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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
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4000 rpm is the "Transition Point", it's a function of design, and is built-in to the carb. It is not adjustable. (Similar but different... the Dellorto DHLA Idle Circuit feeds the engine up to 3200 rpm.)
Remove the progression hole covers, look in, and you'll see a number of small holes drilled through the wall of the 'tunnel' into the throat. Open & close the throttle, and you can observe the edge of the butterfly sweep past the drilled 'Progression Holes', progressively opening more and more hole area to the flow of fuel into the throat. Some Webers will have as few as two progression holes (very crude, coarse metering), and some Dellortos will have up to six.
As the edge of the butterfly sweeps past the holes to the closed-throttle idle position, the last of the progression holes is very nearly (maybe even completely) closed off such that it's not flowing enough fuel into the throat to keep the engine running.
But notice that the fuel delivery 'tunnel' continues past the progression holes to the Idle Mixture Screw at the carb's mounting flange end... well beyond the reach of the butterfly's edge. The Idle Mixture Screw is in the last fuel feed hole, not affected by butterfly position, and it's there to provide an adjustable fuel flow into the throat AT IDLE. Being adjustable, it's essentially a secondary throttle for that Closed throttle zone after the butterfly has swept past the last of the progression holes as it closes.
The Idle Mixture Screw is only used to adjust the closed-throttle mixture at idle rpm. It is not for adjusting O2 sensor readings at any other rpm! Use your CarbTune to 1) balance the carbs, and then 2) set the idle mixture screws as necessary to produce peak intake manifold vacuum. (Note: you do NOT use the Idle Mixture Screws to BALANCE the carbs!) Make th balance and peak manifold vacuum adjustments at an artifically low idle rpm... as slow as possible consistent with smooth running... no stumbling or misfires. After the Idle Mixture Screws are adjusted for peak manifold vacuum, set the idle rpm back up to 900-1000 rpm.
After that, HANDS OFF on the Idle Mixture Screws.
If you don't like the O2 sensor's reading at idle, then change the Idle Jet and/or Idle Air Corrector leaner, then re-adjust the Idle Mixture Screw to once again achieve maximum manifold vacuum at slow idle with the new jetting.
Adjusting mixture/ O2 sensor readings is a function of replacing jets and air correctors, not turning a screw. And no, it's not convenient. You don't adjust the flow rate thru your lawn sprinklers by turning the fawcet in your bath tub. You don't adjust O2 levels at rpm above closed throttle idle rpm by turning the 'IDLE' Mixture Screws.
Installing an O2 sensor does not transcend the need to understand your carbs, and how to tune them. And no, Webers do not have high resolution similar to a fuel injection system with sensors and feedback loops delivering real time status to an ECU. You may think you have a scalpel, but Webers are really more like an axe.
Each throat has a replaceable choke, main emulsion tube, main jet, main air corrector, idle jet, idle air corrector. Those are the six playing cards you get to shuffle. And once in, they're not variable... no adjusting screws. And if you choose incorrectly in pursuit of some optimal Air-Fuel ratio/ O2 sensor reading, then you can easily introduce an off-idle hesitation, a lean transition stumble approaching 4000 rpm, or a rich bog-down after 4000 rpm.
Regards,
Tim Engel
Last edited on 06-26-2020 08:40 pm by Esprit2