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Needle profile, SU carb, guide to throttle opening
Here is a rough guide of where to file the carb needle (assuming the carb throat diameter is suited to the engine cc):
Top (thick) 10% idle < 2000rpm
Middle 30% mid 2000 - 4000rpm
End (thin) 60% top end > 4000rpm
Take care when filing the idle area. It is very sensitive. To leanen an area, file the rest and wind the mixture screw out slightly.
Needle codes for SU carbs
Two choices here - either have a look at the selection of needles codes and the vehicle specs that they were used in below, or alternatively visit www.winsu.co.uk and download WinSU. This is a program written by the IME creator, Scott Beavis, to help visualise and compare needle profiles. It also has a "needle-picker" to help pick a decent needle for your engine spec (trial version gives a limited number of suggestions)
Single carbs :
|EB||850cc and rebores, 920cc, 1.25SU, 9:1 CR|
|ADE||998cc, HIF38, Metro inlet, std filter, free flow exhaust|
|ABY||998cc, 12G295 head, HIF-38, Stub Stack, K&N, 3 into 1, RC 40, std cam|
|AAU||1100cc, 1.5in SU, 3into1, heated inlet, RC40, K&N|
|AAA||1275cc, HIF38, 9.5:1CR, conical filter|
|BFY||1275cc, HIF44, Standard Rover Cooper setup|
|BEJ||1275cc, Standard 1.3 Metro <??>|
|BDD||1275cc, ERA Turbo|
|BCA||1275 MG Metro, standard head, HIF44, K&N, stub stack, Maniflow LCB, single box side exit exhaust|
|BDL||1275cc, HIF44, 9.75:1CR, MG inlet, LCB, RC40, K&N, std head|
|BDL||1275cc MG Metro standard|
|BDL||1293cc, HIF44, 1:10, heated inlet, LCB, K&N, stub stack|
|BDL||1380cc, HIF44, 1:10, heated inlet, LCB, K&N, yellow spring|
|BDL+||1310cc, HIF44, stg2 head, 276 cam, big bore exhaust|
|BDL+||1330cc, HIF44, 11:1CR, MG inlet, LCB, RC40, K&N, 37x29, 286|
|BBZ||1330cc, HIF44, maniflow LCB, 276 cam, playmini exhaust|
|BDR||1380cc, HIF44, alloy in., LCB, K&N, stub
stack, 285, MG Metro head.
|BBZ (x 2)||1330cc, 2 x HIF44, maniflow LCB, 276 cam,
'+' denotes that the profile was filed at the top end slightly to richen it.
The Axx and Bxx letters denote jet size. A for HIF38, B for HIF44. A will work in B (rich) but not vice versa.
Needle richer than BDL
The BDP is significantly richer in the top 3/4 of the range. BDN is leaner at the bottom and richer at the top than the BDL. I think BDN would be my choice short of a rolling road.
Needle richer than BDL (II)
BDR is only very slightly richer in the low to mid-range and then becomes insanely rich at the top end. There are only about 2 or three needles richer than BDR at the top.
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