Tuning SU carburettors on a Jaguar XK

Almost everything written about tuning SUs is written with confidence. A surprising amount of it disagrees with the rest, and some of it disagrees with the manufacturer. This page gives the procedure, names its source at every step, and says plainly where the record is divided — including two places where SU's own printed manual and Burlen's current website give different numbers.

Burlen own the SU marque and make the parts. Where they are quoted below it is as the manufacturer, not as one opinion among several.

For a checklist written on request from the symptoms you describe, see SU carburettor troubleshooting — its text is generated when you submit, not quoted from a published source.

Which carburettors an XK140 has

Twin SU H6, 1¾ in. That is the standard fitment and every source agrees. The H8 is the 2 in carburettor. The SU type number decodes as eighths of an inch over one inch, so H6 is 1¾ and H8 is 2 in exactly.

Were the 2 in H8s part of Special Equipment?

It is very widely stated that they were. The factory's own wording points the other way.

Service Bulletin 168, June 1955, issues a needle recommendation for "XK140s with C type heads, standard carbs, and disc type air cleaners". A bulletin phrased like that is only coherent if C-type-head cars normally left the factory on the standard carburettor. It does not define "standard carbs", so this is an inference — but it is an inference from a factory document.

It points the same way as a physical identification: Roger Payne examined the carburettors from an XK140 MC engine, number G3614-8S — C-type head — and identified them as H6 units correct for a car of that date.

Four positions exist:

A further reason to doubt the first: Wikipedia's own XK120 article states the XK120 SE kept the H6, and that H8s appeared only on dealer-modified C-type-head racing cars. The two Wikipedia articles are inconsistent with each other on precisely this point — and "SE included 2 in H8s" is the line most often repeated.

How to tell whether a given car ever had them. An H8 installation requires a mechanical distributor, a special coil mount, a mechanical choke and cable, a blanking plate over the otter switch mounting on the inlet manifold, longer carburettor studs, manifold openings machined out to two inches, and a unique fuel intake line. One owner checked his own car against that list and found none of those parts present — so the H8s had never been on it.

That list matters for the rest of this page. A standard H6 XK140 has no mechanical choke, because cold-start enrichment is by a separate auxiliary starting carburettor with a thermal switch.

How an SU actually works

A fixed-jet carburettor has a venturi of unchanging size. Because the depression across it varies enormously with speed and load, such a carburettor needs a battery of separate circuits — idle jet, main jet, power jet, accelerator pump, air correction — each covering a slice of the range, and each a separate thing to get wrong.

The SU does the opposite. Burlen date the idea to George Skinner's 1905 patent for a carburettor in which both the air and the fuel passages varied together.

The venturi is not fixed. It is the gap between a moving piston and the bridge. As airflow rises, the depression increases, the pressure above the piston falls, and the piston is pushed up — enlarging the venturi until the depression returns to its nominal level. The depression therefore stays roughly constant at all speeds and loads. Hence the name: constant depression, variable jet.

The piston is a light alloy slide running in the suction chamber. Its upper face sees manifold depression, its underside near-atmospheric. Because it is a pressure-balanced device rather than a mechanically driven one, anything that adds friction to it corrupts the mixture across the entire range. This is why Burlen's servicing instruction is to clean the suction chamber bore and the two diameters of the piston with a cloth moistened in petrol or methylated spirit, and to oil only the piston rod. No abrasives. No oil on the piston skirt. No polishing.

The needle hangs from the underside of the piston into the jet. As the piston rises the needle withdraws and the annular gap grows. Since the depression pulling fuel from the jet is constant, fuel flow is set almost entirely by the needle's diameter at whatever height the piston has reached. The needle profile is the fuelling map — one component covering idle, cruise and full throttle.

The damper is a small plunger in the oil-filled cap, working in the hollow piston rod, and it resists upward movement far more than downward. Without it, a snapped-open throttle would let the piston fly up, the venturi would enlarge before fuel flow caught up, and the engine would go momentarily weak. The damper is the SU's accelerator pump, and its oil grade is a tuning variable rather than a service consumable. Too thin gives a flat spot on snap throttle; too thick gives an over-rich transient every time.

Note that some SU carburettors have a plain cap and no damper at all. Those still want oil, but it only lubricates the guide tube — a flat spot on such a carburettor is not a damper fault.

Three consequences shape everything below:

1. There is one mixture adjustment per carburettor — the jet height relative to the bridge. There is no separate idle mixture screw. Changing jet height moves the whole fuelling curve; changing the needle changes its shape.
2. Mechanical condition is mixture condition. A sticking piston, an off-centre jet, a worn needle or the wrong damper oil each falsify the mixture continuously. On a fixed-jet carburettor these would be separate faults. On an SU they are the same fault.
3. The carburettor is a depression-measuring instrument, so it faithfully reports the engine's faults. An air leak, a burnt valve or retarded ignition change the depression it sees, and it obediently mis-fuels in response.

What to fix first

Burlen's tuning instruction opens by requiring the tuner to rule out valve clearance, spark plug condition, contact breaker dwell, and ignition timing and advance before touching a carburettor.

The reason is consequence three above. A badly timed engine presents the carburettor with the wrong depression, the carburettor fuels for what it is told, and the tuner then adjusts the jet to compensate for a fault that is not in the carburettor. The result is a car that runs acceptably at the one condition it was tuned at and badly everywhere else — and a jet setting that will be wrong the moment the real fault is fixed.

Order:

1. Compression test — a burnt valve or a tired bore cannot be tuned around.
2. Valve clearances. On an XK140, 0.004 in inlet, 0.006 in exhaust, per Service Bulletin 156 of November 1954.
3. Points, plugs and leads.
4. Ignition timing and the advance mechanism.
5. Air leaks.
6. Then the carburettors.

The tuning sequence

1. Damper oil

SU and Burlen specify monograde SAE 20. SU publication RZX 1002, Type H Carburetters Tuning & Servicing, gives "thin engine oil grade S.A.E. 20".

Check both dampers hold the same oil, at the same level, of the same grade. Unequal dampers give a car that idles perfectly and hesitates unevenly under acceleration — and balancing at idle will never reveal it.

2. Check the pistons fall freely

With the jet screwed fully up, raise each piston by its lifting pin and release it. It must fall freely onto the bridge with a click. Burlen require this with the jet fully up and with the jet lowered.

A soft landing, a hesitant fall or no click means either a dirty piston and suction chamber, or a jet out of centre. Go no further until it clicks.

3. Centre the jet

The step most often skipped, and the one with the largest effect. If the jet orifice is not concentric with the needle, the needle rubs, the piston hangs, and the mixture is wrong at every throttle opening.

Burlen's procedure: remove the jet control linkage where fitted; mark for reassembly and withdraw the jet, removing the locking spring; replace the adjusting nut and screw it fully up; reposition the jet with the head aligned to the control lever; loosen the locking nut so the bearing can float; remove the piston damper and apply light axial pressure to the top of the piston rod — SU specify a pencil, the point being a light non-marking push so the needle seats itself and drags the bearing into alignment; tighten the locking nut with the jet hard up against the adjusting nut; re-test the drop, reassemble.

Verify at both extremes. The piston must fall freely with the jet fully up and fully down. If it falls freely only with the jet down, it is not centred.

On an XK140 the access is genuinely awkward. Owners report reaching the jet height screws by removing a wheel arch, working from above with a shortened screwdriver, marking a screwdriver with paint for reference, or adjusting by feel. Several note the fixed head coupe is worse than an XK120.

4. Float level

Set before mixture — a wrong float level shifts the effective jet height and defeats everything after it.

Burlen's H-type figure is a 7/16 in (11 mm) round bar between the forked lever and the lip of the float chamber lid, with the prongs just resting on it. Any rod of the right diameter will do — a bolt or a drill shank.

A useful diagnostic pattern. If one bank of cylinders sooties and the other does not, suspect a float level or jet leak on that carburettor rather than a needle or a general mixture error.

5. Initial jet position — and here the manufacturer contradicts itself

Screw each jet adjusting nut up until the jet is flush with the bridge, then turn it down by a set amount to get the engine running. The amount is disputed, and not by amateurs:

We cannot resolve this and neither should you. The two-turn and one-and-a-half-turn figures come from the same company — SU's printed H-type manual against Burlen's current H-type web page — and we found no statement of a revision. Burlen's HS page still says two turns while their H page says 1.5, so it is not a blanket policy change.

What every source agrees on is that this is only a starting point. The final setting is arrived at by response, not by counting turns. Set both jets the same — on an XK140 there is no interconnection to enforce it.

6. Balance

Balance before mixture. Disconnect the throttle interconnection so each throttle can be set independently on its own idle screw.

Listening tube. Compare the intensity of the intake hiss at each carburettor through a short length of rubber tube held to the ear. The traditional method, and the one the Jaguar manual uses. Harper is candid that it is inaccurate for the non-expert.

Piston height comparison — free, and specific to SUs. Remove the suction chamber caps and attach a piece of stiff wire bent into an L to each piston rod. Airflow is balanced when the two indicators sit level. This works precisely because of the constant-depression principle: equal depression means equal piston height.

Airflow meter. Bobbin meters, rotor meters with a pointer scale, and battery anemometers. Harper prefers anemometers, but warns the electronics can be confused by interference from a nearby magneto.

Vacuum gauge. Note that on a twin installation with a balance pipe a manifold gauge measures the pair, not each carburettor.

When balanced, tighten the coupling, then move both idle screws by the same amount to set idle speed.

7. Mixture, by the lifting pin

Lift the pin 1/32 in (0.8 mm), taken up after the pin's own free movement.

Why it works: lifting the piston enlarges the venturi and reduces depression at the jet, weakening the mixture momentarily. If the engine was slightly rich, weakening improves it and revs rise and stay. If correct, the small weakening gives a brief rise as extra air arrives before fuel falls off, then settles. If already weak, weakening further drops revs at once.

The iteration order on a twin installation is the part most often got wrong. Harper's loop:

1. Do the lifting-pin test on the front carburettor. Note the response and adjust nothing.
2. Do it on the rear. Note the response.
3. Adjust both according to the combination.
4. Repeat until one gives the correct response.
5. Leave that one alone and adjust the other on its own.
6. Re-test the first to confirm.

His critical caveat: if lifting the front piston has no effect at all, the rear must be weakened — no response means the engine is effectively running on the other carburettor alone. Exactly this has been reported on an XK.

8. Idle, and the interaction

Burlen's figure for H types is 500 to 600 rpm. That is the carburettor maker's default. We could not establish Jaguar's own idle figure for the XK140.

Mixture and idle are coupled, because enriching from weak raises idle speed and enriching past correct lowers it again. So: balance at idle with the interconnection disconnected; set idle roughly with both screws equally; set mixture by the lifting-pin loop; reset idle with both screws by the same amount; re-check balance; re-check mixture. Normally two or three passes.

Do not chase idle speed with the jet nuts. The jet nut is a mixture control that happens to affect idle. The throttle screw is the idle control.

9. Linkage, and cold-start enrichment

With balance set, tighten the spindle coupling, leaving a small clearance so neither carburettor is pulled open before the other. Speedsport give 0.012 in (0.30 mm) between the link pin and the lower edge of the fork; Burlen's H-type page gives no figure, only a geometric instruction. These are different instructions and we could not establish which applies to the XK140's rod-and-coupling arrangement.

The safe check: after connecting the accelerator lever, re-check airflow is equal at higher speeds. If balance holds at idle but goes off as the throttle opens, the linkage is at fault — or the throttle spindles are worn, which presents identically.

Enrichment on a standard XK140 is the auxiliary starting carburettor, not a choke. A solenoid-operated valve and a fuel metering needle, with a thermostatic switch in the cylinder head coolant jacket calibrated to operate below 35°C. Manifold depression positions the needle, so it weakens itself automatically as the throttle opens.

Adjustment is by one screw only — the stop nut limiting the needle's downward movement. Burlen: bring it into operation, blip the throttle to lift the valve, turn the stop nut clockwise until running becomes erratic, then back off until the engine sits between 800 and 1,000 rpm with noticeably black exhaust. Experienced owners add that the stop turns eight full turns but is better left with two still in the body, that up enriches and down weakens, and that it is worth setting it down for summer and up for winter.

One fault deserves prominence. A stuck-open auxiliary starting carburettor presents as a persistently over-rich engine that no jet setting will cure. The usual causes are the otter switch or a sticking solenoid. Verify the enrichment is completely inoperative before doing the lifting-pin test.

Needles

The needle profile is the fuelling map, and the factory specified it by carburettor part number, not by car model. Identify the AUC number stamped on the carburettor body before ordering anything.

Jaguar Service Bulletin 206, January 1957:

Service Bulletin 168, June 1955 records the change that produced the C-type figure: for XK140s with C-type heads, standard carburettors and disc air cleaners, the recommended needle became WO2, having been SL, from engine G3250 and some earlier engines — made to cure spitting back, and applying whether 7:1 or 8:1.

Note the factory's two statements about SL read oppositely: in 1955 it is the needle being replaced, in 1957 it is the weak alternative. Both are recorded here as found.

Corroboration from the marque catalogues, which also show the AUC dependency:

The SJ/WO2 split matches Service Bulletin 206 exactly. SR for AUC737 does not appear in the bulletin digest and we could not establish which application it corresponds to.

For the 2 in H8, the catalogues give VR (220339) standard, 75 (493460) rich, VE (501868) weak.

A warning before changing needles. Needle choice is the most heavily disputed area in SU tuning and depends on the entire engine specification — compression ratio, camshaft, cylinder head, exhaust, air cleaner type and the fuel in the tank. Service Bulletin 168 specifies its needle for disc type air cleaners particularly. Start with the factory profile and change only if symptoms persist after everything else is right.

Faults

Burlen's own fault table for H types:

Note that the same four symptoms cover four different causes in that first row. The symptoms do not discriminate; the drop test and the centring check do. That is why they come first.

Sticking pistons. Varnish and dirt in the suction chamber bore, or an off-centre jet. Clean with petrol or methylated spirit only — no abrasives — and oil the piston rod alone. Oiling or polishing the skirt makes it worse and makes it permanent.

Worn jet or needle. Because the needle hangs in the jet without positive centring, any mis-centring wears a flat on the needle and bells the jet orifice. A worn needle is not repairable. Fit a new one and re-centre the jet, or the new needle will wear the same way. Burlen changed the needle material from brass to nickel silver.

"Perished diaphragm" — check which car you have. The H-type carburettor on the XK120 and XK140 has no diaphragm in the carburettor body. Its jet is sealed by cork glands — which is why Burlen's H-type fault table lists top and bottom gland failures. Diaphragms appear in the HD-type on the XK150, in SU electric fuel pumps, and in Zenith-Stromberg CD carburettors if fitted as a conversion. So on a standard XK140, a perished diaphragm is a fuel pump fault, not a carburettor fault.

Air leaks. High or hunting idle, an idle that will not come down, a mixture that reads weak however much you enrich it, or a balance that cannot be achieved. Places to check: the manifold-to-head and carburettor-to-manifold joints; both ends of each throttle spindle; the distributor vacuum advance take-off on the front carburettor; the auxiliary starting carburettor's fittings under the inlet manifold; and the balance pipe if fitted.

Worn throttle spindles — the fault that most often defeats an otherwise correct tune. The signature is carburettors that appear well balanced at idle only to go completely out of balance at the touch of the throttle. Three tests: engine off, rock each spindle in its bushes — any perceptible radial play means worn bushes; the leak test at each spindle end; and balancing carefully at idle then re-checking at a higher steady speed. The fix is dismantling and rebuilding with oversize bushes, new shaft, butterfly, seals and gaskets, with the holes reamed. Burlen sell a spindle bush fitting tool, which suggests the job is within reach at home given the tool. The reaming may not be.

Wrong damper oil. A flat spot on snap throttle means too thin or empty; sluggish, over-rich response on every transient with poor economy means too thick. Because the damper is the accelerator pump, these are fuelling symptoms that no jet adjustment will cure.

Ethanol, unresolved

This archive has already published a claim about ethanol and carburettor floats as though it were settled, and had to correct it. It is not settled. What follows sets out the positions and attributes each one. It does not adjudicate, because nothing in the public record would justify doing so.

Burlen — the manufacturer, and the largest single repository of failed parts.

No damage has been seen with the traditional brass floats and there have been no signs of biofuel attack on the solder used to join the upper and lower sections of a brass float.

They have nonetheless changed a good deal else: rubber fuel hoses to Viton, needle valve tips to Viton, fuel pump diaphragms to Viton, and the metering needle from brass to nickel silver. They have developed plastic floats tested in E85. They state that biofuels have adverse effects on some metals including brass and copper, and that there is no additive that fixes material compatibility. Cork, and fibre or paper washers, are not mentioned in that article at all.

Guy Lachlan, Classic Oils, quoted in Classic & Sports Car — a direct contradiction on the exact point:

The other thing ethanol really doesn't like is solder. If you are running a soldered float in your carburettor then think about carrying a spare.

He is a fuels specialist and a trade body officer, not a forum poster.

Neil Cairns, MG Car Club Y-Type Register, goes further and offers a mechanism: that ethanol dissolves the lead and tin in the solder joining the two halves of a brass float, un-solders joints in fuel pipes, and — directly relevant here — dissolves the glue binding the cork particles together in jet tube seals.

In fairness to both sides: Cairns cites no experiments and no measurements; his article is assertion and reported observation. But Burlen's statement is also observational — "no damage has been seen" — rather than experimental. Neither position rests on published testing.

Why this matters more to XK120 and XK140 owners than to almost anyone. The H-type carburettor seals its jet with cork glands. The XK150's HD type does not. If the cork claim is right, it bears on this car specifically; if Burlen are right, it does not. The archive cannot tell you which, and anyone who tells you confidently should be asked what their evidence is.

What this page cannot tell you

• Jaguar's own idle speed figure for the XK140. The 500 to 600 rpm above is SU's default, not Jaguar's.
• Whether the initial jet setting is two turns or one and a half. The manufacturer's printed manual and current website disagree.
• Which throttle linkage instruction applies to the XK140's rod-and-coupling arrangement.
• What application carburettor AUC737, and therefore the SR needle, corresponds to.
• The float level figure for an XK120 — one experienced source says 1/2 in rather than 7/16 in, unverified.
• Whether ethanol attacks solder or cork, on which named specialists directly contradict the manufacturer.
• Any tolerance figure for throttle spindle play.

The XK140 Service Manual would settle several of these and this archive has not read it. Every Jaguar-specific figure on this page comes from the Service Bulletin digest, from marque parts catalogues, or from named individuals — none from the factory manual directly. Check anything here against your own copy before acting on it.