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:
1) Turn both carbs' adj. nuts all the way in (lean)
2) Turn both carbs' adj. nuts out three turns (richen)
Re-start, and:
3) Set balance with Uni-syn
4) Turn in one carb 1/2 turn, and listen for decrease in RPM
5) Turn the other carb in 1/2 turn, and listen for the same decrease in RPM
a) if the RPM difference isn't the same, you may need to turn one in more than the other to get them the same. Don't do more than about one full turn difference. The differences are pretty slight, and if your carbs are worn out a bit, you may be unable to detect a difference. If you can't, don't worry too much....
6) Repeat 4 & 5 until the car stalls
a) it may not stall, if the carbs are worn out. If it doesn't stall, turn the adjustment nuts all the way in, and continue
7) Turn both carbs out two full turns
8) Rebalance, and see how it runs
a) depending on the condition of your carbs, you may need to go richer than that. I.E., on my L28/E88/SU, with good carbs, I'm at about 2.5 turns out from stall. My wife's L24/E88/SU, with worn-out throttle shaft bushings, has to be set at about 3.5 turns out from all the way in out to get off-idle performance acceptable. Although, if my car stalls at 1 turn out, our cars are set the same (1.0 + 2.5 = 3.5)
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? |
Pinging |
Smoking |
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 |
Application |
Comments |
20 wt |
Factory Original |
Probably works great on brand new carbs, but there aren't a lot of those around these days! |
ATF |
Performance |
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. |
20/50 |
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. |
NEW!!
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:
Do you use something different in your SUs? Other carb related info you want to
share? Questions or comments? Bring 'em on!