Austin Champ, Repairing the carburettor

This is an article I wrote for CHAMP NEWS , issue 101. Comments always welcome

{{All the numbers in brackets refer to the 'Illustrated Spare Parts List (ISPL), 1962', illustration no. A14 - the parts are listed on ISPL text pages A24 & A25. At the bottom of this web page is the illustration from the EMER, illustration A14, it might be an idea to print the whole webpage or just the photo (to print the photo: right click on the photo, 'save picture as', then print picture.)}

To write this article I started with eight second hand unknown carbs and tried to get them all working properly. I believe the only way to properly test a carb is to put it on a B40 engine - I have heard many tales of carbs being sold as reconditioned or being sent away for repair and the carbs were still faulty. In one case an owner had a problem with engine running and suspected the carb, he had it 'reconditioned' and the engine would still not run properly, he then suspected some other fault and spent a lot of money. He eventually sent the carb to another repairer who found the 'reconditioned' carb had been assembled wrongly.

The usual reason that a carb is no longer attached to a Champ is that there has been a problem with it - would you change a perfectly good carb? Not only that, the chances are that someone has had a go at 'mending' it, as I found out. All the faults shown here were real, none have been staged.

1. Position a fire extinguisher near the engine bay.
2. Remove the rubber hose between the air cleaner and carb, especially if an unknown carb is fitted, because a carb can pour petrol out of the intake, as shown later, and if the hose is on then the petrol won't be seen, until it may be too late if the engine backfires.
3. Remove the short hose from the nearside of the engine that connects to the metal air pipe that goes across the bulkhead to the snorkel position. Reason: if this pipe is blocked it causes back pressure on the engine and bad running that can be mistaken for a carb problem.
4. When working on the carb put an old towel or sheet under the carb to catch any dropped parts - if you don't you will drop something and the chances are that the bits will end up in the front axle cradle.

If working on carbs it is well worth reading the manuals* first and getting to understand the way the carb works and this can give an idea of where to start on a problem. *EMER D572, page 42, para. 158; illustrations pages 91\92\93.

The carb can be divided into two main parts:

1 - when the engine is hot.
2 - when the engine is cold and starts up.

1. The part used when the engine is hot, i.e. normal running. This part can be subdivided into three:

- slow running\tickover.
- at steady engine speeds above slow running.
- when accelerating

2. The cold starting device (choke) on the top of the carb that the choke cable goes to. This unit is a carb in it's own right and can run the engine even when other parts of the carb are not working but, of course, the petrol\air mixture supplied is very rich and could choke the engine when it warms up. When the choke is 'off' this unit does not play any part in the running of the engine excepting a good seal between the choke unit and carb (there is no gasket).

PART 2, DIAGNOSIS In the cases of these unknown carbs it was easy to identify that any engine problems were the carb, because my engine was running perfectly well before fitting the carb to be tested. If a Champ is a non-starter or is running badly the problem, be it ignition, carb etc. has to be diagnosed and this can be the hardest part. But, this article concentrates on carb problems.

Unscrew the mixture screw (5) and check the taper, if the taper has a shoulder where it has been over-tightened it will affect the slow running. Check it has the spring (6).
For the initial setting, gently screw all the way in and undo one turn.

TOP OF THE CARB - take the small 'L' shaped metal cover off (58) the top of the main body of the carb. - examine the flexible membrane (60, called a 'gasket' in the parts book), if that membrane is damaged the engine won't run.
- remove the membrane, on top of the emulsion tube (54, 27 points to the tube) there is a rubber 'O' ring* (55) (called for some unknown reason a, 'Washer, emulsion tube'). It is 12 mm outside diam, 6 mm inside diam.), this is critical to the running of the engine.
- remove the emulsion tube (54) (it is a double tube, an inner tube and outer tube) and check if either tube is blocked, if it is blocked the engine will certainly not run - this was the fault on one carb. Inner tube: blow through the thin bottom tube and it should be clear - if not, use a thin wire (15 Amp fuse wire works) pushed through from the top and, if necessary, from the bottom and ensure it is clear, (don't worry if the wire, when pushed up from the bottom doesn't come out of the hole on the top because the wire usually hits the flat on the back of the nut but the wire must go all the way through from the top.)
- blow through one of the two holes in the nut (cover one with a finger) to check it's clear, air should come out of the two holes in the sides, see pic. below.
- check the jet (59) in the top cover (58).

carburettor emulsion tube
(above) the emulsion tube (54) shown removed, (below) the left red arrow shows the hole in the side of the nut which connects through the outer tube to the holes arrowed on the right. This tube was blocked, a wire had to be pushed all the way through to clear it. carburettor emulsion tube

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(above) Jet (59) in the small top cover (58), the photo shows part of two different covers, the jet on the left is good, the one on the right is corroded and seized-in, someone has tried to remove\'mend' it.

The float and float fingers are the reason to leave the air intake hose off whilst working on a carb. On one of the carbs the engine ran and was left to tick over and within 30 seconds petrol flooded out of the carb intake, the reason is either the float fingers (43, 'Toggle float') or the float (39). - remove the two float chamber bolts (57) on top of the carb and drop the chamber off. - shake the float to see if there is any petrol inside. - look at the float fingers and check if they are straight, also compare with the photo below and that the fingers are fitted the right way up so that the dimples rest on the float. The fingers are easy to remove by unscrewing the screw - it comes back far enough to get the fingers out. The fingers need to be removed to take the valve 45\47 out. If the fingers have been bent down the valve will shut too early and fuel in the float chamber will be too low and could cause fuel starvation. If bent up the valve will not close early enough and cause flooding.

carburettor emulsion tube
(above) the float fingers (43) shown with the carb upside down. The inlet valve (45 to 48) under the fingers can be seen, (below) if something can be assembled wrongly, someone, somewhere will do it, as here, the one on the left with the petrol pipe attached is the wrong way around. carburettor emulsion tube carburettor emulsion tube

carburettor emulsion tube
(above) the fingers on the left are correct, the right hand side one has been bent up (left) the result, petrol leaking. carburettor emulsion tube

JET CHECK There are 3 jets on the main body, these should be checked. Do not touch the plug (30) on the bottom of the float chamber.

The butterfly spindle (3) that holds the butterfly (2) passes through bushes in the throttle chamber (1). If the spindle is worn air will bypass the closed butterfly and simulate the butterfly being opened and the engine would increase revs. When a worn throttle chamber was stripped the wear was on the spindle, not the bushes - the spindle is the same diameter throughout its length and when pulled through the spindle went tight. The usual way of repairing this fault is to get a new complete assembly, as seen on many carbs. Or, the spindle is listed with its own part number but whether it is still available is not known. It may be possible to machine down the spindle to a uniform diameter and make new bushes. Check the rubber 'O' rings on the outside ends of the spindle.
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(above) the red arrows show the wear points on the spindle, the blue arrow shows one of the bushes

If the accelerator pump is failing or failed the engine will hesitate and even cut the engine when the accelerator is pressed, even with just a hesitation it makes a Champ unpleasant to drive. Another problem is they can leak fuel and the hardest thing is see where the fuel is leaking from.
If the pump is leaking it will usually be the diaphragms failing but just check the four brass screws at the corners and the two set screws that hold the outer cover in place for tightness (the fuel pipe may need to be undone to get to one of the corner screws.)
Note: the chamber in the pump is divided into two parts by the diaphragms and the inner chamber is wet and the outer chamber is dry (see page 16 for illustration), when the diaphragms fail the outer chamber becomes wet and on one carb petrol leaked through the metal, it was porous.
Other checks: (the washers mentioned in the following are a special size and have to fit over a lip just under the head, it is less likely that a washer has failed but if it has, and if a new one is unavailable, use jointing compound to seal the washer).
1. check item no. 61's washer and for tightness. (There is a reason for calling it 'item 61' not, 'Jet economy, which is explained later.)
2. check the non return valve (74) (see left, the plastic ball can just be seen) and its washer and the valve for tightness. The non-return valve has a little plastic ball in it and the ball should be intact and free, shake the valve and you can hear the ball.

From here the accelerator pump has to be removed by undoing the four brass screws. Note, the carb you are working on may have been wrongly assembled, (see left), check with the photos in this article.)
1. check the gasket between the pump and carb body.
2. with the pump on the bench, remove the two set screws holding the outer cover (72) on - the spring inside (71) will probably push the outer cover off.
3. split the centre section (70) away from the pump body - the membranes usually stick so work around them to unstick them.
4. unstick the membranes from the centre section, the membranes can be pulled through and out of the centre section.
5. Inspect the membranes and replace if necessary.

carburettor emulsion tubecarburettor emulsion tube
(left) The washer has to go over the lip right check the non-return valve.

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The two diaphragms just fell out when the outer cover (72) was removed, they were not attached to the bolt (68)

Note: on some originals there were two diaphragms and on others there were four. On replacement kits there are four, two in each position.
1. the gasket between the pump and carb body can be fitted wrongly, check the alignment of the holes with the body.
2. the unit can be fitted upside down - the single rib goes to the top.
3. the centre holes in the membrane seem too small for the bolt, the membranes have to be screwed onto the bolt - don't forget the fibre washer under the bolt head.
4. when assembling the membranes, check one against the body to make sure it is the right way round.
5. lay the parts out in the order to be assembled as per the top photo.
6. push the bolt through the diaphragm and as the second one goes on put the four mounting bolts in each corner of the diaphragms to keep them aligned.
7. the diaphragm assembly can now be fitted to the centre thin metal section (70) (see left) by pushing through the outer two diaphragms.
8. Fit the spring and end cap, push the four mounting screws through, fit the last body part over the screws, fit the gasket and screw the assembly to the main body of the carb.

carburettor emulsion tubecarburettor emulsion tube
(upper) Starting with the bolt on the right, pick up each part as it is and put over the bolt. All chamfers on the washers are put against the diaphragms. The nut on the left had no washer on the originals that have been striped. lower (photo, Nick Wolsey) Aligning the diaphragms using the four accelerator pump mounting screws


There are no notes here on the choke unit (23) itself because I have never known one to fail and there are no main user serviceable parts in it, the only critical item is the joint between the choke unit and the carb body - there is no gasket.
However, it is important that when the choke cable is pushed in that the choke on the carb is in the fully 'Off' position. There is a rubber 'O' ring under the lever (20) shown in the photo.

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The blue arrow shows the 'O' ring. The red arrow shows the lever locating ball that then went 'walkabout' Oh dear oh dear!!


Run the engine to check for petrol leaks.
Put the air intake and engine breather hoses on.
With the air mixture screw (5) one full turn out from all the way in, start the engine, wait until the engine is hot, set the RPM to 500 on the adjustment on the outside of the butterfly spindle, press the accelerator (called 'blipping' the throttle, this is easier done under the bonnet by holding the rod crank that goes to the carb), if there is any hesitation adjust out the idle screw by a small amount and blip the throttle. The maximum the screw should be turned out is between 1 1\2 and 2 turns.
Slow running: there is a note in D574 (Part 1.), page 9, "2. The float guide and slow running tube (in the float chamber) are matched and must be retained/replaced together. Looseness between the two components would destroy the required depression in the tube and cause faulty slow running." Ed: Access to these components would be through removing the plug, which says on it, "DO NOT REMOVE". My guess is that this is a rare problem, perhaps, in very high-mileage carbs. If the components are worn I doubt they are repairable. If a problem on these was suspected then this could be tested by substituting another float chamber, which is one reason why it is always worth buying an old carb for spares.
1. Corrosion: aluminium can disintegrate into flakes and dust and this is said to have happened on some carbs and debris has blocked internal channels on the carb. This was not found to be a problem on any of these carbs.
2. EMER D573, page 13: "From the replacement carburetters remove the transportation piece (fitted to retain the float during storage and transit)." So, if you buy a NOS carb, remove the float bowl and check for this transportation piece before complaining to the eBay seller!
3. The correct carb on a Champ is, Solex 40WNHEO-2, FV144776, the NATO no. LV6/MT12/2910-99-807-7278 - the carbs are the same on the early and later engines. There are similar looking carbs, one of which is 40WNHP0 and it doesn't work on a Champ.
4. How much difference does a good carb make to driving a Champ?, the difference is unbelievable. Testing these unknown carbs on-the-road, after an engine running safety check, but without any maintenance or adjustment, I immediately began to compensate for the carb, especially when the engine would hesitate when the accelerator was pressed by increasing the rpm before letting in the clutch and also to change gear at higher engine revs. The ultimate test of a carb was done at 20 mph in top gear and then (trying) to accelerate, good carbs were like driving silk and, overall, the whole Champ felt different and it was much, much more relaxing to drive.
5. Rebuilding a carb is not technically difficult and is a better alternative, in probably 95% of cases, to paying around 195 for a NOS carb. A NOS carb is going to be, probably, 50 years 'young' with its original diaphragms and there have been problems with NOS carbs failing, and the only way of saying a NOS carb is good is to run it for, say, a 100 miles. But, on balance, most NOS carbs will be OK but it is still better and cheaper to try rebuilding your own carb.
6. There are companies that say they will rebuild these carbs for 95 + 10 (UK) postage each way. One owners experience was, to say the least, very sad. When the carb was refitted the Champ was difficult to start and would not rev up properly, a friend of the owner stripped the carb and it was wrongly assembled. The only way a carb can be tested properly is to put it on a vehicle, which repairers can't do.
7. 'Over to you': I would be grateful for your carb experiences; do you know of a better way of doing something?; or?
Illustration A14 (next page): shows two large cross sections of the carb and in between them is a part cross section and shows 37 vertically on the side of the float chamber yet there is nothing like it on the float chamber of the WNHE0 - 2?
What may be a clue is the part number of 37, LV6/MT12/SX50676/2/90.
There are 3 jets on the carb (ignore the accelerator assembly):
A. A14, 32, main jet.
B. " , 35 'JET METERING'
C. " , 37 'JET, speed', looking at an actual carb there is a jet marked , '90'. in a position just left of where 74 is. All the manuals, including the EMERS show this part cross section, my guess is someone changed the specification of the carb and it was never included in the manuals?
Illustration A14 item no. 61: the ISPL, says on page A25, Item No. 42 (left hand column) that 61 on the illustration A14 is a 'jet economy' yet, on actual carbs the so called 'jet economy' has 'BLANK' engraved on it. The Austin 'DISMANTLING INSTRUCTIONS FOR MAJOR ASSEMBLIES', page 3, may explain this, it says, that the same accelerator pump unit was used in different models of this carburetter and this item (the 'economy jet') may sometimes be calibrated in order to fulfil a special function but, in the WNHEO/2 it is merely a blank to seal the casting at this point. (carb illustrations on page 15 &16)

EMER D572, page 93
The importance can be seen of the emulsion tube with the hole in the top, which acts as a jet, the pilot jet air bleed, diaphragm and 'O' in the idling\slow running circuit.
Fig. 52 can at first seem confusing but the illustration shows the carb after the carb has reacted to the accelerator being pressed.
Looking at Fig. 52 (left) the green arrow points to the inner chamber of the accelerator pump, which is empty because the fuel that was in it has been sent up the emulsion tube. The small inset illustration shows the second chamber as it would be before pressing the accelerator.
Problems with an engine can be: it just stops; it will not start; it misfires; will only run at above 1000 RPM; it cuts out or hesitates when the accelerator is pressed; it will not tick-over. The following are the first checks to do if presented with an engine with any of these problems:
1. Fuel:
- has the vehicle just been filled up?, has diesel been put in? - one of the commonest reasons for breakdowns.
- be suspicious of strangers offering you 'petrol' from a can, how old is it?, is it contaminated with diesel or water?
- is there petrol in the vehicle?
- remove the float bowl (2 x 57) and the fuel level should be nearly to the top of the float chamber.
- while it is off take the float (39) out and look for water globules in the petrol and if there is a lot of muck\grit.
- shake the float to see if there is petrol in it.
- when the float chamber is taken off the fingers (43) of the brass float control lever will come down, feel for the needle of the valve, push it up, let go and it should come down.
If the chamber is empty then it could be the fuel pump or a blockage in the carb or on the vehicle. Refit the float chamber, slightly undo by half a turn the fuel pipe banjo (53) on the carb (from the fuel pump) and use the fuel pump primer lever to pump fuel to the carb, fuel should spurt out with a vengeance. If there is - but no fuel in the bowl - then it is either the filter (52) that is in the banjo bolt (53) or the needle valve (45\47) has jammed in the closed position (which I have never heard of happening, yet).
- test the engine.
if a vehicle has been left anywhere or at a show, check the weak mixture screw (5) under the carb on the inlet manifold end. By hand, turn the screw gently clockwise until it stops and then unwind it one turn - if a malicious person at a show wanted to cause aggravation to an owner they could reach under the carb and turn the weak mixture screw fully-closed and then the engine won't start properly.

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