Crude needle selection method SU carbs

[PeteStupps]

Evening all,

Just wanted some 2nd opinions on the half-arsed way I'm choosing needles for my twin HS2 SU setups. My mk3 Spitfire and 1300fwd heads are both vaguely non-standard, each having larger valves, some gentle porting and a bit of a skim to raise comp ratio. For both I started with the standard BO profile needle (fixed, not biased type).

Basically I'm examining the plugs after different running scenarios. So I set it up at idle as per the manual using the lifting pins, then do some long runs on a suitable fast road where I can shut off the engine abruptly and see how the plugs look. Did this with the Spit last year and went through 3 sets of progressively richer needles, ending up with CT profile. These gave a nice biscuit brown after a good session at 60 mph in top gear, and also look a nice (slightly darker) brown after chugging through london traffic for an hour or two.

The fwd is on leaner BW needles but looked too lean after bombing around the country lanes and now are sooty after 4 hours of stop-start London driving.

Is this a reasonable method? I do intend to have the Spitfire on a rolling road once i've got it running again but can't really be bothered for the little saloon. AFR gauge is another option but would require work and money...

For reference here's the standard BO profile compared to what's in each car currently. Spitfire has a tubular manifold and bigger valves than the fwd, incidentally.

 

[PeterC]

The mixture is not proportional to annulus area, and it will also richen disproportionately as the needle lifts out of the jet. Also once the butterflies are more than 30deg open pulses of pressure from iv valve operation can reach the jet and richen the mixture. This effect is useful to know; tune the mixture when running at <30deg butterfly and it will richen  more at full throttle, which is desirable. Your steady 60 mph probably meets the <30 deg condition.  The pulsing action is partly dissipated by the balance pipes on the manifold, so to make full-load mix richer they can be eliminated (but this is not widely known).

SU theory is here:https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/

Peter

[PeteStupps]

Thanks @PeterC, I have read that excellent write up before but will do so again. Appreciate that there's more going on than first appears with these (aptly named) constant depression devices....