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| Features |
| Tech Talk: Tom Barth’s Carb Tech Series – Part 2 |
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by Tom Barth
Green Flag Motorsports
http://www.greenflag.net
When we talk air/fuel ratio we are referring to a combustible ratio of air to gasoline. The big fancy scientific word for this is stoichiometric. Or, a ratio that burns perfectly without leaving any unburned fuel (rich) or excess air (lean). This does not generally occur especially in the two stroke carburetor engine because of porting, fuel timing etc. The ratio can vary from 14.5 to 14.8 parts air to 1 part pump gas.
Normally the ratio is richer for idle, acceleration and full power. For mid range at constant throttle, it is leaner. The tuner must recognize that the correct ratio is controlled by three separate systems: the power of the engine (throttle), metering of the fuel into airflow (jetting), mixing of fuel with air for combustion (venturi /vacuum). Tuning of each system may be required to achieve maximum performance.
Since part one of this tech series mentioned only the vacuum system for the idle circuit, a description of the vacuum required for main circuit operation is in order. This can be described as the air drawn by the engine (airflow) through the venturi section, under the throttle valve and creating a depression at the nozzle. At full throttle opening the step of the nozzle creates a depression at the atomizer for fuel delivery into the venturi.
The emulsified or atomized fuel (see attachment for position of port) is dependent on the vacuum for getting into the airflow and on to the engine. At idle or small throttle openings the vacuum is higher than when the valve is partly or fully open. As the engine picks up RPM from the progression (from closed to about one-quarter throttle) vacuum begins to strengthen. For main circuit operation, the key component is the tapered needle, hence the ‘needle’ carburetor.
If the fuel metering system responded only to vacuum the engine would be too rich at low throttle and too lean at open throttle. The fluctuating vacuum signal needs the function of the tapered needle moving with the throttle valve to meter the fuel correctly at the various main circuit positions of throttle operation.
We have looked at the vacuum and air/fuel ratio requirements for the Dell’Orto carburetor. Next, how does the tuner apply this knowledge for getting maximum performance and reliability from the engine? Tuning the carburetor by each throttle phase and recognizing that power control, fuel metering and vacuum all play a part in achieving proper performance.
Knowing when the air/fuel ratio is correct or a combustible mixture is important when tuning for performance. Each phase of the carburetor’s operation is independent of the other and should be tuned separately. Remember that tuning the acceleration phase with the main jet can be dangerous.
Next in line for this Ekartingnews Tech series will be idle, progression, high speed and WOT phases.
Part one can be found at http://www.ekartingnews.com/news_info.php?n=2632
Tom Barth with Green Flag Motorsports in Waterford, Mich., has gained his experience and knowledge from 35 years of racing, maintaining, designing and building competition karts. The last 10 years has been focused on the shifter kart through sales and service to the karting industry. This experience is also backed up by 40 years of automotive product, manufacturing and assembly engineering experience.
Green Flag Motorsports offers additional information on the Dell’Orto Carb through the sales of The Dell’Orto Primer available on his website as well as sales and service of Dell’Orto parts for both ICC and TaG engines. The Michigan-based company is also a dealer for TM, Motori Seven and Pavesi engines in addition to the Birel and Topkart chassis and numerous other product lines. Barth offers rebuilding services and arrive and drive programs in addition to parts sales. For more information on how to order the Dell’Orto Primer or to learn more about his products and services, visit http://www.greenflag.net or call (248) 674-2325
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