How long are your needles ?

[tim wood]

Hi, I'm setting up my SU,s and have used a depth gauge to set the jet depth at 65 thou as seemingly I read this somewhere. I thought that if jet depth was so critical then how about needle length . So I measured these. The amount protruding from the base of the piston varies by about 29 thou across all three.
I could spend time equalising this but will it be worth the effort?

Any advise or thoughts welcome.

Tim

[Heuer]

Tim

There are several excellent SU instructions in the Forum Knowledge Base>Manuals, Technical Guides; I particularly like the period SU Tuning Guide.

[christopher storey]

It is absolutely vital that the needles are set correctly. The shoulder of the needle should be exactly flush with the base of the piston

[Nick]

I recall that the needle diameters are measured from the shoulder not the pointed tip of the needle, so you should set all 3 with the shoulder flush.

Nick

[tim wood]

All, thanks for your replies. The older needles did indeed have a shoulder which made things easier but the later ones have a groove. Thus the question is:
To the centre of the groove or the edge. We are only talking thousandths so it may not be critical?

[Nick]

Tim,

The lower edge of the groove should be flush with the base of the piston (from Jaguar service manual). You need to equip yourself with some books !
:)

Nick

[tim wood]

Nick, I have loads of books but this later needle didn't feature in any of them. However to complete my education and add to my resource library I have purchased (and it's been delivered today) the SU carburettor manual.

As you say it references to the lower edge of the groove.