Expert Advice
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The carburation of the KZ engine
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Carburetion (i.e. the mixture of petrol/air) is a key aspect of 2T racing engines. In particular, it greatly affects the performance and reliability of karts, in single-speed engines, with cast iron cylinders and L-shaped pistons, and those with gears, with the barrel of the cylinder coated in Nikasil and a rectangular piston ring. However, the lack of injection systems and cleaning of the combustion chamber devoid of valves, but with lights adjusted by the piston, make the carburetion more difficult to control. There are many interventions that can be performed to regulate it (evaluation of the spark-plug, flue control, noise and performance of the engine on the track, telemetry, etc.), but first you need to know which direction to go, understanding whether the carburetion is rich, lean, or correct. Danilo Rossi, DR Racing boss and five-time world champion on karts, explains the methods he thinks are the most effective ones to adjust the carburetion of the KZ: evaluating the piston and testing of the engine with the kart on the carrier trolley.
Clearly engineers already know the basic parameters for carburetion, but it is better to get there gradually, perhaps starting with carburetion that is a bit richer, and therefore safer, to then make it more lean and move towards more correct values. A test that is often done is to start the engine with the kart on the carrier trolley and, after it has warmed up, repeatedly accelerate: when listening, the noise must be fluid and without any strange crackling, or as it is called in the jargon, no “rattling”. In addition, with the telemetry system mounted, you can see how the revs go up each time you accelerate, and how many laps are reached keeping the wire throttle pulled out for a few seconds.
There are many possible systems: the spark-plug evaluation depends too much on the petrol and oil used, and with small electrode spark-plugs it also becomes very difficult. The best method is “evaluating” the piston crown. You can do it through the spark plug hole, inserting a tiny light onto the tip of a flexible arm. Of course, the evaluation is more accurate and complete if you have time to dismantle the cylinder and directly check the piston crown.
The principle is quite simple: if the carburetion is oily, the piston crown is cooled and coated with the oil-petrol mix: the temperatures reached are not too high and the aluminium surface is glossy and has its original colour. As the fuel mixture gets leaner, the mixture wets and protects the piston crown less and the temperatures soar. Combustion thus reaches the metal, heats it up and, as a result, the aluminium changes colour, with shades ranging from yellow to green to blue to violet. The high temperatures burn the oil-fuel mixture to the point that carbon deposits will attack the piston crown colouring it black, especially in the vicinity of the side and centre transfer ducts (TT). The shape of the deposits is affected by the flow of mixture, from the transfer ducts to inside the combustion chamber.
Finally, when the fuel mixture becomes too lean, the piston crown begins to go a brown colour in the middle as well and you begin to see signs of detonation, the most critical and dangerous aspect of carburetion that is too lean and the overheating.
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