Model cars that have nitromethane based fuels can be a daunting and frustrating proposition when it comes to setting the carburettor on the engine. This is in part due to some engines being temperamental, but new car racers are often put off by lack of assistance in what can be considered an art.
In this article we will hopefully guide you through the identification of the various parts and progress with basic instructions on how to set the fuel/air mixture on your engine.
Model cars can (and do) suffer severe, if not terminal, damage if run with poorly set engine mixtures.
Whilst we are happy to provide what knowledge we have, this article is a guide only and we will not be held responsible for any damage you do to your engine.
There are two different types of model car carburettors, one is called the slide carburettor and the other is called the barrel (or rotating) carburettor. Both are introduced below.
A top view of a typical slide carburettor is shown in the animation to the left and if you Play the animation you will see how the slide part of the carburettor moves to control the air and fuel flow through the carburettor.
The animation can be Stopped if required.
If you Play the animation of a top view of a barrel carburettor to the left you will see how the barrel moves in relation to the body of the carburettor. you will note that this movement is at 90 degrees to the movement of the slide in the above example. The animation may be Stopped if required.
If you Play the animation of a side view of a barrel carburettor shown on the right you will see demonstrated the movement of the lever that opens and closes the throttle. The animation may be Stopped if required.
Using the key on the right will allow you to identify the positions of the adjusters on the carburettors pictured in this article.
Not all carburettors are fitted with low end/speed adjusters as it makes it simpler for the engine mixture to be set and cheaper for the manufacturer to make.
Top End/Speed Mixture.
Idle Speed Screw.
Low End/Speed Mixture.
There are many, many different carburettors out there in the model car world fitted to numerous different types and sizes of engines. Therefore we cannot offer universal settings, as the weather, altitude, climate and type of fuel also affect the settings on a model car engine.
The only setting on a carburettor that seems to fall between close dimensions is the one that sets the tick over or idling speed.
This dimension, which is marked A, is shown in the image of the slide carburettor to the left and is the gap between the slide and the body of the carburettor.
Due to the way that barrel carburettors work the, gap is usually toward the corner of the carburettor body as shown in the image to the right.
Confusion is sometimes being caused, though, by the manufacturers literature describing this as the low end throttle, which in truth it is, but people confuse this with the similarly named low end mixture.
This setting is, in my experience, always between 1 and 1.5mm and is adjusted by the screws marked in red on the images to the left and right. Turning the screw clockwise will open up the gap and and turning the screw anticlockwise will close the gap.
The low speed mixture screw (see note above about idle settings) is normally located at the end of the carburettor body where the servo linkage moves the barrel/slide and is responsible for controlling the amount of fuel that enters the carburettor at throttle openings up to the point where the slide or barrel is around half way open.
As stated above for ease of use and cost considerations not all carburettors are fitted with this adjustment screw.
This setting is adjusted by the screws marked in light blue on the image to the left and turning the screw clockwise with lean the mixture and and turning the screw anticlockwise will richen the mixture. No settings are given for this adjusting screw due to the variations between engines and conditions.
The high speed mixture settings are usually located at the end of the carburettor body where the fuel pipe connects and can be in the form of a screw or a knurled adjuster that allows you to use just your fingers to alter the setting.
This setting is adjusted by the screws marked in green on the image to the right and turning the screw clockwise with lean the mixture and and turning the screw anticlockwise will richen the mixture.
The terms lean or rich are ones that refer to the ratio of fuel to air. A rich mixture has a higher proportion of fuel mixed with the air than the optimum ratio. Conversely a lean mixture setting has a lower proportion of fuel mixed with the air.
Before making any adjustments to your engine make sure you have either the manufacturers factory settings or you write down the settings that are on the carburettor. This is because you will have a base line setting to which you can return even if the engine is not running well.
All carburettor settings (apart from the idle speed) are quoted by the number of turns out from a closed position. This is achieved by turning the screw gently in a clockwise direction until it stops. Applying too much force in tightening the screw risks damaging the parts into which the screw goes and can lead to problems with the carburettor sticking or operating inconsistently.
Which order you set the needles in is dependant on what state of tune the engine is presently in. If the engine will not start at all you would either reset them to the factory settings or record the settings (see general notes above) and then start with the idle speed. The high speed needle would then have to be approximately set which would then allow you to set the low speed mixture. It needs to be done in this way due to the reasons given below;
The low speed mixture setting works by limiting the flow of fuel set by the high speed mixture needle and so is directly influenced by the high speed mixture setting. The reverse is not true so the high speed needle should always be adjusted first. This may be expanded upon at a future date.
Obviously, if you have identified the carburettor as being of the type that does not have a low speed adjustment feature you don’t have to worry about this.
Some carburettors are also fitted with a ‘third needle’, which isn’t a needle at all, it is an adjuster that allows for the position of the jet assembly to be altered in relation to the flow of air through the carburettor. More (maybe) in a later article.
As for tuning your engine when it is either sluggish or keeps cutting out, that is an entirely different article that will be published at a future date unless sufficient requests are received.
Information about setting the engine mixtures on the petrol (gas) powered engines used in the largescale cars can be found in our Large Scale Engine Mixture Settings article.
Please check our Glossary for explanations of any unfamiliar terms.