I have finished rebuilding two 4 screw SU carbs. In the manual, it indicates that the needles should be flush with the piston surface. However, I've been advised to leave a fraction of the needle shoulder, 1/16th of an inch, still visible. Anyone have an opinion?

The manual says it, and Scott Bruning of Ztherapy.com said it, so i installed mine flush. I suppose either way will work, as you'll compensate when tuning the carbs. FWIW, Jeremiah

I was told the reason you do not want to leave a shoulder on the needle is that it will require increasing the setting on the fuel mixture nut to accommodate the increased restriction in the nozzle. This increased setting will cause the carbs to then be too rich when you open the throttle (effectively changing the taper of the needle).

Seems logical, but I can't prove it. I would think though you would causing some added wear to the needle and nozzle, thereby shortening their live.

In his ZTherapy video-tape, Scott Bruning offers a simple, no-tools method of setting the needle "height". Insert the needle with "excessive" shoulder exposed and tighten the set screw just enough to hold it in place. Put the piston in place and press down until it bottoms out - the needle will bottom out and slide up in the piston to the "correct" position. Carefully remove the piston and tighten the set screw.

I use this method with my SU's and the car runs great.

Hi all,

just restarting a old post.

What is the surface of the piston, is the outmost face or is it the rebated groove where the "hole" is???

Just about fell out of my chair reading this thread mentioning Scott Bruning as "the source". Then I caught the dates of the posts.

The bottom of the piston is the "bottom" of the piston..... Any adjustment up and down is as previously stated, like putting in a different needle.

Just use a straight edge across the face of the piston , have the shoulder of the needle touch the tool and tighten the screw. That's it . Gary:)

Just use a straight edge across the face of the piston , have the shoulder of the needle touch the tool and tighten the screw. That's it . Gary:)

That's how I did mine. Worked fine.

thats how ive always done mine too but I always wondered if i was doing it wrong. thanks guys

I'm looking for some help with carbs on 72 Z.

I removed pistons on carbs to check needle settings, found the rear carb has been replaced with "rebuilt" carb. Has small rebuilt sticker on back of vacuum tower.

Needles and spring are different, both shorter in rear than front.

I need to correct problem with carbs before going to next problem with restoration.

Car runs pretty good and idles smooth, carbs respond to adjustment and are presently balanced.

Do I need to correct the needles and spring differences????

I want to make this car right.

... let us know what length spring is in the un-rebuilt carb and the length of spring in the rebuilt carb. Buy a set of matched needles of your choice and a correct spring or set-of springs, and reassemble.