Originally Composed by Don At
Edited for clarity by some anonymous poser.
You'll need a vacuum gauge, not a good one, I got mine at Baxter's
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.
Now let's 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. Let's 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, that's 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.
This is where you need the karma of the Skuza's monkey.
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 porcelain 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 secondaries 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
secondaries 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 diameter 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
porcelain 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
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 consistently 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 mileage and it picked up from 14.4 to 18.2! Not
important to me but he liked it.
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
and springs lift the metering rods and dump the required fuel demanded by the
Cause 2: Not enough initial timing in the motor.
Cause #1 Cure: As the vacuum drops in the manifold it allows the spring under
metering rod to lift the rod out of the seat and dump fuel. So if you having a
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
WOT, of course this is assuming that everything else is in good working order
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
and then a bog until the engine picks up enough RPM to burn all the fuel…sound
Cause #2 Cure:
To fully explain the theory behind this I want to revert back to the basics of
overlap and the science of the 4-stroke performance engine.
For those engine Guru's out there you can skip this part….
Overlap definition: The position of the valves on the exhaust stroke where both
and exhaust valves are open. This is what creates that lumpy idle and reduces
pressure on a race cam at idle and virtually destroys the power band at low RPM.
So what happens?…..When the piston reaches almost TDC on the exhaust stroke and
spent fuel is being forced out of the combustion chamber the intake starts to
fresh fuel to rush into the cylinder across the piston and be scavenged by pure
the exhaust valve basically flushing the combustion chamber clear of exhaust
So now based on the ramp speed of the cam lobe and the exhaust valve slams
the intake can refill the swept volume of the cylinder on the down stroke. Now
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
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
measurement of volume and we make more HP. A blower or turbo charger just
So how does this affect the carb tuning?
In an overview we can say that we've increased the amount of fuel in the
volumetric efficiency and we've decreased cylinder pressure at idle or as they
the cam comes in, so we need to burn all that fuel we've stuffed in there with
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
duration in the cycle to burn the fuel, this is why at idle if you start to turn
your motor it will pick up RPM, you not adding any fuel or air your just giving
the enginemore time to burn what's available.
In conclusion what we've done by advancing the idle timing is burn all the fuel
combustion chamber, eliminated the rich idle condition and removed the stumble
Depending on how radical the cam and the volumetric efficiency you've created
combination, this will determine the amount of advance at idle is required for
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 racecar…you may have to go up to 30*
Just one more note here…. 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
relationship to the max timing and the RPM you want all this to happen at.
Let's use this quick scenario…. 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
so now at 2500 RPM your max timing is now 48*...look out a melt down is about to
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