Whether you are preparing your car for an upcoming show, working on an engine rebuild, or simply cleaning up the engine compartment, you pay careful attention to what you are doing; it’s your hobby, it’s important to you, and you love it.
That attention detail is just one of the things that sets you apart from the average driver in their modern “econo-box,” or minivan. The cars of today are very reliable, and very efficient; but often that comes with a loss of character.
On the other hand, our cars possess a great deal of character; that’s a huge part of their charm, and of course, the occasional frustration. But taking care of things correctly pays dividends in satisfaction, reliability, economy, and general peace of mind. Our cars are designed to be enjoyed, not worried over.
Checking flanges for flatness of an HD8 from a Jaguar E-Type , using a Dial Test Indicator
“God is in the details” that famous quote by the American
architect Ludwig Mies van der Rohe is our credo when it comes to
rebuilding S.U carburetors. Sounds a little corny, I know, but
nevertheless it’s true. Rebuilding a carb. is far more than simply
removing it from the engine, rinsing it in a little kerosene, installing
a rebuild kit, and hoping for the best. Chances are you will get away
with it, at least for a while, but the carburetor will certainly not be at
Almost all of these carbs are now more than 30 years
old, many are over 50, and too many of them have seen very little, if
any, real maintenance or care. The key to a true rebuild is to take the
carburetor back to the condition it left the factory in, or as close to
that as possible.
So, what’s involved? It is quite a lengthy process, and some of these steps are covered in the Our Services section, but the intent here is to focus on the smaller details, that can yield big results. So, if you’ll stay with me, I’ll walk you through some of them, and tell you why they are important.
The first stage after we receive the carbs is to
identify them by the SU part number, either from the tag which should be
(but often isn’t) attached, or if not, by a little research. As an
aside here, please NEVER discard that little tag, it may not seem
important, but it is when it comes to correctly identifying your carbs.
After that we thoroughly clean the carbs to remove all of the surface
grunge before we proceed.
Once the carbs are cleaned to the point
were we can actually see what we have, we initiate a work order
card, on which we record any external issues, or shortages before any
dis-assembly takes place. This card follows the carbs until they are
We then check the linkages. Are they free to move, do
they have the correct springs, and clevis pins installed? This is
particularly true on the “H” series carbs; where the main pivot hole in
the brass choke operating levers has a 5/16” hole, in which a 3/16”
clevis pin is used to allow a little float. This “float” (or more accurately "lost-motion") is designed to
allow the fast-idle cam to engage before the fuel mixture is enriched.
It’s not uncommon to see a set where the owner has fitted a oversize
(usually 5/16”) clevis pin to eliminate what they see as “slack” in the
system. That’s a mistake, but one that is easily remedied.
Initial check of the linkages prior to disassembly
Another common mistake is an over exuberance when it comes to installing throttle, and choke return springs. Each setup is of course different, but in general terms, we simply want to ensure that both the throttles, and chokes return to their stops when released, we don’t need to install extra springs unnecessarily.
On the “HS” series for example, we use three springs,
all on the throttle; one on the center cam, and one on each of the
individual throttles levers. The choke return on these carbs, is taken
care of by the radial spring under the fast idle cam. The “HIF” series
has a similar albeit slightly larger spring. We sometimes see 5 or more
springs on a twin carb setup; it’s just not necessary.
The later HS carbs also use a very effective radial spring on the throttle, which eliminates the need for the traditional return springs.
The image above shows a typical spring arrangement on a pair of HS2's
A Quick Tip – On many of our cars, it
takes quite a bit of effort to pull out the choke, overcoming the
friction of the choke cable etc. So, before you do that, press the
accelerator pedal down to the floor; that way you are taking the
throttle spring pressure off the cable, and it’s much easier to pull out
the choke. Just remember to take your foot off before you start the
Alright, so once the linkages have been checked we move on
to the pistons. We check to see if they rise, and (and even more
importantly) fall smoothly, and note the result. We will conduct a
drop-test to check if the piston falls at the correct speed, but much
later in the process.
We then remove the suction chambers and
measure the height of the jet from the bridge, using a digital vernier.
If we know the set came from a satisfactorily running vehicle, we will
use that setting when setting up the carbs on the test bench, as this
speeds up the installation process for the owner. However, if they came
after sitting unused on a shelf, we use the original factory settings,
but with a slight revision (see Quick Tip below)
A Quick Tip – It’s worth remembering here that the settings shown in the original owners manuals are based on the fuels of yesteryear, which were often more potent (read higher octane) than the ethanol-laced “stuff” we are sold today. The result is that these original settings tend to give a weaker than desirable mixture. A couple of more “flats” is usually sufficient to compensate, and to some degree it depends on the quality of the actual fuel you are using. (Note: When rebuilding we don't count the flats - we use a digital vernier to set the jet height, and on most vehicles we use an initial setting of 0.085" below the bridge)
With the suction chamber removed, it’s time to check the condition of the springs. It is the compression of the spring, combined with the weight of the piston that provides the correct amount of depression (partial vacuum) at the throat of the carburetor. When they leave the factory these springs color-coded, either Blue, Red, Yellow, Green, or Red & Blue. However, the paint does not last very long, so in order to determine if the spring is correct for the application we need to test it, simply measuring the free-length is irrelevant. We place the spring on a test fixture, and then place the appropriate weight (which range from 2 ½ oz to 11 ¼ oz) on top of the spring, and measure the length of the spring under compression. A spring can normally be brought back into specification, but if not, or it’s corroded, it is replaced.
The needle is then removed, and checked firstly to make sure it is the correct needle for the application, and secondly to make sure it is undamaged, and straight. The needle securing screw is then removed. If the needle is one of the later spring-mounted type, we also remove the spring, and the sintered collar, after making a note of the direction of needle bias.
Carburetor body reamed for new bush
A New Super Oilite® Phosphor Bronze Bushing
We now turn our attention to the throttle shaft. All
throttle stops, return springs etc (model dependent) are removed from
the shaft, and we then check for wear. Even though we always replace the
throttle shaft this check gives us a feel for what is likely to be the
overall condition of the carb. We then remove the throttle plate
(butterfly), and the throttle shaft. The throttle plate screws are
always discarded. The original bushes are cast into the body during
the die-casting process, and if worn, must be removed by line-reaming.
This is a job which cannot be done one side at a time; the result would be bushes
that were not in line, and a throttle spindle that would not want to
rotate. Once the line-reaming has been completed (image above left) new Super Oilite® Phosphor Bronze bushes (image above right) are pressed into the body, and then
checked for both fit, and alignment, with a special plug gauge which is long enough to pass through
the carb body, and simulate the shaft.
NOTE: We never use oversize throttle shafts. If the bushes are worn we replace them, and use new standard size shafts. The goal here is to bring the carb back to as close as possible to the way it left the factory. If you are contemplating using oversize shafts; remember you can only ever do it once. On HD8 carbs we ALWAYS replace the PTFE bushings, and at no extra cost.
An example of a frozen piston
Checking shaft bores using a plug gauge
Beyond this point, the design, and therefore the order of disassembly, varies between the various types of carbs, but instead of getting these differences, we will focus on the key issues that are important to the functionality of the carburetor.
Now the suction chamber and piston are apart, we can make sure there is no damage to either the bore of the chamber, or the outside diameter of the piston. The fit here is critical, which is why these are only sold as matched sets. Minor damage can usually be repaired, and if this is required it is taken care of at this point. Once the mechanics of the repair are taken care of, the fit between the chamber and the piston must be restored to ensure: a) smooth operation over the length of the stroke, and b) and that the drop-test results are within specification. Each size range of carburetor has a different rate at which the piston should fall. For example, a carburetor with a bore size of 1 ½”” or greater, should fall within 5-7 seconds. To measure this, the two piston transfer holes are temporarily plugged, the assembly inverted, and the time for the piston to fall, carefully timed.
Preparing for the drop-test
The float chamber alignment lug referred to in the text below
We now turn our attention to the float chamber.
We do not rely
on the fact that the float does not appear to contain any fuel; it could
been sitting unused on a garage shelf for years, before we see it.
Regardless of the type, the float is removed, weighed on a digital scale
accurate to 1/10 gram, and then subjected to a
pressure test for 24 hours. Afterwards, the
float is weighed again to ensure it truly is leakproof. A
leaking float is discarded, and replaced.
The needle valve is removed, and
discarded. Float chambers
are examined to ensure they are undamaged, even a small nick in one of
the mating surfaces can result in a fuel leak. On HIF-series carbs the
rubber sealing ring to the bottom flange of the body is removed, and
discarded. On HS-Series carbs we also make sure that the lug cast on to
the side of the carburetor body, is intact. Occasionally, these are
broken off, which means there is no way to align the float chamber at
the correct angle. We can repair these, and this is offered as a
service. The rubber adapters (which actually determine the float chamber
angle) are checked, both to ensure they are the correct type (angle)
for the application, and also for any deterioration. The rubber adapter
is used to reduce the amount of vibration that is transmitted to the
float chamber, which would result in frothing of the fuel.
The jet is removed and discarded, as are any washers, or sealing rings, associated with it. With the H-Series, all of the jet components that are retained for the rebuild, are carefully examined for any damage before being put aside for cleaning. The HD, HS, and HIF series jets are both one-piece items, which are always replaced.
The HD Jet
The image below shows an HD-series Jet, which is very different from any other SU jet. The one on the left is a new, correctly manufactured jet with a Viton diaphragm, the one on the right, a used aftermarket version. Notice the way the inferior rubber diaphragm has warped, and curled, causing it to leak. The high ethanol content of the fuel today demands the best quality materials; in this example, the blue-colored rubber diaphragm gives it away, the higher quality items are always black.
I have also found when removing aftermarket jets, the length of the the jet tube is incorrect; often by as much as an 1/8", causing severe mixture problems.
At this point the disassembly is complete. Every
component is then cleaned in several stages, and using different
methods, depending upon the material it is made from. All steel and
aluminum components are glass-beaded to restore the finish, and this
process has the added advantage that it actually seals the pores of the
material. The last stage of cleaning for every component is an agitated
alcohol wash, before being routed to the air-conditioned assembly area,
as a boxed kit.
The first stage in the assembly process, regardless of carburetor type, is to install the new throttle shaft, and butterfly. On the H-Series, HD's, and some HS carbs, the throttle stops are secured to the shafts by a tapered pins. These must be driven out, and the new shaft drilled to take the new pins. The important part here is to ensure the stop is in the correct position on the shaft, prior to drilling the hole. We never re-use the original pin, and always replace it with a 1/8" diameter roll pin to eliminate any slack that would have existed.
It is important that the butterfly fits snugly in the venturi. They are not round, are chamfered on the circumference, and will only fit one way. Once in place, we fit the screws, but do not tighten them. We use a halogen lamp through the bore to ensure there is no light showing around the circumference of the butterfly, and then hold it tightly closed, while finally tightening the screws. We then check again with the light, before opening up the split ends of the screws to prevent them backing out.
We do not rely solely on the split screws to secure the butterfly in the throttle shaft. In addition, we also use (blue) removable thread-locker to ensure the screws never back out, which can have serious consequences for the engine, particularly at high speed.
As part of our attention to detail, we always use brass split pins on the H-Series, and earlier carbs. These are perfectly satisfactory for these low-stress applications, and enhance the appearance of the finished carbs.
To be continued as I find the time to write...........
In the meantime:
you have any questions about our services, or you simply have a
question about your carburetors, please don't hesitate to e-mail me; I
will help in any way I can.
You can e-mail me at firstname.lastname@example.org or, if you are stuck and could use some quick help, call or text my mobile at (336) 587-9623 (09.00am - 10.00pm Eastern Time
British Classic Motors - More than 40 years of SU Carburetor Rebuilding & Restoration experience..............