Edelbrock Tuning


Originally Composed by Don At FBO Systems
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.


Secondary circuit

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 45.

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 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 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.


Part 2

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 fuel…sound familiar?

Cause #2 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 part….

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?…..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 enginemore 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 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 it's 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 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.