here we go, get out your note pads and there will be homework
err ahh trackwork for ya'll this weekend.

You'll need a vacuum gauge, not a good one, I got mine at Sears
here in town for $9.95.

Plug the gauge on the lower port on the front of the carb, this should
NOT have your vacuum advance unit hooked to it.....right? Ok
Now lets get the idle down as low as it'll go without stalling, now start
turning one of the idle screws in until it starts to loose vacuum or the
engine starts to sputter, .....now count the
turns as you turn the screw out until the same thing happens. lets say
your number is 2 so now go in 1/2 of that or 1 right....OK now same
with the other side.

Remember to keep bring that idle down to compensate for your tuning, you want it as low as possible so you can get a true reading on the idle mixture.

Now start all over again..Idle, in, out, count and set.

By now you should have a pretty good smooth idle easy huh....

Now lets watch the vacuum gauge 1/2 turn in 1/2 turn out slowly turn
the screw back and forth no more than a 1/2 turn in either direction
until you get the highest reading on your gauge, thats it your
done.....runs good now doesn't it.....set the idle up to your desired
RPM for most of you using a Edelbrock your probably running a pretty
mild engine so 750-950 is probably about right.
Now crack the throttle off idle...sounds pretty good huh :-))

OK now check your timing and set it where you want it.

Start over and do the whole process again...Idle down..the old in and
out...vacuum gauge and now your done.

Secondary circuit...

Grab a couple of new spark plugs and a wrench, we're going on a road
test. Find a nice lonely road somewhere, install a new plug in any hole
that's easy. Now stand on that throttle run it all the way through first
and about 1000 RPM below your shift point in second, turn off the key
and release the throttle at the same time. Coast to a stop and remove
the new plug, is the porcelin black brown or white?
Brown is good and probably good to go on the other hand if the plug is
white it's lean and if it's black it's rich. We just tested the WOT
mixture and we now have a baseline on that end of it.

OK put in that other new plug, this time drive the car hard but do not
let the secondary's open, you may want to leave the air cleaner off so
you can hear the carb working and keep it off that deep howl that the
secondary's make when they open. This time run it all the way up to
your max RPM and shut down the same way. Pull the plug and install
one of your old ones and head for the shop.

Remember the two different diameters on the metering rods. The rods
are pulled up out of the jets as you accelerate, the first stage or
primary circuit is the larger diameter of the metering rod, the smaller
diametyer or the tip size is your secondary or WOT circuit.
So let's think about this...the SMALLER the diameter of rod the more
fuel right....remember the rod is lifted out of the jet and because it's
thinner at the end as it's pulled up and out of the jet seat, more fuel
is allowed to flow into the engine....make sense?

For this example we'll use a metering rod numbered 40/60, 40 being
the tip of course.

So let's say for example the #1 plug in our plug test was a little dark
brown almost black and the # 2 test plug is almost white. By our test
we know that #1 plug was wide open throttle so our engine needs less
fuel at WOT so the smaller number of 40 needs to increased to say a

But, the primary throttle circuit is showing a lean condition so it needs
more fuel..so...yup, you guy's are smart, we would drop it to say a 55
so now we need to go looking for a set of metering rods 45/55. and
start all over with a couple of new plugs and back to the road.
This is how I do it and to super tune one of these carbs you really
need a spark plug reader and look way down at the bottom of the
porcelin but for most simple applications this will get you so close
you'll never feel any improvement in the seat of your pants.

I used this method 2 weeks ago on my neighbors 80 El Camino (sorry
but someone has to keep them running or they'd be cluttering up all
the roads)

It's a basic stock 305 last winter we put a Edelbrock RPM manifold, 600
Edelbrock, curved the distributor, headers and he tuned it up to the
best of his ability...it ran 16.40's pretty consistantly but always had a
stumble and was hard starting. I got tired of hearing him crank and
crank the thing and convinced him to let me tune the carb...it didn't
take much convincing....last week the car ran 15.87, 15.91, 15.88 and
broke out in the 1st round with a 15.87 on a 15.88 dial :-((
He checked the milage and it picked up from 14.4 to 18.2! Not
important to me but he liked it.

There's alot more fine points to these carbs but my fingers are tired
and I need to get my ugly sleep. Besides you have lots to work on for

Next lesson we'll discuss:
What those springs that the metering rods sit on do and .....what are
those weights and how do they work?


  • muffinsmuffins Regular
    edited July 2010
    Well I guess it's time to start getting everyone dialed in on their Eddy carbs, so here we
    go. In previous postings I've gone over fuel delivery and basic set up so I'll presume at this
    point everything has worked out and the carb your working on is in top condition.

    In this chapter of the infamous book we'll look at the baseline of the carb and fine tuning
    the transition from idle to WOT.

    I continually see Guy's buying a new Eddy and immediately going inside and putting in
    bigger jets and smaller rods thinking that this will get them more fuel for their motor, when
    in actual fact it gives you more fuel but your not changing the air volume so you just end
    up with a rich condition and a "Bog on acceleration"

    Let's look at one of the most common complaints that I run into on Eddy carbs.

    1. Stumble off Idle

    Cause 1: Usually caused by a lean condition for a fraction of a second while the plungers
    and springs lift the metering rods and dump the required fuel demanded by the engine.

    Cause 2: Not enough initial timing in the motor.

    Cause #1 Cure: As the vacuum drops in the manifold it allows the spring under the
    metering rod to lift the rod out of the seat and dump fuel. So if you having a little stumble
    or hesitation it's usually the spring rate. If you go to the next stronger spring it will allow
    the metering rod to open quicker and dump fuel sooner in the transition between Idle and
    WOT, of course this is assuming that everything else is in good working order such as
    accelerator pumps and timing curve.

    By changing jets or richening the metering rods (smaller) you will increase the fuel to air
    ratio but not change the timing on the metering rods so you may end up with a stumble
    and then a bog until the engine picks up enough RPM to burn all the fuelÂ…sound familiar?

    Cause2 Cure:
    To fully explain the theory behind this I want to revert back to the basics of camshaft
    overlap and the science of the 4-stroke performance engine.

    For those engine Guru's out there you can skip this partÂ….

    Overlap definition: The position of the valves on the exhaust stroke where both the intake
    and exhaust valves are open. This is what creates that lumpy idle and reduces the cylinder
    pressure on a race cam at idle and virtually destroys the power band at low RPM.

    So what happens?Â…..When the piston reaches almost TDC on the exhaust stroke and the
    spent fuel is being forced out of the combustion chamber the intake starts to open allowing
    fresh fuel to rush into the cylinder across the piston and be scavenged by pure velocity out
    the exhaust valve basically flushing the combustion chamber clear of exhaust gases.

    So now based on the ramp speed of the cam lobe and the exhaust valve slams closed so
    the intake can refill the swept volume of the cylinder on the down stroke. Now if everything
    is right and the valve size and carb can create enough velocity (or air speed) as the piston
    heads toward BDC the fuel charge will continue to fill the cylinder when the piston stops
    and hesitates for that split second at BDC basically overfilling it or actually building a slight
    amount of pressure in the cylinder. The intake valve slams shut and captures the
    pressurized fuel charge. Now we have more cylinder pressure than a static or ambient
    measurement of volume and we make more HP. A blower or turbo charger just exaggerates
    this phenomena.

    So how does this affect the carb tuning?

    In an overview we can say that we've increased the amount of fuel in the cylinder through
    volumetric efficiency and we've decreased cylinder pressure at idle or as they say, before
    the cam comes in, so we need to burn all that fuel we've stuffed in there with relatively low
    compression or cylinder pressure.

    The only way to do this is to give the flame in the chamber a longer shot at it, so we do
    this by increasing the idle timing therefore allowing the combustion chamber a longer
    duration in the cycle to burn the fuel, this is why at idle if you start to turn advance into
    your motor it will pick up RPM, you not adding any fuel or air your just giving the engine
    more time to burn what's available.

    In conclusion what we've done by advancing the idle timing is burn all the fuel in the
    combustion chamber, eliminated the rich idle condition and removed the stumble off idle.
    Depending on how radical the cam and the volumetric efficiency you've created with your
    combination, this will determine the amount of advance at idle is required for the best
    response off idle for your engine.

    Stock: 10-12* or maybe even less
    Mid range cam combo maybe around 18-22
    Stout runner, try around 25
    Full blown racecarÂ…you may have to go up to 30*

    Just one more note hereÂ…. before you can get your distributor curved correctly you need to
    go through this tuning process to determine where to set the idle advance and it's
    relationship to the max timing and the RPM you want all this to happen at.

    Let's use this quick scenarioÂ…. you set your engine up incorrectly and you've got the idle at
    1200 to make it run smooth, the idle timing is at 12* and max advance of 38* comes in at
    2500 RPM and you send you distributor out to have it curved to this spec.

    Then I come along and retune it correctly and advance the timing to 22* and idle it down to
    1000 RPM, when I moved the idle timing up it also moved the timing up on the other end
    so now at 2500 RPM your max timing is now 48*...look out a melt down is about to occur.
    This also reinforces the need for a timing light that you can dial in to determine the MAX
    advance, setting a performance motors timing from the idle mark is Russian Roulette.
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