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Setting the mixtures & idle speeds on the flow-benchOur Services

Most of the SU carburetors we see today are of course at least forty something years old, and many of them have received little if any real maintenance, and quite possibly, a fair amount of abuse.   

Here's a quick summary of the work we do, followed by the process in more detail:
1)  New gaskets, seals, jets, and Viton-tipped float valves  
2)  New standard-size throttle shafts
3)  New stainless steel float chamber, and suction chamber screws (where appropriate)
4)  Polishing of suction chambers *
5)  Glass-Beading of all components
6)  Wet-Testing, setup, and flow bench testing to allow easy installation,
     and initial start-up - (includes air-flow balancing of all multiple carb setups)
7)  Easy-to-Follow Setup & Tuning instructions are available for download
8)  Ongoing Tech Support  

* On all H-Series carbs, we also polish the brass throttle stops/levers, and choke levers.


Have a question? Maybe I can help. Call or text me (336) 587-9623

A Quick Overview of the Process 

The first thing we do, after we receive your carbs, is to assign a unique work order record, and then thoroughly clean the exterior, before any dis-assembly is carried out. 

As the carburetors are being dismantled, every component is carefully examined for wear, and a list made of every component that needs replacing. We do this because we often find something that needs replacing, or repairing, that is not included in the standard rebuild kits.

NOTE: On HS-series carbs, there is a small rectangular lug cast onto the side of the body, which fits into the rubber adapter used to determine the correct angle of the float bowl. If you have one which is broken off, we can repair it. More information can be found in our SU Pricing & Order Form (see section D)

This stage also includes determining whether or not the throttle shaft bushings require replacing. The image on the right shows a typical worn throttle shaft. This is the type of wear which will create a rough, and inconsistent idle, as air is drawn in around the shaft.

We never use oversize throttle shafts. Either new throttle shafts are all that is required, or we line-ream the carburetor bodies, and press-fit new Super Oilite® Phosphor Bronze bushes. These bushes are manufactured to extremely tight tolerances, and contain 19% SAE 30 oil for very long life.

While some rebuilders do offer the over-sized shafts, together with reaming the original cast-in bushes to suit, please remember that this process can only be done once; the next time they wear, the bushes will have to be replaced. 
An example of a worn SU carb throttle shaft                   Typical worn shaft

The Glass-Beading Process

Once the disassembly is complete, every single component goes through a multi-stage cleaning process to remove all traces of its past life. The processes differ of course depending on whether they are brass, aluminum, or steel components, but the final stage for all is an agitated alcohol wash to remove any trace of contaminants, before they are moved into the air-conditioned final assembly area.

The carburetor bodies, float chambers, etc. are all glass-beaded to restore the finish to as close as possible to that of the original, they too then receive the same agitated alcohol wash. The images below show the results of the cleaning, and glass-beading process.

This is a "Before & After" shot of an HIF4 body, from an MGB. The one on the left is typical of a carb just received, the one on the right is after the thorough cleaning process, and the final glass-beading, immediately prior to re-assembly.

Finish Detail

Here's a close-up shot of the bead-blasted finish. We use very fine glass beads (they have the texture of flour) and this process actually seals the surface of the metal; which means they are much easier to clean; an oily smudge can be simply wiped off.

Suction Chambers

The suction chambers and pistons are a matched set, and that is how they originally left the factory. These two items cannot be changed independently of each other; in fact SU will not sell you one without the other. The reason? The fit between them is absolutely critical to the correct operation of the carburetor. If the fit is too loose the piston will rise too quickly under acceleration, causing a excessively weak mixture, if the fit is too tight it will lift too slowly, resulting in an overly rich mixture. To ensure this fit is correct we conduct a bench “drop-test” and accurately time how long the piston takes to fall a specific distance within the suction chamber. This time is then checked against the factory specifications to ensure all is well. If not, adjustments to the fit are made to bring the assembly back into spec.

A Polished SU piston ready for reassembly

We also refinish the pistons, which dramatically improves the appearance, as you can see in this image.

This work is always done prior to the "Drop-Test" to ensure there is not change in the performance.

The suction chambers are then rotated at high-speed in a custom-built fixture, and wet-sanded to prepare them for final polishing. Various grades of emery cloth are used, depending on the condition of the actual piece, but the final finishing is done with 1000 grit size paper. Once the wet-sanding is completed, they are machine polished to a high finish. Once the polishing is completed, the suction chambers and pistons go through the same agitated alcohol wash.  Machine polishing of an SU carburetor suction chamber

ORDER NOW: To go directly to our Pricing Schedule & Order Form section, please click here

The Reassembly Process

All assembly, and testing work, is carried out in an air-conditioned environment, adjacent to the main workshop area.


Every component is reassembled in a strictly adhered to sequence. Here we see the new throttle shaft, and disc after installation. Each disc must close completely to ensure a slow, smooth, idle.

NOTE: In addition to the traditional mechanical method of securing the new throttle disc screws, we also apply removable thread locking liquid as an added security measure.

Once installed, we check each assembly with a Halogen light through the bore, to ensure it is closing completely.
New SU Throttle Shaft & Disc after installation

A new SU Float Valve installationHere, the new Float Valve has been installed. Once the carb has been fully assembled, we do a full wet-test to ensure it closes completely, and there are no leaks.

The new Jet Assembly after installation, with the choke pickup lever attached.

One way to identify a genuine SU Jet Assembly is the steel braided hose cover from the float bowl to the jet, and the color identification rings. Many aftermarket kits use a simple piece of push-on fuel line, and a plastic float bowl connector.
The SU Jet Assembly after installation

Checking that the new SU jet is correctly centered.

Once the Jet has been installed, it must be centered to ensure the piston will fall smoothly, and completely. Here the setting, and the smooth operation of the choke assembly, is being checked.
The finished carburetors are then wet-tested to ensure there are no leaks, set up to the original factory settings for each specific application, and then tested on our flow-bench before being packed ready for shipment. Multiple carb sets are also air-flow balanced on the bench at this time.

Once you have reinstalled your carburetors, all that is normally required is to just re-check the fuel mixtures, following the simple step-by-step instructions provided, and then set the idle.

For more details of the entire rebuilding process, click here to go to our "It's All in the Details" section

ORDER NOW: To go directly to our Pricing Schedule & Order Form section, please click here

* GUARANTEE NON-TRANSFERABLE: Applies to original customer only - no longer applicable should either the carburetor(s) or the vehicle be sold.

British Classic Motors - More than 40 years of SU Carburetor Rebuilding & Restoration experience..............

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