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The only thing the needle does is to regulate how much fuel gets into the engine at various vacuums, Tony - which depends on rpms, throttle position and engine load. The 'profile of the needle is the calibration device - carefully computed to ensure proper air/fuel mixture.
Engine power comes from burning petrol - but to burn petrol you need air. The ideal mix is 14,7 parts air to 1 part fuel, by weight - more fuel without necessary air does not increase power; unburn fuel is simply expelled with no power contribution.
Too lean simply does not fully utilize the available air - power loss.
Basically, xk engine were set up to run slightly fat, the xk tolerates fat mixture with ease - but does not lie running lean.
A ‘fat’ set-up also compensates for carb imperfections, ensuring that the mixture does not go ‘lean’ under any circumstances. Being mechanical/analogue carbs are simply not able to ensure a consistent response to carb airflow.
The only drawback with running fat is pollution and excessive fuel consumption; in Europe with no emission regulation they did not care about the first. And Jaguar disregarded the latter, Jaguar drivers, at the time, presumably wanted performance over economy. The advent of emission regulations in the US, a very important Jaguar market, forced Jaguar to ‘do something’.
However, leaning out the mixture was one option, and necessary, to make other emission control hardware perform properly - but leaning out mixture was not the main reason for power loss. Other factors, the emission control hardware and resetting required for the lower octane rating played a part. The xk was designed for 98 octane available for longer time in Europe, hence the usual higher compression ratio. So needle profiles was one factor, but carb choice another - Strombergs, with their better emission features were not fitted to Europeans…
All the above is rather crude assessments excluding finer points on very complex interactions. In your case, disregarding emissions, you may prefer ‘fat’ - a secondary element is that fuel act as a coolant. However, ‘fatter’ beyond ‘ideal’ will not give more power - but will ensure against flatspots. The best choice of needle profile is more a matter of trial and error - Jaguar chose their set-up based on perceived use of a Jaguar - taking market specifics into account. Your driving style and modification may benefit from a different than standard needle profile…
But do not expect miracles from needle choice - some experimenting with ignition timing won’t come amiss…
Frank
xj6 85 Sov Europe (UK/NZ)
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