elanner wrote:(Snip)... I vaguely assumed that the engine RPM while on the starter motor is so low that there isn't a lot of vacuum (especially when air is being drawn through the enrichment device) so the role of the idle jet would be incidental. Meanwhile, the advantage of the pump, of course, is that it doesn't need any vacuum at all.
Nick,
Venturi type carbs like the Weber and Dellorto have two basic circuits... the Idle Circuit and the Main Circuit (Constant Depression carbs like SU and Stromberg are a different can of worms).
At 'closed' throttle and small throttle openings, the engine is sucking against a 'closed' plate, which is a pretty effective way to create a vacuum. As the throttle opens more, the throat is exposed more and more to atmospheric pressure, and the vacuum weakens. You can't suck a strong vacuum against a closed plate when the plate is open.
However, by the time that vacuum is weakening, air flow velocity through the throat has also picked up, and the venturi is creating more and more vacuum. At some point, the rising venturi vacuum exceeds the falling sucking-against-a-plate vacuum, and takes over the carb's operation.
Sucking vacuum drives the idle circuit. Venturi vacuum drives the main circuit. The height of the fuel level (float setting), the fixed lift-over height, and the strength of the available vacuum determines the transition rpm between the idle and main circuits.
In a Dellorto DHLA, the designer selected a lift over height that results in a transition point at about 3200 rpm (if all other guidelines were followed in matching a carb to an engine). The idle circuit feeds the engine up to ~3200 rpm, at which point the venturi-fed main circuit takes over and feeds the engine up to red line.
In a Weber DCOE, different dimensional design choices were made, and the transition point is closer to 4000 rpm. It just is what it is. You can agree with a carb's design specs, or you can buy another brand of carb, but designed-in/ machined-in features are not adjustable. I like Dellorto DHLAs more than Weber DCOE's for a number of reasons, but mostly for their basic design architecture on a street engine.
The above design features are the designer's tools and choices.
Throat and choke sizes for a given engine size also affect vacuum strength, and can also have an impact upon vacuum strength at engine rpm, and upon transition point; but these are the engine builder's/ carb tuner's choices.
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For starting, the throttle is (nearly) closed, so the engine can draw a good, strong vacuum even at cranking speed. And it's the Idle Circuit, working on 'sucking' vacuum that feeds the engine during starting and up to the transition rpm.
The Idle Circuit and the Enrichment Device are both driven by 'sucking' vacuum. So when the 'choke' is pulled on, everything in the Idle Circuit continues as before, and the Enrichment Device kicks in to add and additional amount of fuel to the same airflow, creating a richer mixture. For the most part, it meters like a faucet... the more you pull it on, the more fuel is dispenses. It's not smart... at least no smarter than the operator. It's just a pre-set orifice and a vacuum source peeing additional fuel into the air stream. And it works as long as the 'sucking' vacuum supply is strong... ie, and small throttle openings.
Regards,
Tim Engel
PS...
I know, 'sucking' vacuum isn't a technical term, but it creates that mental image that helps with understanding how the carb works at low throttle settings.