SU Carburettor

This is a very basic guide to maintaining SU carbs. The XK120 and XK140 are fitted with a

pair of H6 SU carburettors - H defines horizontal and the 6 denotes the bore size. SU's go up

in size in 1/8" divisions so that an H2 has a 1 1/4" bore, an H4 1 1/2" and in the case of the

120 and 140 the H6 which is an 1 ¾" carb. The 150s are fitted with the more complicated

HD6s and HD8s which I will cover in a future issue.

The Basic SU Carburettor

The basic SU carb is very simple but extremely cleverly designed, with a variable venturi that

ensures both performance and economy. As with all good designs the SU has few moving

parts. The largest and by far the most important is the piston, which is what governs the size

of the venturi and therefore maintains the wonderful driveability at low engine speed.

Suction chamber ( Dash pot )

Inspecting the piston and suction chamber is the first job, making sure they are scrupulously

clean. Check the larger diameter of the piston, where the small groves are, for scores. Do not

attempt to remove these scores, just remove any burrs formed at the side of such marks. The

piston needs to seal in the suction chamber, which can also suffer from scoring and will need

the same treatment. Fine wet and dry can be used to remove any burrs. The grooved part of

the piston should never touch the side of the dash pot and is guided by a chrome piston rod

that runs in the suction chamber. This should look like new to ensure that the piston moves

smoothly up and down the dash pot. (The piston and chamber were probably matched at the

factory, so it is good practice to avoid swapping pistons and suction chambers)

The Damper

The damper has the job of enriching the mixture on acceleration. This is done by retarding

the rate of acceleration of the piston and causing a greater depression in front of the jet - in

short, reducing the air and increasing the fuel. Check that the damper is not too long and that

the piston can go fully into the dashpot when assembled. On most H6s there should be a

small vent hole in the top of the damper. If not, drill a 1/6" hole to one side of the top.

Main jet & needle

The main jet should have a perfectly round 0.100" hole in the top of it - if not, it will be

impossible to tune the engine. It should be changed if there is any doubt. It is harder still to

recognise wear on the needle so change it if you are not sure of its condition.

Centering the jet with the tapered needle centre (this should only need doing if new jets

are being fitted)

Remove the dash pot and piston, then adjust the jet so that it is flush with the top of the

bridge (the bridge is what the piston falls on to at its lowest point). Refit the dash top and

piston and let the piston fall slowly on to the bridge. The piston will probably stick just before

the bridge. The needle will be fouling on the main jet. Loosen the large nut under the carb,

which allows the jet to move, with the piston sitting now on the bridge re-tighten slightly, lift

the piston up and allow to drop again. Repeat this until the piston drops easily then tighten the

nut fully. Check again to make sure that the piston drops positively on to the bridge, then

adjust the jet back to the basic setting of 2 1/2 turns anti clockwise (down)

Float chamber

Check for any corrosion at the bottom of the chamber. Ensure that the drilling at the base of

the chamber is clear and wash with petrol. The float is soldered together and it is not

uncommon for the float to fill with petrol. Do not try to salvage this as it is a lost cause (get a

new one). Make sure that the float slides easily on the 1/4" centre pin.

The float level is governed by the float lever, which in turn shuts the needle valve and

reduces the amount of fuel. The needle valve is extremely important as it can cause

unwelcome flooding and petrol smells, so if there is any doubt as to its condition change it.

The float level is adjusted by bending the lever, it should be set with a 7/16" bar placed

between the lever and the float chamber top. (See sketch No1) The float chamber top is held

on by the centre 1/4" tube nut, which in turn holds the overflow pipe on. It is vital to use the

special fibre washer under the overflow (breather pipe) as this will cause severe flooding if

not used.

Throttle spindle and disc

The throttle spindle simply rotates in the throttle body and therefore the bushes in the body

need replacing occasionally. The disc will also wear. Inspect it and change it if necessary.

The bushes can be gently tapped out with a flat-ended drift and replaced in a similar way. If

you are careful, the bushes will not need to be reamered. The spindle should pass through the

bushes nicely. The disc can now be pushed into the slot of the spindle. The slotted screws can

be fitted loosely and the spindle rotated so that the disc is closed. Look through the

carburettor body to see if you can see a light past the disc. If so, move the disc around until

there is virtually no light seeping through, then tighten the screws. Check once again, then

bend the split ends of the screws apart. Never use second-hand screws.

SETTING UP THE CARBURETTORS

Cylinder compressions, spark plugs, contact breaker points, ignition timing, air filter and

condition of carburettors must be checked before any attempt is made to set up the

carburettors. Before adjusting the mixture it is good practice to set the jets to the

recommended level. Remove the dash pots and pistons, then the dome tube nut at the bottom

of the jet, screw the jet adjuster up so that the jets are level with the bridge. The basic setting

is then 2 1/2 turns anti clockwise. Refit the dash pots and start the engine.

The next job is to balance the airflow though the individual carbs. This can be done by

placing a rubber tube in the intake of the carb and listening at the other end so that the suction

can be heard. First loosen the clamp bolt of the universal jointed connector between the two

carburettors so that they are working independently of one another. Rotate the throttle

adjusting screws so that each carb is sucking equally and that the tickover is approximately

600 RPM. Tighten the connecting bolt and check the balance once again. To adjust the

mixture, turn the adjusting nuts up to weaken or down to enrich until the fastest idling speed

consistent with even running is obtained. The mixture can now be individually fine-tuned by

using the lifting pins on the side of the carburettor body, listen to the engine speed and lift the

pin slowly, the engine revs will either:

Increase (Rich Mixture)

Increase and then decrease (Correct Mixture)

Immediately decrease (Weak Mixture)

The carburettors are now set up. The mixture will be correct at all engine speeds, as long as

the original needles and the original engine set-up are used. Changes such as air filters,

camshafts, high compression pistons and large valves will dramatically change the air-fuel

ratio. Any one of the above can damage the engine, as too little fuel will cause increased

combustion temperature. This will burn pistons and valves. Too much fuel will wash the oil

from the cylinder walls and increase bore wear. If your engine is modified in any way, you

have no choice but to put your car on a rolling road. The mixture can then be checked at all

speeds and all throttle positions. Adjustment can then be made with a huge choice of needles.

Symptom Cause Remedy

Erratic running

Stalling at idle

Lack of power

High fuel

consumption

Sticking piston

Dirty piston and dash top

Jet out of centre

Worn jet or needle

Clean

Re-centre

Fit new

Hesitation on

pick up

Low damper oil level

Incorrect oil grade (too thin)

Top up

Replace with correct

grade

Float chamber

flooding

Dirty or worn float chamber

needle valve (dirty fuel)

Punctured float

Incorrect fuel level

Sticking float

Clean or renew

valve (flush system)

Fit new

Check and reset

level

SU H and HS carburettors.

1: Unscrew throttle adjusting screw until the face just clears the stop. Open by

screwing in 1 1/2 turns.

2: Remove Piston and Suction Chamber, disconnect mixture control wire, and screw

jet adjusting screw until jet is flush with carburettor bridge (or full up if you cant).

Replace parts, and check Piston falls freely on the bridge (by using piston lift pin).

Turn jet adjusting screw 2 complete turns (or 12 flats if accessibility and visibility is

restricted, eg on Minis!).

3: Start engine, and adjust to desired idling speed (eg, warning light glowing).

4: Turn jet adjusting screw to obtain fastest idling speed WITH EVEN FIRING.

5: As the engine speed may have increased, readjust Throttle Adjusting Screw.

6: Check mixture strength. Use Lifting Pin to lift piston about 1/32”.

o A) If the engine speed increases, and continues to increase, the mixture is to

RICH.

o B) If engine speed decreases, the mixture is to WEAK.

o C) If engine speed momentarily increases slightly, the mixture is CORRECT.

7: The exhaust note should be regular and even. If it sounds „splashy,‟ the mixture is

too weak. If there is a rhythmical misfire, together with blackish smoke, it‟s too rich.

8: Re-connect mixture control wire with about 1/16 inch free movement before it

starts to pull on the cam lever.

9: Set Choke knob (on dash) to max. movement (about 5/8 inch) without moving the

carburettor jet, and adjust the fast idle cam to give a fast tickover, about 1000RPM.

SU Tuning

This isn't the definitive guide, but it works for me. It‟s a combination of several different

pieces of literature and also personal experience.

Before you tune your carbs you need to make sure that points, plug gaps, ignition leads,

timing, valve clearances and the general fuel and exhaust system (i.e. leaks) are in good

condition. Otherwise it‟s a head ache to tune them.

For spark plugs I would recommend nothing else but NGK in your Triumph Spitfire...

BPR6ES Copper Core are probably the best... but BP6E & BP6ES are fine too.

NGK Technical Information Pages - lots of technical information on spark plugs

NGK UK Technical Information Pages - not as detailed as above - but with good examples of

plug colour and simple to understand information

Check that your needles are correct and also that they‟re properly in the pistons. Note:

Piston's work exclusively with their carb body - do not mix them up!

Make sure you have the correct springs and also that there‟s some oil in the dashpots. If you

are unsure of what oil to use, just use engine oil until you know any different - or can judge

what to use. A thinner oil will richen the mixture on acceleration.

1. Go for a hearty drive to get the car to operating temperature.

Go on! Drive it like you stole it!

2. Balancing the amount of air that is being taken in by each carb.

Take off the air filters.

Loosen the clamps (1) on the throttle linkage so that you can adjust each carb's

throttle separately.

Turn each throttle screw (2) the same amount until the car idles at around 800rpm.

Use the cross on the screw as a guide – I always turn them so that they end up at 1/8

increments. As a rule of thumb, for both the throttle screws and mixture screws

(which we‟ll get to later) – when looking from above – clockwise is to increase, anti-

clockwise is to decrease.

Next you need to adjust each screw individually until they are both letting in the same

amount of air – you need some sort of manometer for this - either use one you have

bought (follow the instructions) or make your own using a piece of windscreen

washer pipe - put one end in the carb the other in your ear – listen to the hiss –

constantly comparing the “note” of the hiss for each carb.

If the engine idles quicker after balancing, turn the screws anti-clockwise by the same

amount.

Before moving on to the next step, rev the engine up to about 2-3000rpm for a couple

of seconds. You should repeat this every 2-3 minutes to keep the engine warm. As the

throttle linkage clamps have been loosened the throttle pedal won't work, so push

evenly on the throttle actuators (arrows) with a finger on each.

3. Adjusting the mixture.

You will be using each carb‟s mixture adjustment nut (4) to change the amount of fuel

that the carb is mixing with the air. You need to determine whether or not the carb is

rich or weak – and adjust it accordingly. To do this you must use the lifting pin (3)

located underneath each carb.

Push in one pin at a time and listen to the engine speed. If the speed rises then the

mixture is too rich and you should turn the adjustment nut anti-clockwise whilst

looking from the top. If the engine speed drops then the mixture is too weak and you

should turn the adjustment nut clockwise to richen it. When the mixture is set

correctly the engine will momentarily rise and then remain the same. Turn the nut one

flat at a time until you get a feel for how much to turn the nut. Again - I like to turn

them one flat at a time - so then its easy to figure out how much to turn them next

time.

If your lifting pins are seized, use a screwdriver to carefully lift up the piston. Make

sure you only lift it up about 1 or 2mm at most. Obviously making sure you lift each

one up the same amount.

You now need to go back to step 2 and check the air flow of each carb – it is likely

that it will have changed. If it has then adjust the throttle screws and then check the

mixture again.

Now do up the throttle linkages – making sure there is a slight clearance so that the

throttle cable isn‟t pulling on the linkage the whole time.

4. Once you think everything is done, go for another drive.

Then remove the spark plugs and take a look at the colour.

I'm not sure what's up with the one on the far right, but the rest look as they should.

The mixture has ended up overall slightly too rich because the air filters lessening the

amount of air entering the carb - however an anti-clockwise turn on the mixture

adjustment nut should cure that.

5. Now all you need to do is adjust the choke and you're done...

Basic Service Guide Part 4

Tuning SU carburettors

Tools required

Screwdriver

Spanner (size)

Air filters

Sparking plug spanner

Jet adjusting spanner (early cars)

Length of tube or hose

Carburettor damper oil

Carbalancer tool

Colortune

Gastester

SU Carburettors The SU carburettor is relatively simple in design but very effective in operation. Often

underrated, the SU offers extraordinary versatility. In his book, 'How To Power Tune MGB

4-Cylinder Engines,' Peter Burgess points out that. "A correctly set up SU has more ability to

specifically alter fuel delivery to suit varying load and throttle positions, than a computer

controlled fuel injection system."

SU carburettors work on the constant depression or constant vacuum principle. Inside the

body of the carburettor is a piston, which reacts to the air flow into the engine. Attached to

the piston is a tapered needle which alters the fuel delivery as it rises and falls. This

combination provides variable amounts of fuel and air to cope with the changing demands of

the engine.

SUs are simple to adjust, but beware of the temptation to fiddle with them unnecessarily. If

your engine is not running correctly, always check your ignition system first of all. Most

engine problems are caused by incorrect timing or points out of adjustment. Only when you

are completely satisfied that the every component in the ignition system is

working correctly, can you turn your attention to the carburettors.

Twin HS4 carburettors with standard air filters.

Checking adjustment The colour of the sparking plug electrodes and the colour of the exhaust tailpipe provides the

most obvious guide as to how well your carburettors are adjusted. Take your MG for a short

run and thoroughly warm up the engine, then remove each plug in turn and note the colour of

its electrode. A good quality plug spanner is a worthwhile addition to your tool kit and will

enable you to take out the plugs without damaging them or burning your fingers. Too rich a

setting will give black sooty deposits on the plug electrodes, a weak setting will give white

ashen deposits; while the ideal setting is a pale biscuit colour. When replacing spark plugs

take great care not damage the threads in the cylinder head.

Carburettor adjustment Twin SUs are straightforward to set up, with three main areas of adjustment.

These are:

synchronising the carburettors, adjusting the mixture, setting the idle speed.

Before attempting to make any of these adjustments, start the engine and

run it until it reaches normal operating temperature, then remove the air

cleaners.

Air cleaners removed.

Air cleaners Unbolt the four bolts that hold the air cleaners in place. The front mounting bracket holds the

choke cable. The rear bracket is not attached to anything, so it can fall in the engine bay when

the bolts are withdrawn. With the standard air cleaners removed from the car, renew the air

filters. Prise open each air cleaner casing and remove the old air filters. Refit the new filters

and reassemble the casing.

Unbolt the air cleaner assembly.

Removing old air filter.

Synchronising twin carburettors For best possible performance synchronise the operation of both SUs. The linkages between

the carburettors are adjustable and you can reset them by slackening off the

clamping bolts on both the throttle and jet interconnections.

Undo the throttle clamp bolts.

With the clamping bolts loosened, restart the engine and adjust the throttle idling screws on

each carburettor to give the desired idling speed. Set the engine idle speed as

low as possible, but keep it fast enough for the engine to run reasonably

smoothly.

Set the throttle adjusting screws.

Listening to the intake 'hiss' with a length of plastic tubing.

With the engine running use a length of rubber tube to listen to the intake 'hiss' from each

carburettor. Compare the hissing sound made by the air being sucked into

each carburettor. You can buy specially made equipment for this

comparison check, such as the Carbalancer, which shows the level of air

intake on a simple meter. It is important to be consistent about where you

position the rubber pipe, when you measure the hiss from each carburettor

in turn. Measure the hiss until it is the same for each carburettor, and then

set the throttle adjusting screws.

Carbalancer gives a visual reading from each air intake.

Adjusting the mixture If a check of the colour of the spark plug electrodes reveals the need to adjust the mixture,

now is the ideal time to do so.

On early MGs, the jet adjusting nut is clearly visible beneath the body of the carburettor.

Turning this adjusting nut upwards will weaken the mixture and turning it downwards will

make the mixture richer. You can obtain an SU jet adjusting spanner to

make this adjustment easier.

Adjusting the mixture on an HS4 carburettor.

On an HIF carburettor the mixture adjustment is by a cross head screw situated in a recess in

the base of the body

. On later cars, jet adjustment is controlled by turning a screw on the side of the body of the

instrument. Turning the screw clockwise will enrich the mixture, while turning the screw in

an anti-clockwise direction will weaken the mixture.

Carburettor lifting pins At the side of each carburettor is a lifting pin, these pins enable you to adjust the mixture and

check the movement of the piston. First check that each carburettor piston is rising and falling

correctly. Press the pin up to lift the piston and then check that the piston falls freely by

listening for a discernible click as it comes down on the bridge. If the piston

moves only very slowly and you cannot hear a click, then the jet needs to be

correctly centred.

Lifting the pin at the side of the carburettor to raise the piston.

Once the carburettors have been synchronised, the lifting pins can be used to check the

mixture adjustment. Raise the lifting pin on the front carburettor to lift the piston by 1/32 in

(0.8mm).

1. If the engine speed increases, the mixture strength of the front carburettor is too rich.

2. If the engine speed immediately decreases, the mixture strength of the front

carburettor is too weak.

3. If the engine speed momentarily increases very slightly, the mixture strength at the

front carburettor is correct.

Repeat this operation for the rear carburettor, and after adjustment re-check the front

carburettor, since both are interdependent.

When the mixture is correct the exhaust note will be regular and even. If it is irregular with a

splashy type of misfire and colourless exhaust, the mixture is too weak. If there is a regular or

rhythmical type of misfire in the exhaust beat, together with a blackish exhaust, then the

mixture is too rich.

Carburettor tuning equipment A popular device for setting the mixture is to use a Colortune. This consists of sparking plug

with a transparent top which enables you to see the colour of the combustion taking place.

You can the set the mixture by observing the colour changing in the plug. It

is best to do this in a shaded area, although the Colortune comes with a

viewing tube.

The Colortune plug provides a visual guide to combustion.

There is also the Gastester which provides a measure of the carbon monoxide or CO level in

the exhaust. CO is a by-product of the combustion process and there is a known relationship

between the amount of CO present in the exhaust gas and the air/fuel ratio. The Gastester

provides a measure of CO levels and enables you to adjust the mixture to conform to MoT

specification.

Reset idle speed Usually as a consequence of making adjustments to the intake and the mixture, the idle speed

will have increased and it will be necessary to readjust the throttle adjusting screws. You will

find a recommended setting for the idle speed in the data specification of your workshop

manual. As an approximate guide, the idle speed can be set at the point at which the ignition

light is just beginning to glow.

Throttle interconnection clamp bolts Finally set the throttle interconnection clamping levers so that the link pin is 0.12 in

(0.30mm) away from the lower edge of the fork. Tighten up the clamp bolts.

Jet interconnection clamp bolts With both the jet levers at their lowest position, set the clamp bolts so that both jets will begin

to move simultaneously. The choke cable is fitted to a bracket that is connected to the air

cleaner bolts, it will be necessary to refit this bracket with a suitable shorter bolt, so that you

can carry out the following adjustment with the air cleaners removed. Connect the choke

control so that there is approximately 1/16 in (1.6mm) free movement before it starts to pull

the jet levers. Then pull out the choke control until it is just about to move the carburettor

jets. Adjust the fast idle screws by comparing the intensity of the intake hiss to give an engine

speed of about 1000 rpm when hot. Finally refit air cleaners.

Damper oil Top up each of the piston dampers with carburettor damper oil. Unscrew the damper cap,

withdraw the damper and top up the reservoir until the oil level is 1/2 in

(12mm) above the top of the hollow piston rod.

Topping up the dashpots with damper oil.

Pancake air cleaners Many owners fit pancake air cleaners which improve performance if used in conjunction with

richer carburettor needles. Richer needles become necessary because the reduced resistance

of the slimmer air filter will weakens the mixture.

Pancake air filters usually have foam filters which can be cleaned with petrol., while K and N

air filters have special cotton elements which are pre-oiled and require cleaning and re-oiling

at only 100,000 mile intervals depending on conditions.

Float chambers The float chamber of the carburettor contains a float and a needle valve. The float works

rather like a ball valve in a water cistern. As the level of fuel in the float chamber falls, the

needle opens and allows in more fuel. If the float sticks or becomes punctured, fuel will

continue to enter the chamber and eventually pour out of the overflow pipe. Sometimes the

needle valve itself can stick or become blocked and either not allow in any fuel or fail to shut

off and fuel will overflow. On early cars the float chambers are mounted at the side of the

carburettors but on later cars the float chambers are built-in to bottom of the main body.

Butterfly valves Rubber bumper MGs have carburettors fitted with overrun depression limiting valves fitted to

the butterfly or throttle disc. The idea behind these valves was to clean up exhaust emissions

by drawing in more air on the overrun. If the springs become weak and the valves fail to

close properly an erratic tickover with a rising idle speed will result. This can be remedied by

replacing the valves with new ones or by reverting the earlier specification

butterflies without the overrun depression valves.

A butterfly or throttle disc fitted with an overrun depression valve.