Why we should all show carburettors a little love

If there is one part of classic car ownership, we get asked about more than anything it is about tuning Carburettors. For many even quite practically minded people the carburettor is mysterious object, prone to faults, confusing to diagnose and impossible to set up correctly.

We are often asked to tune and set carburettors only to find that they have had no TLC for many years and have major faults. But, often the Carburettors are perfectly OK and further investigation typically reveals that the car has an ignition fault which has nothing to do with the much-maligned carb.

When you think about, the carburettors job is both straightforward and very complex.  It must deliver the right amount of fuel no matter what the car is doing.  The optimum air to fuel mixture for a petrol engine is about 14.7:1. For those who don’t speak ‘engineer’ it means that for every gram of fuel used about 14.7 grams of air are required to burn all the fuel. It is precise, and the carburettor must deliver that mixture consistently and constantly. It must give us enough fuel to start the car, allow it to idle smoothly, it needs to respond quickly to our throttle input and provide the right amount of fuel and air as we accelerate away.  As we slow down it needs to restrict the supply.  It does all this no matter if the car is travelling up or down as steep hill, leaning into a corner or even very hot or cold. And it does it all mechanically with no clever electronic brain to help it out. Take that on board and you realise that these are marvels of engineering that deserve our respect rather than our than our mistrust or anger.

When the carburettors were first fitted to the car in the factory, they were precision and highly efficient pieces of kit that were set up for optimum performance. But many decades of use will have degraded them, causing smooth-running problems and at worst persistent performance issues. The precision nature of their design and their many moving parts that are reliant on each often mean that it can be confusing to figure out why they are not performing as they should, and which parts are responsible.

With parts that have been rubbing against each other for decades, wear is inevitable in every part of the carburettor. The gaskets that once kept the carb body air-tight may be leaking, the fuel float could be corroded or cracked and filling with fuel rather than floating on it. Running repairs over the years may have resulted in the Carburettor bodies being screwed too tightly to the manifold, distorting them, and causing even more leaks. Butterfly valves will have scored the surface of the bores allowing air in. The spindles on which the butterflies are fixed to will have worn their bushes and be loose. Jets and passageways may be clogged with dirt. The jets may even be wrong for the carb (the result of a previous bodged attempt to make the carb run better).

Classic car owners experiencing problems often buy and install a carburettor service kit only to find little improvement.  The truth is that installing a rebuild kit a won’t attend to any of the above common problems which require the carburettors to be rebuilt by a professional.

People are often shocked by how much this can cost but there can be a lot of re-engineering involved in a rebuild. The carbs will need to be completely stripped down and ultrasonically cleaned. Depending on the make of model if the throats have ovalised, they will need to be rebored to round, this requires new over-size butterflies to be fitted. The spindles will likely be replaced which will require new bushings. The original plated finishes will be replicated, and new jets and emulsifiers fitted. New gaskets, seals and pump parts will also be installed. In short, a proper carburettor rebuild is a fairly major re-engineering exercise taking many weeks and the skills of many tradespeople.

Our customers are frequently amazed by the difference a full carburettor rebuild can make to their vehicle’s performance. After a rebuild we always think we have rebuilt both the carburettor and the trust people once had in them! A properly engineered, rebuilt, and correctly set up carburettor is an engineering masterpiece that will give years of faithful service.

Do yours need a little love?

Copyright © 2019-2020 SILSOE CLASSIC AND MODERN LIMITED