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Carburettors with more depth of discussion

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K2-K6:
To discuss and, hopefully inform, with more in depth discussion of carburettor function and how it relates to these SOHC bikes along with their setup.

Rozabikes Tim:
Great idea. How about people like yourself copy and pasting some of your previous helpful descriptive posts on the topic to this section Nigel?

K2-K6:

--- Quote from: Rozabikes Tim on October 04, 2024, 08:50:12 AM ---Great idea. How about people like yourself copy and pasting some of your previous helpful descriptive posts on the topic to this section Nigel?

--- End quote ---

 ;D literally at just this moment thinking the same to capture items already discussed.

I'm out today with some car bods but will get something done later.

K2-K6:
In response to the role played by idle jet function specifically around hanging idle effect

I'll try to pick the right wording to describe what their influence is.

Its situational rather than hard rev band or throttle position.  This because the engine is responding to volume of ai, while the carburettor responding primarily to vacuum (the drop below atmosphere via venturi design) therefore it'll flow differently for different situations.

Example is if you rev the engine then drop the throttle instantly/fully, the air volume stops but the vacuum now climbs as the rpm decays according to flywheel energy. That provides a spike in fuel with the slide closed, making it temporarily rich which helps to quench combustion, finally to recover as these two influences equal out. All in a very short space of time. Failure to do that by having  restriction in delivery jet, then it'll go lean, relatively, and promote "hanging" rpm.

Anther example .... if you're approaching a long sweeping bend and go down a couple of gears, throttle off to near closed, then say it's running 5000 rpm that'll be pulling tgrough the idle jets as vacuum is high, this facilitates it going immediately you then open the throttle, if it didn't do that you'd get a flat spot up there that's really annoying as initially it would be lean. So a much higher influence than most would suspect.

It's also why you get popping on overrun if there's an exhaust header leaking air into system. The fuel that's in excess from moving towards rich with throttle closed passes into the exhaust unburnt in some quantity, availability of leaked air then brings that back to combustible in the hot exhaust pipe, and so you get popping from air leak but supplied by excess fuel from idle jets working correctly.

K2-K6:
The effects described above you can observe, in vacuum terms, when you're running with vac gauges hooked up.

Running at idle, give the throttle a quick blip to lift revs to say 7000 rpm, watch the vacuum drop as the slide comes open, then when you drop it straight away the vacuum hits a high point as slides close with engine rpm still elevated as it's now drawing against a closed venturi.

These are some of the fundamentals that design of carburettor has to work round.

I've referred to fuel injection system sometimes, which may bring the question of "what's that got to do with our bikes ? " to which the answer is, the injection system has to get around the same fuelling idiosyncrasies (it's not working on vacuum to meter fuel ) as carburettor, but with the added detail that each "workaround" has to have a specific method and control regime to achieve.  It just shows a bigger picture in examination of fuelling system and their strategy.

The combustion chamber "sees" only fuel air ratio from either system, being effectively blind to what's delivering that mix.

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