It seems to be that time of year again, so here goes....
Tuning Your S.U. Carbs
-Roger Garnett Rev. 4/19/90
Well, it's not really that hard to set up SU's, just different. Of course
it always gets more interesting when you have more than one...
There is a very good Haynes SU carb manual available, reccommended reading.
Here is a laymans guide to adjusting SU's (long):
step 1- Tune up the rest of the engine- REALLY! clean or replace, and set
the points, set the timing, plugs, valve lash, and remove the air filters.
(have new ones ready) All of these things can affect the setting of the
carbs, which should be done LAST, (if at all). The carbs rarely need to be
adjusted, once set. Also replace/install the gas filter. Of course, it helps
if the carbs are in good mechanical condition as well. But you can consider
a rebuild once you have gotten things working first!
step 2- clean the carbs! use gum-out or similar stuff, clean all external
linkages, shafts, and stuff.
step 3- Remove the float bowl covers, clean the float bowls, remove old
sediment, and check/adjust the float setting. (turn the cover upside down,
and get a 1/8 in drill bit, set the drill bit accross the cover, the float
tab should just touch the bit.) Make sure the needle is moving and seating
properly. This is just like *most* floats. Replace the cover.
step 3b- Go get a pint of ale, or something close, and set it nearby.
step 4- Remove the piston covers. CAREFULLY remove the piston, DO NOT
BEND THE NEEDLE. Set the piston down on a clean wadded rag to prevent
rolling. Clean the inside of the carb. Check operation of the throttle.
Check the throttle shaft slop- this is the most common place for wear
on an SU, and is often where air/vacumn leaks occur. The bushings and
shafts can be replaced, but it requires some machining. A small amount
of leakage can be tolerated, the car just won't idle as evenly.
Clean the piston. Stare in awe at the odd carborator design, simple and
effective, (constant velocity). Dump the old oil out of the damper if
you haven't already spilled it. clean. Reassemble, check piston movement,
raise it, then let go, it should fall freely. If not, check assembly
again, make sure the piston isn't binding against the carb body, it should
ride only on the damper shaft. Do not strech the spring. When all is operating
properly, fill the damper with Marvel Mystery Oil for light damping, or use
motor oil for heavier damping. (I use MMO)
step 5- Start the car and warm it up, then turn off/disconnect/otherwise
disable the choke mechenism. This will get set later- There are two
types, thermostatic, and REAL (cable controlled).
step 6- Check coarse throttle adjustments- make sure the throttle cable
pulls on both carbs equally, and returns completly when released. This
is adjusted by loosing the set screws on the throttle shaft and matching
the two sides. You can also adjust the cable length at this time, using
the cable set screw/retainer at the end of the cable. You can check the
float adjustments now by removing the piston & cover, and looking at the
fuel level in the needle seats. Both carbs should be about even, a little
below the top surface of the jet. If not, readjust one or both floats to
match the level.
step 7- Syncronize the throttles- if you have a uni-syn, here's your
chance to use it, (air flow guage), if not use a tube and listen to
the airflow. The Uni-Syn is much easier to use, and can result in
better balance. Alternatly adjust the idle screw on each carb, attempt
to set the idle as low as possible (~800-1000 RPM). Adjust until the
airflow is *close* to the same at each carb. The engine may now be running
rough, just keep the idle speed high enough to keep running. Give the
throttle a quick snap to make sure everything is settled, then check
sync again. Periodicly snap the throttle to make sure everything is seated.
Magic Time- Relax, and shake your voodoo rattle...
step 8- Adjust the mixture- this is done with the spring-loaded hex fitting
under the carb, where the fuel supply tube enters from the float. Turning the
fitting raises and lowers the needle seat. Pick a carb, and turn the fitting 3
flats (1/2 turn), first in one direction, then back 3, then 3 in the other
direction. Note where the engine runs better, idle speed should increase.
Turn to the best setting. Repeat this proceedure until you get the best
operation you can, (higest idle speed), keeping track of flats turned will
help you remember where you were. If you get lost, turn all of the way in,
then back out 12 flats and start again. Periodicaly snap the throttle and
push up on the fitting to make sure everything is seated.
When you think you're close, stop, uncramp your fingers, breath deep,
and do the same to the other carb. Then retune the first carb, and then the
second again. This serves to match the mixture of the 2 carbs, and prepare
you for the beer sitting over there in the sun. (why do you think the British
drink warm beer?)
step 9- repeat step 7, setting the idle speed as low as possible, and
re-syncing the idles. Now go back and readjust the mixtures. After a couple
of iterations, the engine should be running smoothly (controlled by mixture)
and at a low idle. Repeat as necessary. Set the final idle to 800-1000 RPM,
depending on the condition of the rest of the engine.
step 10- DRIVE! Take a quick spin up the lane, you need the reward of driving
a properly tuned car.
step 11- Adjusting the choke- I won't get into the temperature compensated
needle in the type HIF, or the Thermostatic choke in the later strombergs.
Check the manual for more info.
The choke is supposed to do two things; the first half of travel moves a
cam on each carb which opens the throttle, for warm up. The second half
pulls down on the needle seat to enrichen the mixture, for starting.
Start with the choke in the off position (knob in). Adjust the
so that the cam only starts moving the throttle after you start pulling
out on the cable (adjust with shafts and adjusting screws). Try to get both
carbs adjusted the same, you can check this with your unisyn or hose, but the
choke adjustment is less critical than the operating settings. After the cable
is about halfway out, it should start engaging the lever which pulls down on
the needle seats. Adjust the linkages so both carbs are acted on equally, you
can do this by adjusting for even running of the engine. Of course, for a warm
engine, the richness of this mixture will cause some roughness. Make sure the
needle seats return freely when you release the choke.
step 12- Drink that warm beer (only one, no DWI now...) it will taste great
at this point!, go wash up, and go for a ride.
The first time through carb adjustments can be confusing, once you've done
it, all of the stuff in the manuals makes sense. Go back and read them again-
As always, I reccommend Bentleys, which is a repro of the original factory
manuals, and then Haynes, and throw out the Chiltons. (orginal factory manuals
are to be read in a clean enviroment, repros are for smearing grease all over,
except, if that's all you got, use it!) Haynes has an excellent manual just
for SU carbs, it covers operation, theory, rebuild of all models, and has
needle charts for hundreds of car/engine/carb setups. They also have a
manual for Zenith-Strombergs, which, while similar, are a whole 'nother beast.
________
/___ _ \ Roger Garnett (garnett@tcgould.TN.CORNELL.EDU)
/| || \ \ Agricultural Economics | The All New:
| |___|| _ | 3 Warren Hall | South Lansing Centre For
| | \ | | | Cornell University | Wayward Sports Cars
\| \ |__/ / Ithaca, N.Y. 14853 | (607) 533-7735
\________/ (607) 255-2522 | SAFETY FAST!
|