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| Features |
| Tech Talk: Tom Barth’s Dell’Oroto Series – Part 3 |
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by Tom Barth, Green Flag Motorsports
http://www.greenflag.net
Dell’Orto describes the control of engine power as four phases of throttle operation. Idle – Progression – High Speed and Wide Open Throttle (WOT). Understanding these phases as a throttle valve position can help the tuner determine which carburetor components may need adjusting or replacing for maximum performance.
Beginning with the idle, the throttle position is open just slightly for proper idle. The variable components for setting idle are: idle air screw, throttle valve adjustment screw, idle jet and the idle emulsifer tube. The idle air screw is located on the side of the lower carburetor and functions by increasing or reducing idle air to the idle circuit.
Turning screw in richens idle mixture and out leans mixture. The idle emulsifier tube is positioned in tandem under the idle jet. It emulsifies the idle mixture and is also a well or reservoir for fuel needed during the progression phase. The throttle valve adjustment screw raises or lowers the valve for desired idle speed. The idle jet can be replaced to richen or lean idle fuel. Tuning components: air screw and idle jet.
The progression is described as a throttle position to one-quarter open. It might be helpful to mention here that the Dell’Orto carburetor is designed with two separate operating circuits - the idle circuit and the main circuit. The progression phase is sometimes referred to as the transition from idle to main circuit. Or, to provide additional fuel mixture to get the acceleration started as the engine RPM has not, at this point provided sufficient vacuum to pull fuel from the main circuit.
A note here: when using a very lean main jet, a condition can occur called ‘lean peak’ and can damage the engine after continual use because of low vacuum (during progression) each time the throttle is opened. Remember the main jet always controls the amount of fuel entering the engine during main circuit operation. With a lean main jet, lean peak will occur every time the throttle valve is closed, then opened again, such as in shifting. How many times a lap? Take note tuners. Tuning the progression phase is done with the idle jet and/or the throttle valve cutaway.
Hi Speed is described as the throttle position from one-quarter to three quarters of throttle opening. From one-quarter throttle on, vacuum is strong and the main circuit is pulling fuel necessary to meet the engine’s power needs. The tuning in this phase is done with the atomizer, and/or needle and needle clip position. The atomizer design should also be taken into consideration during hi speed tuning. The length of the atomizer, either the lower end that is emerged in the fuel or the length of the stem end that screws into the nozzle will determine the fuel timing as it enters the engine. If the fuel gets to the engine sooner it is richer, later it is leaner. Of all the tuning components the atomizer plays a major roll.
Wide Open Throttle or WOT is the throttle position at full open. The WOT phase is tuned with the main jet only. When you get to WOT phase in your carburetor tuning program the hard work has been done. If the idle, progression and high speed is right, select the biggest main jet that the engine will take and go race fast and long.
Tuning Tips:
Understand the features of the tuning components: needle – straight section length and diameter – tip diameter – taper length – atomizer orifice diameter – length of submerged section – length of stem the screws into nozzle.
Don’t lean down the main jet until you work with the high speed phase first.
Tune to A/F (14/1) ratio for all four phases.
Knowing fuel timing will help the tuner get top performance quicker, i.e. from progression to high speed the atomizer round section length and stem length will determine how crisp acceleration is.
Needle taper is used to introduce acceleration earlier or later.
Realizing that much of the series is focused on the ICC carburetor because of rules restrictions to Max and TaG engines it should be noted that the basic principles are the same for both carburetors. TaG’s and MAX’s are difficult to dial in because the tuner has very little room for changing operating characteristics.
Much has been written about the ‘high cost’ of the ICC. I don’t subscribe to this. From what I see, most damaged ICC’s are the result of improper carburetor settings, i.e. too lean! Burned crank big ends and scored cylinder walls are significant warnings that in the quest for power, the trade off is expensive rebuilds. That mentality pretty much destroyed the high performance moto market. Let’s not do it to the ICC too. With proper carburetor adjustment the ICC will perform at maximum until the required rebuild time.
The carburetor is the single most important component on the ICC and Max/TaG engine in terms of tuning. The rules restrictions allow very little to be gained by expensive engine modifications. Attention to detail and track testing to get driving, handling and power to the track will pay big dividends to the low budget privateer.
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