SU carburetors got you down?  Does it seem like everyone you ask for help says, "Those carbs are junk."  If that's you, read on:

    When I had my first Z, the ill-fated '73, I had no luck getting help in setting them up.  Although it had the earlier '72 (Round-Top) SUs, I couldn't figure out how to set them.  I ended up installing the Holley Four-Barrel, which worked great.
    However, I have since learned how to set up the SUs, and I'm very happy with them.  There are good write-ups on balancing the carbs found at the Internet Z-Car Club web site, so I won't cover that here.  Setting balance is fine and dandy, but the mixture is equally important.  Some folks may tell you that you need to spend $50 or so for a Color-Tune.  Bah Humbug!!
    On IRC one night, Doug Antelman told me how to set the mixture without a Color-Tune.  Having given this information out on several instances, I decided to do post it here for the world to see.

Warm the engine up, then shut it off: Re-start, and:     Additionally, elevation and temperature can affect tune.  The best way to get them spot-on is to get the car running properly, then adjust the carbs as necessary to get the best performance.  Also, the front or rear carb may need to be richened or leaned to match the other from step eight.  Once you get it running properly, drive it for a while, then pull the plugs.  You want all six plugs to look the same.  If three of the plugs are darker (dull black - glossy black is probably oil), lean that carb out 1/2 a turn  and recheck.  Of course, any modification requires rebalanacing the carbs....

    Here's some symptoms of incorrect adjustment.  Of course, all these problems can be the result of other problems, so consider this chart when attempting to diagnose problems that appear after tuning the carbs.

Too Lean?

Too Rich?

Poor low RPM performance
Fouling plugs
Running hot
Poor mileage

    And now a few words on carb fluid.  I've found the level and type of fluid in the carbs to be of the utmost importance.  What concerns me is not the fact that it's critical, but the fact that it's seldom mentioned.  I've corrected some pretty major-apperaring drivability problems, just by adding oil to the carbs.  I've heard of people running the carbs dry, and others who used water.  I can't vouch for their success, only my own.  So here's what I've found:

Type of fluid



20 wt
Factory Original
Probably works great on brand new carbs, but there aren't a lot of those around these days!
Works very well.  Thin enough to be responsive, but thick enough to smooth out off-idle performance.  Requires that carbs be in good tune, and that throttle shaft bushings not be too badly worn out.
Moderate Driving / Worn Carbs
Thicker than either of ATF or 20 wt.  Overall performance is similar, but responsiveness suffers.  Makes the vehicle run better at low RPM.  Thick enough to counteract leaking throttle shaft bushings and poor tune.

After passing this on countless times via email, I figured I'd post some info on setting the needle position.  Many of these cars have been around the block a few times, so often times, the carbs are way out of whack - too far to straighten out with the above procedure.  F'rinstance, on my '66 1600, the rear carb was about five turns out, whereas the front carb was at around two.  Yikes!  So, when you encounter something like that, or when you just can't get them lean enough, try this:
  1. Warm up the car
  2. Shut it off and remove the carb domes (take out the dipsticks first)
  3. Remove spring from dashpot (the piston-thing that lives inside the dome)
  4. CAREFULLY remove the dashpot - see the needle on the bottom?
  5. Loosen the set-screw that holds the needle so that the needle can move a bit
  6. Turn the adjusting nuts all the way in (located underneath the carb throat, where the fuel line enters from the float bowl)
  7. Pull the needles out about 1/8" or so, you should see the shoulder of the needle sticking out past the dashpot body
  8. CAREFULLY reinstall the dashpot, and push down ever-so-gently to seat the needle fully in the nozzle (that's the hole the needle sits in)
  9. Pull the dashpot back out, snug down the needle set screws, and put them back together.
  10. Set the balance and mixture as above.

Do you use something different in your SUs?  Other carb related info you want to share?  Questions or comments? Bring 'em on!
Also, see ZTherapy for more info on SU carbs.