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
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)
3) Set balance with Uni-syn
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....
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
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 =
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.
Poor low RPM performance
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
||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:
Warm up the car
Shut it off and remove the carb domes (take out the dipsticks first)
Remove spring from dashpot (the piston-thing that lives inside the dome)
CAREFULLY remove the dashpot - see the needle on the bottom?
Loosen the set-screw that holds the needle so that the needle can move
Turn the adjusting nuts all the way in (located underneath the carb throat,
where the fuel line enters from the float bowl)
Pull the needles out about 1/8" or so, you should see the shoulder of the
needle sticking out past the dashpot body
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)
Pull the dashpot back out, snug down the needle set screws, and put them
Set the balance and mixture as above.
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.