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Tkart magazine

The Expert Advices | The art of carburetion

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28 January 2016

All about CARBURATION!Drivers certainly help in giving mechanics feedback on how they’re sensing engine performance at lows and highs (with their feet and ears). If you want foolproof data, however, telemetry instruments are the way to go; the only “problem” is that the data itself is hard to decipher and time-consuming to gather.
There is another infallible method that rests on direct visual proof, provided you havethe expertise to intepret it: inspecting the piston crown. To understand what it entails, we consulted with TM Racing’s official tuner Franco Drudi, the “super guru” of 2 stroke engines.

1 Off the bat: how does one check carburetion?

A very effective method consists in opening up the engine and “reading” the piston crown and ring. The piston is the moving component of an engine that sits inside the cylinder and transfers motion to the crankshaft. Apparently simple, it is composed of a number of accurately engineered components. In 2-stroke engines, it is responsible for opening and closing the transfer ducts and exhaust ports - basically, it does what valves do in 4-stroke engines.

Low rpms
High rpms

2 In detail, how exactly do you do this?

First, you analyze the piston crown. The premise is that wherever you see brown coloring, it means those sections are reaching very high temperatures; viceversa, the areas with a clean surface are keeping “cold”.
When “reading” carburetion from the crown, there are two sections to consider. To inspect it at low rpms, you want to focus on the transfer ducts area opposite the exhaust. If the piston is new or almost new, it will be easy to “read” in terms of color: you want it to be clean, clear. If you see dark brown stains, it means carburetion is too lean at low rpms causing the piston to overheat in the ducts section; so you’ll need to make the lows a bit richer.

3 What about high rpms?

You focus on the central section of the piston crown, following the same “read the color” principle: if it’s too clear and shiny, it means the carburetion is too rich. This portion of the piston should reach higher temperatures compared to other segments, so you want it to be slightly brown – the opposite compared to the reference section for the lows.

Drudi’s inspection

4 What about the spark plug?

It’s less indicative. And when it’s new, it really doesn’t tell us much of anything. I personally rely on the piston crown and ring.
The piston ring is very helpful in telling us if carburetion is right or not, especially at high rpms. If it features “micro-welding” marks, it means carburetion is just right, i.e. as lean as it can be for maximum engine performance. When you see these marks, you’re good to go... just avoid going any leaner though!

5 Are there other things to watch out for?

Yes, the other key elements to inspect are the outer ring and its seat. Both should feature slight heat markings, called micro-welding marks. If you see them, it means that carburetion, especially at high rpms, is just right; i.e. right at the limit for maximum performance. Basically, the marks “equal” extra horse power.

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