What is this part? (SU Carbs)

[DBarr]

Hey everyone!

I'm adjusting my SU carburetors and have a specific question about what I believe to be some kind of positive stop for the fuel ratio adjustment?? (see picture).

I installed a new header from Dean as well as a Petronix electronic distribution and then everything started running a little rough. I had a good idle, but as soon as I hit throttle I had shutter about 18-22 hundred rpm. Once I got up to 3k it settled down. I had the engine out and the intake off, and it wasn’t tuned all that well before so I figured I'd give a carb adjustment a go.

I got the carbs balanced to one another, but had to adjust the little knurled ring on the underside of the dashpot on the rear carb to richen my fuel mixture, presumably because of decreased exhaust resistance. I had to put it so low that the "stop" (whatever it is) had to be backed way down so that it wouldnt interfer with the knurled adjustment. Anyway, I got the rear carb adjusted and the idle smooth out at ~2k-~3k rpm and it is running really well now. My only concern is that the stop(whatever it is) is set very low so that there is minimal grab by the threads. I cant help but feel that having it "bottomed out" like this may be an indication that some other setting is wrong.

I should add that the forward one was already set to a similar position (nearly bottomed out).


So,
What is this part?
Does it matter that it grabs so little?
Opinions from the experts? From the peanut gallery?


Thanks so much!,

David



screencapture

[jrusso07]

From the peanut gallery. It is as you suspected a positive stop for the mixture adjust/jet height. Some of my cars have it, some don't and I don't think it matters. My guess, it was added as a emission control item on late model cars to prevent people from setting the mixture overly rich. But as you see it has it own adjustment....not as fool proofed like some of the English Zenith Stromberg carbs of the era that plugged the mixture adjust access.

[dbrick]

I'm guessing many have vibrated loose and fallen off and nobody noticed. If it's only function is a stop, I would remove it. BTW, if you still have a stumble, try thicker oil in the carbs and be sure your ignition is advancing properly.

[DBarr]

Is the fact that its near the end of travel evidence that I need a different jet?

[spl310]

Possibly you need different needles and jets. That is a smog part that was only on the 68 and later cars. Your car being a 66 means that it has had the carbs changed out at some point, and they likely still have smog needles in them.

[Mainer311]

Glad you posted this. Mine has them too and I never knew what they were.

[SDkvrd]

Early 240Z carbs (SU's) have them too. Removing them won't affect anything unless you happen to dial it out so far that the finger nut comes off!