Carburation is one of the most delicate aspects of racing engines. A good part of the performance depends on it and, if executed incorrectly, it can also compromise the internal components of the engine. It is no coincidence that when TKART invited readers to address their doubts and questions to Franco Drudi, head of the TM Racing engines development department and one of the most experienced tuners in the paddocks, carburation was one of the most popular topics. Here are all of Franco’s answers.
1What do you need to do to the carburetor according to the revs you prefer? Benjamin L.
To adjust the gasoline amount which increases the RPM from one rev to another can be done by working on the various parameters of the carburetor such as the atomizer diameter, the conicity of the pin valve, jets, guillotine valve level. The air screw, on the other hand, only refines the jet of the minimum. Here is a practical example: I use a DQ 268 atomizer with a k 23 pin at the first notch and a maximum jet of 175. What can I do if I need to give more max float? I switch to DQ 268, K 28, the needle all the way up and a 175 maximum jet. In this way I am able to achieve more max float with the same jet and the same atomizer. How? Using a needle with a shorter conicity and getting more from the atomizer. Since the needle is conical, I increased the passageway between the needle and the atomizer when the carburetor is fully opened.
"Ask Franco Drudi": this was the invitation addressed by TKART to its readers. Many replied
Dell'Orto VHSH carburetor
2What could be a list of tasks that a kart driver should know by heart? Charlie M.
As a rule, you first should remember that if I lift the needle (I pull it more out of the atomizer) then I grease the entire carburation curve. Vice versa if I want to lower it. Other components are only used on parts of the delivery curve. If I increase the atomizer diameter, for example, then the charge becomes richer when the engine is working when you are on the power and vice versa. Almost in the same way, the max float valve needle (the bigger it is the richer it is, and the smaller it is the leaner it is) affects the supply from ¾ of the aperture to fully opened. This is because in other phases, the richness of the mixture is determined by the needle, the atomizer and the rounding of the valve. The idling jet plays an important role as well as, if it is smaller you will have a much more rapid response at the beginning of the opening (but it can be dangerous) while it will be the opposite if it is bigger.
3What is the needle and what is the difference between the various needles (U and K)? Davide P.
The needle is basically a needle free to slide inside the atomizer. It is hooked to the gate valve and when the valve is pulled upwards by the accelerator cable, the needle rises accordingly, leaving the fuel mixture to flow into the atomizer to reach the diffuser. The difference between the U and K type needles are the dimensions. K pins have on average a larger tip diameter and are usually longer which have a different conicity than U needles. These are parameters which affect the delivery with the same atomizer. The finer the tip, the richer the engine will be at higher revs and a longer conicity means having more fuel at low revs. A larger stem diameter means more cleaning at the beginning of the opening as the mixture will be much leaner, but it can be dangerous.
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