MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Newbie SU Question


How difficult is it to change needle in an SU carb? Are special tools required? Any web resources that detail the process?

-Josh
Josh

Not difficult in itself, just a couple of screwdrivers and a few minutes, although it depends on whether you are fitting a fixed or swinging needle. But the big question is 'Why?' If the jet with a fixed needle was poorly centered it could have worn the jet as well as the needle so both must be changed. In any event you really should setup the carbs from scratch after changing any parts, and that means checking and adjusting the valve clearances, plug and points gap, and timing first.
Paul Hunt

Paul

"But the big question is 'Why?"

I was told that I may need to change needles after fitting a free flow exhaust, K&N filters, a warmer cam and an improved head.

"a fixed or swinging needle."

Not sure. What is the difference? It's a single HS6 for a midget 1500. I posted here because responses are faster and it's one of those universal question.
Josh

Josh

A single HS6 on a 1500 Midget ?? Sounds interesting.... Yes, would say needle would need changing with mods you listed, But (and I cannot say this enough) get along to your local rolling road/dyno and get it set up properly. You know it makes sense.

Fixed or swing needle ?? Early SU's have the needle physically fixed in the central position in the piston. This means when you disturb the piston from the body of the carb, like when you inspect or replace the needle, you have to re-centralise the jet to the needle. It's not hard but a bit of a pain. So later SU's had the needle retainer changed slightly. There is a very small spring on the "back" of the needle, and the collar that the needle sits in has two little pips on it's back. The idea is when the needle is fitted, the spring forces the needle onto the pips which means the needle is now sprung to one side. With me so far ?? The upshot of this is that the needle is now biased to one side, you don't need to centralise the jet anymore. Downside ?? Well the needle runs up the side of the jet all the time so after a while could wear the jet slightly oval. In everyday use the SU will correct for this, but for motorsports or a carb of dubious background it might be prudent to change the jet.

Phil.
Phil Hill

This thread was discussed between 15/04/2002 and 16/04/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.