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All About Carburetors

All About Carburetors

The most important part of your engine is the carburetor. That’s a bold statement, but the carburetor is responsible for the mixture and delivery of fuel and air to the engine and it therefore can make the biggest difference to its efficiency. Correct adjustment of the carburetor will give you the optimum performance and economy. Yet many of us neglect them, while others make things worse by fiddling with them when problems lie elsewhere.

Fortunately, parts are plentiful for most commonly used carburetors and specialists can rebuild virtually any carburetor ever made, or supply a suitable alternative.

If your engine is not running at its best, always look at the ignition side first: are the points and plugs clean and correctly gapped, is the timing right, is the coil working properly and not getting excessively hot, is the condenser doing its job, are the plug leads, caps, rotor arm and distributor cap distributing the spark without arcing?

Once the ignition side has a clean bill of health, address the fuel/air delivery to the carburetor. A dirty or restricted air filter will make all carburetor adjustments pointless, so get that right first. A free-flowing air filter can make a big difference and may necessitate a different needle or jet so that mixture does not lean out at high revs, which could cause serious engine damage.

Fuel vaporization is a major issue with many classics, as modern fuels vaporise more readily than when our cars were new. Modern cars keep fresh fuel constantly circulating from the tank to the metering unit and back again, so it does not get as hot as when it is sitting in a classic’s float chamber – and of course, when stuck in a traffic jam, the fuel sits there for much longer than when running at speed. As underhood temperatures rise, the fuel starts to vaporise and pressurise the fuel supply system, stopping any fresh fuel coming through. With many classics having the carburetors above the exhaust manifold, the float chambers get extremely hot; even on crossflow engines problems are not unknown.

Lagging the pipe, ensuring the correct heat soak block is in place between the pump and the block and fitting a heat shield for the pipe or pump will help. If your carburetors are above the exhaust manifold, a heat shield (stainless steel or aluminium – and there’s nothing to stop you making your own) is vital, as are the heat soak blocks between carbs and inlet manifold. On some cars, it may be possible to improve matters by fitting extra thick (or double) heat soak blocks, but be careful that you do not introduce any air leaks, or misalign throttle controls.

After fuel vaporization, the next most common cause of carburetor-linked running problems is an air leak. Because (except with supercharged or turbocharged engines) the fuel system depends on vacuum, with the pistons sucking in the fuel/air mixture, a totally sealed intake system is vital. Any air leak around a gasket or from a vacuum-controlled component such as a servo, vacuum wipers or gauge, will weaken the mixture, with the effect most noticeable at startup.

More commonly the leak is from worn carburetor components, especially the throttle shaft. Once worn, the only way to cure the problem is to replace the shafts and, in most cases, to have the carburetor body jig-drilled and bushes inserted.  This is not a DIY job, as precision can only be achieved with a proper jig. 
If wear is not excessive and mostly on the shafts, new shafts may be sufficient; 
if you want to prove that worn shafts are affecting the engine’s slow running, some grease around them will temporarily stem the leaks and allow you to set the mixture more accurately.

Fuel additives will, over time, gum up your carburetor no matter how many filters you have to keep out dirt. A good dose of a proprietary carb cleaner from time to time will help, though if deposits are heavy it may make matters worse by dislodging them and blocking jets. If the car is left standing for long periods, such as over winter, all fuel in the carburetor will evaporate and leave a film behind like thin foil. The only way to get rid of serious deposits is to remove the carburetor from the car, strip it down and thoroughly clean it out. Remove all jets, needles, plugs, sliders and shafts, soak them in carb cleaner and get every trace of dirt out before reassembling.

A bent needle, especially common on SU and Stromberg carbs, will make the car run like a pig. It’s vital that the piston drops firmly onto its seat and if no amount of jet centralising will achieve that result, fit new needles and jets. Don’t be tempted to replace just one or the other, or just on one carburetor – you will never get even running unless they are perfectly matched. Make sure also that the correct springs are fitted above the pistons and that an appropriate grade of oil is used in the dashpots. Strombergs are particularly prone to losing this oil, so top them up weekly if necessary.

Another Stromberg peculiarity is the rubber diaphragm: if this is in any way perished or damaged, replace it, otherwise the car will never run properly. Beware of cheap diaphragms: they are often fractionally too thick and inflexible, leading to stalling and hot starting problems.

Zenith, Solex, Weber and Dell’Orto carburetors are built on a different principle, with no piston or needle. They rely instead on a fixed choke and different jets, usually with an accelerator pump containing a diaphragm that can give trouble. Provided these carburetors are clean, unworn and correctly jetted, they are very reliable.

Multiple carburetors rely on a well-designed throttle linkage, correctly adjusted – the slightest maladjustment will severely restrict performance. When setting them up, make sure they are completely disconnected and use a flow meter to get the throttle settings identical before tightening the clamps – and then raise the revs a little with the throttle pedal and check that the flow is still equal.

Published Dec 7th, 2015

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