This post follows a pair of HD8 SU carburettors through our workshop — from arrival to completion. The carburettors came from a 4.2 litre Daimler Sovereign, a car that deserves nothing less than a proper professional rebuild.
[INSERT PHOTO 1: The pair of HD8s as received — front view showing throttle flanges and suction chambers]
The Car
The 4.2 litre Daimler Sovereign is one of the most refined expressions of the Jaguar XK engine — a straight-six that powered some of the most important British cars of the post-war era. In Sovereign specification it was tuned for smooth, effortless torque rather than outright power, and the twin HD8 SU carburettors were central to that character. Get the carburettors right and the engine pulls cleanly from idle to the red line without a stumble. Get them wrong and no amount of tuning will fix the flat spots, the rich running, or the hunting idle.
These carbs had done a long and honest stint of service. The owner wanted them properly rebuilt, not patched up — which is always the right call on a car like this.
How They Arrived
The carburettors came in as a matched pair, as they should — HD8s are always set up and balanced together. On first inspection they were complete and structurally sound, which is the best possible starting point. The bodies showed the usual patina of decades in service: surface corrosion on the aluminium castings, old gasket residue on the mating faces, and the kind of general grime that builds up over years of heat cycling and fuel vapour.
[INSERT PHOTO 2: Side profile of the pair showing SU body castings and overall condition before strip]
The HD8 is one of the largest carburettors SU ever made — 1¾" bore — and was fitted to some of the most prestigious British cars ever produced. On a 4.2 Daimler, the pair work in concert to feed a 258 cubic inch engine that was engineered for smoothness first. When they are right, you barely know the engine is working. When they are tired, the car simply does not feel like itself.
Strip-Down
The first stage is a full strip to bare castings. The suction chambers come off first, then the pistons and needles are removed and set aside for inspection. The throttle spindles are checked for wear — a worn spindle is one of the most common causes of air leaks and rough idling on any SU, and on an HD8 the spindle is a substantial item. The float chambers are removed, drained, and stripped.
[INSERT PHOTO 3: Stripped carb body on the bench with suction chamber removed, retaining ring visible]
Once stripped, every component goes into the ultrasonic cleaner. This is where the real story of a carburettor's life comes out — years of varnish deposits from evaporated fuel, emulsified oil contamination, and in some cases the residue of old ethanol-blend fuel that has attacked the original rubber and left debris throughout the fuel galleries.
What We Found Inside
The needle valves told the usual story. After decades of use the valve seat showed the characteristic wear groove where the needle tip had been seating, and there was varnish build-up around the seat that would have been restricting fuel flow and affecting float chamber level. Left in place, this causes an over-lean condition that is almost impossible to tune out.
[INSERT PHOTO 4: Close-up of needle valve area showing varnish deposits and wear on the seat]
The float chamber gaskets had long since hardened and were no longer sealing correctly. On ethanol-blend fuels this is almost universal on original rubber — the alcohol content attacks natural rubber compounds over time, causing them to swell initially and then harden and crack. Both sets were replaced as standard.
Reassembly
With the castings ultrasonically cleaned, bead blasted and repainted, reassembly begins with new components throughout: needle valves and seats, float chamber gaskets and seals, throttle disc seals, dashpot O-rings, and jet gland seals. The needles are checked against the original specification and replaced if worn.
[INSERT PHOTO 5: Carb body during reassembly — internals exposed showing fuel gallery and rebuilt components]
The throttle spindles on this pair were within tolerance and were retained — when spindles are worn we rebush or replace them as required. The pistons were cleaned, lapped lightly to ensure they move freely in the suction chambers without side play, and refitted with new O-rings.
[INSERT PHOTO 6: Choke spindle and throttle mechanism area during strip showing the mechanical detail of the HD8 body]
Setting Up
Before leaving the workshop, the pair are set up on the bench: float levels checked and adjusted to specification, jet centred, and the mixture screws set to the baseline position. Final fine tuning is always done on the car with the engine at temperature, but getting the base settings right in the workshop means the car will run from cold and there is a reliable starting point for road tuning.
The Result
A properly rebuilt pair of HD8s will outperform a new pattern replacement carburettor in almost every respect. The original SU castings are robust and dimensionally stable — there is no reason a well-maintained set of HD8s should not last another fifty years. What they cannot do is compensate for worn internal components, and that is exactly what a full refurbishment addresses.
The Daimler Sovereign is a car that rewards proper maintenance. The XK engine is one of the most tuneable and longest-lived units ever produced, and with correctly rebuilt carburettors it will pull smoothly and cleanly in a way that a poorly running pair simply cannot replicate.
If you have a pair of SU carburettors that need professional attention — whether HD, HS, HIF or H type — our refurbishment service covers the full process described here. Every job includes a full strip, ultrasonic clean, bead blast and repaint of the body, and new wear components throughout, with a 2-year guarantee on all fitted parts.
Interested in a refurbishment for your carburettors? Find out more about our service and pricing here.