Do SU Carb Needles Wear Out?

Started by Rosebud, January 16, 2023, 04:20:37 PM

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I've got 40k miles on my Seven Enterprises built 1380 A+ motor.  Since new, my A/F ratio was always in the 14:1 range on the highway and my plugs were a nice chocolate brown. Lately I've noticed that my A/F mixture is running overly lean at highway speeds. My wide-band O2 gauge now shows 16:1 to 18:1 on the highway and my plugs are overly white. When I go WOT the A/F ratio drops to a reasonable 12:1, but leans out again when I back off.

I've been richening the SUs in an attempt to bring down the A/F mixture, but then I run too rich at idle and lower RPMs. Nothing else has changed. The air cleaner has been serviced, my timing is spot on and my plugs are new. I have no vacuum leaks (that I know of).

It has occurred to me that perhaps the needle profiles have changed over time and need replacing. Is that possible? Any thoughts?

Twin SU® HS4 1 1⁄2" carburetors
Fast road cam
10:1 compression ratio
200+ psi in all cylinders
...the sled, not the flower


Yes, theneedles can wear.  However, worn needles will generally cause the system to run richer:
- Worn needle = narrower needle
- Narrower needle = more fuel exposure
- More fuel exposed = richer mixture

Running leaner = less fuel or more air.

Did you change types of air filters (such as now using K&N filters) that may be allowing greater air flow?
Have you actually checked for any air or vacuum leaks?
Worn throttle shafts or seals?


Needles can wear, but it would take a very long time. I would say your 02 sensor is maybe failing, but if you're noticing the plugs changing colors that reflect the wideband, then that's not it.  However, if you're running a vacuum advance 16-18afr at cruise is a nice lean cruise mixture for fuel economy. If the car stumbles at cruise then richen it up.  12:1 at WOT is too rich and you should be shooting for low 13s. Anyways, you could have a vacuum leak that's introducing air too. 


sounds to me like you may have an air leak somewhere.....
Complete failure at retirement

1989 Cooper Racing Green
2009 Clubman S
2014 Audi Allroad

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