Performance Carb Setups - Part 2
By Ryan Ballou
We left off
last month with an overview of the various circuits within the Weber
carburetor. This month we move on to the installation process and
what it takes for a smooth running engine.
The first thing you need to do is to get yourself a good linkage setup.
The linkage is what can make or break a good running dual carb'd engine.
By far the most popular setup is the cross bar linkage. This consists
of a bar spanning the carbs with an arm on each side and one in the
middle. The middle arm is where your accelerator cable attaches. The
two side arms connect to the carbs via threaded connecting rods. The
three main styles of cross bar linkages are the hex bar, the Tayco
(round bar) and the Berg linkage. Whatever linkage you choose is just
a matter of preference - there is no real best linkage here. Mine
is the hex bar style from either CB Performance or Redline Weber.
They both use connecting rods with left hand threads on one side that
make fine adjustments a breeze.
The second and less popular style is a center pull linkage. It consists
of a center bell crank assembly that pulls/pushes an arm on each side
directly attached to the carb throttles. If you find that you need
to use a center pull linkage for one reason or another, I suggest
the setup made by CSP. It seems to be the only setup out there where
people don't make complaints based on quality issues.
Before you can start the actual process of installing the carbs, you
must first do an initial pre-tune procedure. This will ensure an easy
first time installation. Most important is to first make sure both
carbs are absolutely clean. Any small amount of dirt can easily plug
an idle jet or passage and cause a lean condition. Make sure to use
a carb cleaner that does not leave any residue as this residue can
attract more dirt. The easiest way to clean the carbs is to pull all
the jets and spray cleaner through every passage. Be careful and wear
eye protection. Carb cleaner has a way of splashing on everything
when used this way. It will also eat paint like brake cleaner so be
cautious of your surroundings.
Once the carbs are cleaned, the first order of business while they
are still apart is to set the float level. The up measurement ranges
from 10mm to 13mm. This measurement is taken from the gasket on the
carb top to the outside edge of the float. The adjustment is made
by bending the tab at the needle valve a little at a time with needle
nose pliers until you get the right setting. The other setting is
the down position. This adjustment is made the same way but by bending
a different tab. The book number for this setting is 32.5mm, but can
vary in depending upon application. Consult your engine builder for
help, or just experiment. The absolutely most important part of setting
the floats is to be sure they are the same from carb to carb. Uneven
floats cause a mixture difference from side to side of the engine.
Before continuing with pre-tune procedures, you need to get a baseline
jetting established. This I cannot provide for you on a general basis.
Jetting selection is affected by so many things in an engine combination
that there is no way to know for sure. Your best bet is to speak to
the person selling the carbs, or a VW mechanic versed in performance
of aftermarket based engines. They will help you determine your starting
point, and can help fine-tune once the engine is running. In most
cases, the jetting the carbs come with is too lean. Most people will
find there idle jet size between a 55 and 60. Mains vary greatly depending
upon the engine, as do airs. Rule of thumb from the Weber books is
4.1-4.3 times your venturi size yields a starting point for the mains.
However, for me it's closer to 3.9 as I'm running a 38mm venturi and
a 150 main jet. The rule tends to error on the rich side. Airs are
usually about 50 more than the mains. Again, talk to your engine builder
or someone familiar with dual carb setups on VWs to be safe. Too lean
will cook and engine, too rich will kill fuel economy and cause premature
Now I'll move on to setting the throttle position. Two ways to accomplish
this is by either counting turns of the mixture screw or use of a
feeler gauge. With a feeler gauge you set the throttle plate position
with the idle speed screws so that the plate is open .006". Counting
turns, you set the screw to about 1/2 turn in from just touching the
carbs throttle arm. Throttle plate position is not only important
for setting idle speed, but it determines the position of the plate
relative to your progression holes. When adjusted properly, the edge
of the plate should be just covering the lowest progression hole.
When you touch the throttle, the first hole becomes exposed and the
idle circuit starts metering fuel.
On to the mixture screws. Start with these screws turned in about
2 turns from fully bottomed. When bottoming these screws, just lightly
bottom them. If you cinch them down hard you risk damaging the screw
tip or the seat in the carb. Next to the mixture screw is a screw
with a jam nut. These are the air bypass screws. For now just make
sure they are all lightly seated (all the way in).
Now it's time to begin test fitting the components. The first order
of business is to test fit the manifolds alone. Interference points
to watch for are at the manifold base between the spark plug holes
and clearance of the mounting nuts to the manifold. As far as the
nuts are concerned, the 11mm nuts I spoke of in the last article are
the best choice. Clearance from the manifold to the engine tin can
easily be added by using a flat screwdriver to catch the lip of the
seating surface for the manifold and prying the sheet metal back.
The manifold should seat completely flat and not touch any part of
the sheet metal.
Now bolt the carb to the manifold and test fit again. Point of interest
this time is the choke block off-plates and the carb tops to the fan
shroud. Adding clearance here can literally be an ugly task. A ballpeen
hammer is the easiest route here. Go slow and check the fit often.
Clearance here tends to only be a problem on doghouse style fan shrouds
with heater outlets. How much clearance is needed will depend upon
the manifolds used and engine width, which is based on a number of
Once you've determined that everything will bolt up cleanly and not
cause any interference problems, you can begin final assembly. The
first step is to match port the gaskets. You'll want to match both
the set from the carb to the manifold and the manifold to the head.
Set the gasket where it will be when bolted together and gently press
any excess into the port. This will provide a nice indentation telling
you where to cut. Use a fresh x-acto knife or utility blade and cut
the gasket to match the opening.
Before installing the gaskets, smear a thin layer of grease onto each
side. This will help greatly in preventing any leaks and also helps
to keep the gasket from sticking when you have to remove the carbs
in the future. Now bolt the carbs to the manifolds, then set the carb/manifold
assembly on the head and bolt it down. Do the same with the other
side. Make sure the air cleaner assemblies, linkage plates, and velocity
stacks are not installed yet, just the bare carb.
You may find that the nuts at the firewall side nearly impossible
to reach. What has worked best for me was to use a 1/4" drive
swivel socket, and two 1/4" drive extensions with another swivel
between them. Once you get the socket seated on the nut, tightening
should be easy. Just snug up one side at a time, then go for final
torque. No need to wrench them down super tight, just about 13-15
ft*lbs will do.
Now that the carbs are on you can connect the fuel line. Get yourself
a good quality brass/stainless/metal 'T' fitting to split the line
to both carbs; don't mess with the plastic ones. If you are going
with an electronic fuel pump then most regulators (good ones) will
double a your 'T' fitting.
Time to look at the linkage. Since the most popular style (and easiest
to setup) is the hex bar, that is what I'll focus on. First you need
to install the center pull and arms onto the cross bar, just leave
them loose for now. You want the arms to be in the same plane as one
another, and the center (throttle cable) arm to be down either 45
or 90 degrees depending upon your particular brand. The cross bar
will connect to a mounting base on each carb via a ball socket. These
incorporate a threaded design to compensate for various engine dimensions.
You'll want to adjust the overall width of the bar so it has only
about 1/16" to 1/8" side play.
Once the bar is situated install the air cleaner base and velocity
stacks and bolt them down. Now you need to setup the threaded rods
that connect from the cross bar arms to the throttle arms on the carbs.
Screw both ends onto the threaded rods all the way at first, then
connect them to each side at the carb and cross bar arm. Next spin
the center rod on both sides until the ends are at about ½
of full thread. Now it's time to set the arms in the correct position
on the cross bar. They should both be vertical (as seen when standing
back looking at engine), or as close to it as possible, but most importantly,
they should be a mirror image of each other. Be sure both arms are
at the same angle to vertical. Stand to the side of the engine and
sight the rods inline with each other. If one leans forward/rearward
from vertical, you want the other to match as exactly as possible.
It may be necessary to shim or shave the mounting hardware at the
carb arm to achieve this. Having this set properly will help ensure
that both carbs open up at the same rate. When one carb 'leads' another,
you can end up with a rough running engine as one side will run lean.
Now check the throttle stops, both carbs should be on their stops.
If they aren't, then you need to adjust one rod or the other until
both carbs are on their stops. This is where having those left hand
threads really comes in handy. All you need to do to make the adjustments
is spin the rod one way or the other.
With everything bolted up, and your linkage squared away (for now)
it's time to start the engine up. You will need to crank it for about
20-30 seconds at first to fill the float bowls up. Then it may be
helpful to have someone nearby that can pump the throttle from the
engine side once it does catch. The goal here is just to get it to
idle, but trust me, it'll be rough at first. You may find yourself
having to pump the throttle every few seconds to keep it going, but
it should hold on it's own after a minute or two once the engine starts
to warm up.
As soon as it will idle on it's own, go for the mixture screws. DO
ONLY ONE AT A TIME. Turn the screws clockwise (in) in 1/8 turn increments
until the engine runs rougher than it was. You may pass a point where
it smoothed out a little, that's OK, pass it for now. Once it just
hints at running rougher, back the screw out 1/4 turn. Now repeat
this procedure for the other three screws, doing one at a time. Once
that's done, do it again. Go slowly and give the engine time to react.
Just do a 5 count after every 1/8 turn. Then as soon as it hints at
a lean miss, open it back up about 1/4 turn. When these are right,
then engine should idle nice and smooth.
Now that the mixture screws are all set, it is time to set the idle
speed and synch the carbs. This part is easy, just grab your synchrometer,
my favorite is the air flow meter sold by many distributors, often
called a snail gauge. Stick it in the closest velocity stack of one
carb and note the reading. Now go to the next carb and compare readings.
Bring the higher flowing carb down to the same level as the lower
carb. Now check your idle speed, goal is between 800 and 900 rpm.
Bring them up or down little by little until you reach this speed.
Once there, do the mixture screw routine again. Then check the idle
speed again. It may take some back and forth footwork, but you'll
This is a good time to check for intake leaks. Intake leaks make you
run lean and can cause serious damage from overheating. Take that
same can of carb cleaner you used earlier and spray a little around
the base of each manifold. If there's a change in engine speed, you
have a leak and will have to remove that carb and correct the problem.
The most common cause of leaks is when you have an interference fit
between the carb/manifold and some engine tin. It prevents the manifold
from seating fully on the head and introduces a leak.
Once you're sure you have no leaks, your engine is idling smoothly
and the carbs are synch'd, you just have two more steps to carry out
before you can actually drive it. First you need to check the synch
between barrels on the same carb. This is what the air bypass screws
are used for. Use your airflow meter to check between the two barrels.
If the rear barrel is flowing more air, then you need to open up the
bypass screw on the closer barrel until it reads the same. This will
affect the carb synchronization and you will have to repeat the above
procedures. If the barrel closest to you reads more, then open up
the screw on the rearmost barrel. This has no effect on synch. Something
to note is that these also affect throttle plate position when used
on the closest (linkage side) barrel. When you bypass air (open screw)
you end up compensating by closing the throttle plate more (turn out
idle speed screw). This affects the plate to progression hole relationship.
So you want to try and keep the same bypass screw adjustments on both
front barrels, rears don't matter so much. Basically, if you find
yourself opening the screw on one carb, open the other front screw
on the other carb the same amount. This can get tricky fast and you
may find yourself chasing your tail on this on. Just get it as close
as you can keeping the two front barrels the same. Also, if you find
that you've opened all the screws, then you did something wrong. Turn
them all back the same amount until the one that was opened the least
Now you need to do some fine-tuning with the linkage rods you set
earlier. The best way I've found to do this is to cinch up the throttle
cable in the linkage so that it holds the throttle partly open. You're
looking for a reading of about 10 kg/hr on the air flow meter. It'll
sound like you're really winding it out but that's OK, it's only about
3k rpm, give or take. Now take flow readings on each carb again (front
barrel only). Follow the same procedure for setting the linkage rods
as you did earlier, but make only very small adjustments. Once you
get them flowing evenly at a higher rpm, you'll know that they are
opening at the same rate. Reset the throttle cable and recheck the
synch, they should still be the same.
Time to drive. Put the air cleaners on first and take it easy until
the engine warms up a little. Then just have some fun. After a good
drive and the engine is fully warmed up, head back home. When you
get there keep the engine running and check & adjust idle speed
as needed. It will probably have changed now that the engine is warm.
After that, do the mixture screws again. Once
everything checks out, drive
it like you stole it.
1-Looking down the throat
you can see what a horizontal discharge tube setup looks like.
It's just a straight shot to the heads, no restrictions.
2-Easy way to check for clearance is to slide a feeler gauge into
areas you aren't sure about.
3-Another tight fit at the base. Bending for clearance here cause
some unseen problems. I had to cut some of the internal fins on
my flaps to clear the air flow divider between the heads. The
divider was pushed in so far the shroud didn't fit right without
4-Another potential clearance problem is the air cleaner bases.
These I just cut to fit with a cutoff wheel.
5-Swivel sockets make everything easier.
6-The last (I promise) area to watch for when it comes to clearance.
You really don't want this getting stuck in the 'on' position.
And don't laugh at my foil
7-Completed installation, ready for tuning. Notice the linkage
rods are virtually mirrors of each other. Keep the angles the
same and your engine will run smoother.
8-A good brass 'T' is much safer than the cheapo plastic ones
in most kits.
9-On my engine, this reading corresponds to about 900 rpm. In
most cases, if you go past 5.5 - 6, you're idling too high.
10-On my engine, this reading corresponds to about 900 rpm. In
most cases, if you go past 5.5 - 6, you're idling too high.
to High Performance 101 index page Next >>
us | Contact
| Events | History
| Images | Interactive
| Links | Tech