[dave-r]

To tune a carb you need;
a)A Vacuum gauge.
b)A way of telling how rich or lean your engine is.

It is vital to check that the fuel level and pressure is correct before you start!!!
You also need to make sure your ignition timing is correct. The timing is tied into this which can complicate things.

So assuming there is nothing wrong with the engine/carb and the timing is spot on and the idle speed is set to 600rpm (manual gearbox) or 800rpm (auto).

To set the idle mixture turn the mixture screws lean until rpm drops a bit. Then turn rich until you just get max manifold vacuum or max rpm with the car in gear. If the mixture screws do not seem to do anything your engine may have a long duration cam installed so you need to pay attention to the following.

Bigger duration cams might need a higher idle rpm to run smooth.

Unplug the vacuum advance on the dizzy and plug your vacuum gauge into the dizzy timing port on the carb.
This port should get its vacuum source from above the throttle blades (not manifold vacuum).

At idle there should be no vacuum on this port. Big cams demand a lot of air and fuel at idle. So if a bigger cam than stock is in the engine the following steps may need to be taken

If there is vacuum present on the dizzy port it means that the throttle blades are open enough to allow the mains to operate and supply fuel at idle. If the mixture screws cannot give you enough you need to modify the idle circuit so that it can supply enough air and fuel by itself at idle. You may find that the mixture screws do very little anyway until you fix this problem.


On a carb fitted by the factory on a mid 70s+ stock engine you may need to enlarge the idle restriction hole in the metering block very slightly to allow enough fuel into the idle circuits. Aftermarket carbs designed for performance should be OK though.

Some performance carbs have a screw to allow you to adjust open the SECONDARY throttle blades to increase idle speed. This allows you to then close the PRIMARY throttle blades with the idle speed screw until vacuum drops right off at the dizzy port.
You might also need to drill a hole in the PRIMARY throttle blades (start at 1mm) to allow more air into the engine as well. If you don't have any adjustment on the secondary blades you might end up needing holes bigger than 2mm with a big cam. If so start drilling the secondary blades as well.

When you have this right you should be able to get the throttle blades closed enough to loose the vacuum at the port and still have it idling at the correct rpm.

Next you need to plug the vacuum port (don't connect the dizzy yet) and place your vacuum gauge on a port from below the throttle blades (manifold vacuum).

Check that the rating of the power enrichment valve (holley) or enrichment springs (Carter/Edelbrock) is two inches below this level.

It may help to connect a long hose to this port so that the gauge can be read while driving later.

Now leave the vacuum advance disabled on the dizzy and also disable the secondary actuation on your carb if it is a 4bbl. Remove the linkage if it is mechanical or plug the vacuum pipe if it has vacuum secondaries. Wire the secondaries shut to make sure.

Go for a drive.

If you have a rich/lean indicator check that the mixture is maintained at all times.

If you get a momentary lean condition when you open the throttle quickly you need more accelerator pump shot by installing a bigger nozzle. If the duration of the shot is too short (it was ok initialy but then went lean for a short time) you need to change the little plastic cam that operates the pump on a Holley or use a different lever hole on a Carter.

If the carb runs lean at a constant highway speed a larger primary jet is needed. If it is constantly rich go to a smaller jet.

If it is OK at light throttle speeds but rich at high speed have a look at the vacuum gauge again while it is happening to see if levels are dropping below the valve opening point.

It is important that after making any changes at all you check the idle fuel level again.

When you are happy with all the above reconnect your vacuum advance on your dizzy and make sure the timing curve is ok on your engine with no knocking and the like.

Then check once more that the mixture is right at idle and you have no vacuum at the spark port and that it drives well on the primaries with no lean or rich spots and the fuel level is ok.

Everything ok?

Next step.

Now we re-connect the secondaries and go for a drive again.

Again check to see if the mixture remains stable at all times. If it goes rich at high engine speeds (drive some distance in second gear to simulate high speed without breaking the law) and check the vacuum reading again to ensure the power valve remains closed at normal cruise.

If it goes rich or lean at high speeds now it could be because the secondary jets need changing but this should be rare.

The main problem with secondaries is they can cause a bog when opened quickly.

The fix depends on the type of carb you have.

A Carter/Edelbrock AVS or TQ allows you to loosen the air door tension until the car bogs. Then re-tighten about a 1/4 turn and test again. Aim for the fastest opening without a bog.

Some Carter and Edelbrock AFB carbs have a counterweighted secondary velocity valve door (what a mouthfull!) that you have to trim mass from or add mass too in order to trim the opening rate.

Holleys are either vacuum secondary or mechanical secondary 'double pumper'.

The opening rate of the vacuum secondary is done by swapping in lighter diaphram springs until you get a bog and then go back a spring.

With the double pumper you need a huge shot of fuel if the pedal is mashed open. This is achieved by changing the nozzle size and or pump cam.

[dave-r]

If you do not have a rich/lean gauge the following should help.

1) First thing, adjust the fuel pressure! On carburetors, proceed to set
the float levels too. Everything is relative to the float level / fuel
pressure. If you change that, you start over.

2) Adjust mains by vacuum gauge, pick a RPM (I usually use 2800 or 3200 in
top gear, depending on gearing and where I normally cruse), and adjust the
mains so you get the maximum average vacuum at that rpm.

3) Adjust idle to get maximum vacuum in gear (autos)(Note1) or in neutral
(man). You will have to play with idle speed and richness settings, because
they are interdependent, and when you change one, you change the other.
Divide that number by two, and use that number for power valve selection
(or enrichment activation). If the idle screws have no effect, or a perfect
tune causes more than a 300 RPM drop in RPM when shifted into gear (note
2,4), remove the carburetor and turn over. Where are the throttle plates?
There should be an idle discharge hole, and a transfer slot. If there is
more than .004 showing of the transfer slot (Note 3), you need to drill a
hole in each throttle bore on the transfer slot side, near the center.
Start really small (1/32) and work your way up in size until the idle
screws work again and you can idle properly.

4) Adjust accelerator pump so you have the shortest duration and volume
that won't cause a bog under normal acceleration (no secondaries and normal
throttle action). You should not be able to tell when the pump shot ends
and the mains come on line. If the car bogs then goes, you need to increase
the squirter size. If it goes, bogs, then goes again, you need to either
decrease squirter size or increase pump capacity, possibly both. On FI,
move the dial or map, on a carb, change the squirter or pump cam.

5) Adjust the secondary jets to give maximum acceleration or highest MPH in
the 1/4 mile. Sometimes the secondaries will be leaner than the mains, esp.
if a secondary PV is used. On carbs or FI without secondaries, try a high
flow PV if lean. On some FI units, you can control fuel enrichment. Tune
the enrichment circuit as if it were the secondary jets. Failing that,
you'll have to decide if you want performance or mileage, and tune for
that. This will also be true for 4160 Holleys, and other carbs with
non-adjustable secondary jets, as you will have to tune using a high flow /
low flow / two stage power valve and main jets.

6) Adjust the secondary accelerator pump / diaphragm spring / air door to
give the quickest 60 foot time (also without bog or hesitation). Sometimes,
it requires a lot of fuel to cover up the lean pump shot on the primaries.
On Holley vacuum secondary carbs, the spring will also affect mileage,
lighter the spring, the more performance and lower mileage you will get.

7) Note vacuum on top end at full throttle. If carb is way too small, the
vacuum will climb above the number you got in step number three. If this
happens, take the highest reading you have, and add one. Replace (or add)
the secondary power valve with this number, and go to step 5.

7) Adjust choke so under startup, no black smoke pours out, but no bog
occurs either.

All out drag tuning requires all jets to be equal on squarebore carbs,
jetted for best MPH, and the accelerator pumps tuned for fastest 60-foot
times. Chokes and air horns are normally removed. Sometimes the PV is
(mistakenly) removed. If the PV is removed, the car will load up in the
pits, because jet sizes need to come up about 10, causing an overrich
condition at part throttle.

General thoughts on the matter:

Increasing cam duration requires more jet.

A dual plane manifold can use a much bigger carb.

If you decrease the metering signal, you need to increase the jet (as you
do when converting from a dual plane to a single plane manifold).

A blown power valve will have the greatest effect at idle and low RPM. You
can check the carb by unbolting it from the manifold, putting the nuts back
on the studs, and plopping the carb onto the studs. If you have an electric
pump, turn it on. For a mechanical pump, completely remove the coil wire,
and crank for about 15-30 seconds. The manifold and carb will be separated
by a 1/2 inch or so, enough to see if any gas drips into the plenum when
the engine is off (Don't try to start it!). If gas is present, check where
it is coming from. There is a hole near the throttle plate that leads to
the PV chamber. A blown PV will cause gas to drip from this hole(s). If it
is dripping from the bores, the float level may be too high, or your carb
has a crack.

The biggest city mileage killer is the accelerator pump.

A bigger jet requires less accelerator pump, a smaller more.

A dogleg style booster requires the most pump shot; the annular style the
least.


The higher the velocity through the barrel, the better the atomization is.
Better atomization means more efficiency (more MPG and Power). That's why
dual planes typically give better MPG and performance up to the point where
they start to choke off the upper CFM (and HP).

If you have a 600 CFM 4BBl carb on a true dual plane, the most any one
cylinder can draw through the carb is 300CFM, where a single plane allows
it to draw 600 (At its rated in. Hg, that is).

Note 1 This usually causes the idle circuit on auto transmissions to be
slightly richer than need be when unloaded, but causes it to be perfect
when you are actually driving it.

Note 2 A lean idle circuit causes automatics to drop an excessive amount of
RPM when put into gear and stopped, and may cause stalling. A richer idle
is what is called for, unless you see black smoke pouring out the tailpipe.

Note 3 The transfer slot must show, or you will get a really bad off idle
hesitation. If a non-drilled carb doesn't show the slot, thin the throttle
blade at a 45-degree angle to expose. If that still doesn't work, you
really shouldn't be running a 1050 Dominator on your 2-liter motor anyway.

Note 4 Sometimes, a really tight converter and a really big cam will do the
same thing, there may not be much you can do.

[dodgefreak]

dave, first of all, thanks for all the info, last week I did my ignition, now my carburator... i've got a question: are most mopar distributor vacuum ports timed(no vacuum at idle) or full(vacuum present at idle) ?
i need to know because of the edelbrock caburator ...

thanks again!

david

[dave-r]

Always timed (no vacuum at idle). This is VERY important.

No matter what carb you have the fitting for the pipe is always from above the throttle blades and NOT manifold vacuum from below the blades.

[dave-r]

Found this great little article. Sorry about the Chevy in it.

http://www.chevyhiperformance.com/howto/45638/

[dave-r]

http://members.tccoa.com/392bird/tuning.htm