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A warped carburettor mounting flange can easily be restored to flatness to cure air leaks. This will enable the correct fuel mixture strength to be restored and allow the engine to run at idle speed.

Q.

I have acquired a Riley that has been dry-stored and is in good all-round condition. As it hasn’t been used for many years, I renewed all the normal servicing components and drained/flushed through the fuel tank and supply system, including the fixed jet Solex carburettor. The engine now starts and runs nicely at relatively high revs, but it stalls when the revs fall to tickover speeds. Any suggestions please?

John Appleton, Plymouth

A.

First re-check the carburettor for inner cleanliness, especially the jets. Cleaning through the fuel supply system may have dislodged tiny particles of dirt/debris which may now be blocking a jet.

Another aspect to check is the carburettor’s mounting flange, which may be warped. If this is the case, a serious air leak can occur around/past the flange, thus weakening the fuel mixture. Assess initially by removing the carburettor and placing a straightedge, such as a steel rule, across the flange. If a visible gap exists, this indicates warping. Even if no gap is visible, try sliding a thin feeler gauge (approx. 0.002in.) between the two; if the flange is flat this should not pass between them.

A warped carburettor flange can be restored by placing a sheet of fine grade (ideally 600 or 400 grit) silicon carbide ‘wet or dry’ paper, abrasive face uppermost on a thick sheet of flat glass, and gently rubbing the flange against the abrasive paper using a circular motion. It helps if the abrasive paper is coated with a little water dispellant spray/light oil to provide some lubrication. Make sure that at all times the carburettor flange is held firmly and squarely against the paper.

Frequently inspect the flange during the abrading operations. After a short time the ‘low spots’ will be highlighted (a darker shade of grey than the surrounding abraded metal).

Keep going until the flange surface is the same, uniform colour all the way across, indicating that it is flat − only remove enough metal to achieve flatness.

On completion, remove all traces of abrasive from the flange and surrounding parts of the carburettor. Check the mounting flange on the manifold too, although it is usually the carburettor that suffers most from warping. With the air leak cured the engine should be far happier about running at low speeds.

This magnetic MIG welding earth connector is rapidly attached and removed and provides good contact, even on ‘difficult’ surfaces compared with a conventional earth clamp. It’s always wise to ensure that the surface to which it is attached is clean.

WHAT ON EARTH?

Q.

I have recently started MIG-welding repair plates into the bodywork on my classic but I am having trouble with the earthing clamp supplied with my second-hand welding machine. The clamp seems to be worn and doesn’t always make good contact. Is there a better alternative that is also quicker and easier to use?

Jamie Bishop, Scunthorpe

A.

Many classic car workshops use heavy duty magnetic earthing clamps that instantly provide a good contact. These are particularly easy and quick to attach to/detach from vehicle bodywork and other steel surfaces.

Exhaust gas leaks often occur around joints between adjacent sections of new pipework. It is wise always to apply a little exhaust assembly paste around the joint as the pipes are brought together. This sets rapidly and helps to provide an effective seal.

BLOW IT

Q.

I have just had a new exhaust system fitted to my classic, but my car is now making a loud ‘chuffing’ noise, especially when accelerating. Is this normal and will the new system settle down with use?

Jemma Smith, Huddersfield

A.

A new exhaust system often sounds different in tone compared with the old one it is replacing, but shouldn’t make loud ‘chuffing’ noises. This indicates that your new system is leaking exhaust gases. Such leaks typically occur around pipe joints between adjacent sections of the system, but occasionally it may be found that the new silencer (for example) is faulty and is leaking from a seam. Such leaks tend to get worse, not better, with use!

Mention the problem to the suppliers/fitters of the system; they should be able to fix the leak. It does need urgent attention though, since leaking carbon monoxide − which is poisonous − might enter the passenger compartment. If the leak is at a pipe joint, separating the joining sections and applying exhaust assembly paste should do the trick.

It’s essential that the pipework and joint are not strained − the mountings should be checked − or the system will soon start to leak again.

Sometimes failure of an electrical circuit is due to a simple breakage of a cable or soldered connection, or may be caused by dirty/corroded connectors. Check these aspects first, rather than assuming that the operating switch is at fault.

DODGY LIGHT SWITCH?

Q.

I drive an Eighties classic and yesterday the headlamps stopped working (except if the main beam flasher stalk is operated). All the fuses seem okay. I have been told that the headlamp switch has probably failed. Should I just acquire a new stalk/switch assembly, or are there any other checks I should or can carry out myself before spending any money? The switch forms part of a complex stalk assembly incorporating the horn control, light switch, main/dipped beam control and indicator switch.

George Hubbard, Rhyl

A.

The switch may have failed, but this is not necessarily the case and should not be assumed. Start by removing the cowling(s) around the top end of the steering column/stalk area.  Such cowlings on Eighties cars are usually plastic (sometimes now brittle…), and secured by a series of screws. If in doubt about dismantling procedures, consult your workshop manual.

Once the cowling is out of the way, look closely at all visible wiring and connectors. Watch for broken soldered connections (not uncommon), severed cables and other obvious problems. A broken soldered joint can be carefully re-soldered, taking care not to damage adjacent components.

The use of a ‘heat sink’ to ‘absorb’ excess heat is recommended. For example, a small pair of long-nosed pliers can often be attached to the connector being soldered, and the handles temporarily held together with an elastic band. Alternatively a small self-grip wrench could be used. Check also the multi-cable plug/socket connections linking the switch assembly to the main wiring loom, as dirty/corroded contacts here often cause problems.

If all seems well in the checks so far, use a test meter to establish input/output voltage at the switch. Internal breakdown within the switch is possible, in which case a replacement assembly is usually required as do-it-yourself repairs are not usually possible.

SU carburettors are sensitive to fuel/air mixture and slow-running adjustments. Raise the idle speed a little above normal levels using the adjuster screw, set the fuel mixture to give smooth running, then lower the tickover speed again.

FINE ADJUSTMENT

Q.

My classic is running unevenly, also stalling at every opportunity when the engine revs fall, and can sometimes then be difficult to re-start. Pulling out the choke control a little seems to help (even when the engine is warm). I have tried increasing the slow running speed, but unless the engine is spinning at excessively high rpm it still tries to stall. Could this be a fuel-related problem? The car has a single SU carburettor.

Ben Kersley, Droitwich

A.

Start by ensuring that the valve clearances and all ignition settings are correct. Check too that the fuel has not degraded. Modern unleaded fuel has a very limited ‘shelf’ life, and when it deteriorates (usually indicated by discoloration and/or an unusual smell), it can cause a number of running problems, including a reluctance to idle. However, in this case the fact that pulling out the choke seems to improve the situation suggests that the fuel/air mixture is excessively lean.

Warm the engine to normal operating temperature and turn the slow running adjuster screw clockwise − viewed from the top − to give a higher than usual idle speed. Now slowly rotate the jet adjuster nut at base of carburettor downwards (i.e. anti-clockwise, viewed from below) to enrich the fuel/air mixture. Turn this adjuster nut just one flat at a time, and note the results. If, as suspected, the mixture is over-weak, enriching it will result in smoother running. Note that on a variable jet carburettor like the SU, the mixture setting will affect the engine’s running at all speeds, not just at tickover.

Keep rotating the jet adjuster nut until the smoothest running is obtained. If the engine starts to run unevenly, go back one flat. On completion of the mixture adjustment, re-adjust the idle speed. Check also that the fast idle adjustment is spot-on. Full details of this procedure and correct adjustment of the mixture/slow running are provided in most workshop manuals and some handbooks.

Note that altering the fuel/air mixture affects emissions, and ideally adjustments should be made in conjunction with a CO meter (MoT testing stations should be able to check the emissions levels for you).

Differential units are often stored badly when removed from a vehicle, and can look dusty like this one (purchased via an internet auction site). Clean very carefully before installation. This is time-consuming work, but essential.

DIFF-ICULT JOB

Q.

I have purchased a used ‘low mileage’ final drive assembly which appears to be in good condition, except that the internal components are covered in dust, etc. How can I clean it?

Bill Ainsworth, Portsmouth

A.

Used differentials are often dirty, so ideally a final drive unit should be placed in a sealed, clean, strong polythene bag as it is removed from the vehicle.

Avoid immersing a differential in solvent. One approach for cleaning is to use a soft rag, dipped in some water dispellant/light oil, and very carefully wipe clean all the gear teeth and all surfaces within the unit. As an alternative to a rag, a soft-bristled brush such as an old toothbrush could be used. Slowly rotate the input shaft, a little at a time, to expose all the gear teeth in turn for cleaning.

On completion, carefully lubricate the bearings and gear teeth with a little fresh gear oil of the correct specification for the unit. Rotate the gear teeth and ensure that operation is smooth. Re-check for any signs of dirt, dust, etc. before installing the unit.