The following SU carburettors were used by Rover in the SD1
Most S.U. carburettors have three basic adjustments:
- The mixture adjustment.
This adjustment takes the form of a large hexagon nut on the jet holder itself as (types H and HS)
or simple screw adjustment (HD and HS8). Its function is to position the height of the fuel jet
relative to the metering needle.
- The fast idle screw.
Situated on the linkage which interconnects the throttle valve and the cold start mechanism, this screw permits
a quick and simple adjustment of the increased idle speed which is necessary whilst the engine is cold.
- The throttle adjusting screw.
This screw functions exactly the same as the idle speed screw found on conventional carburettors. Acting against
a stop, it holds the throttle valve open a set amount at engine idle
Don't forget to remove the circlip from the top of the sleeve
when trying to remove the dashpot assembly from an HIF SU.
To determine if the jet is centered properly.
- Unscrew the damper rod
- lift the piston to the top of its travel
- release the piston
- you should hear a soft 'clunk'
- if you don't hear a sound or the piston doesn't fall freely the jet needs to be centered
The procedure to center the jet is quite straightforward:
- Disconnect choke lever
- Mark the position of the jet. It is important that the jet is marked and kept in the
same relative position during the process of centring and final assembly.
- remove the jet, jet adjusting nut and coil spring above the adjusting nut
- replace adjusting nut without the spring and replace the jet according to the mark ( nut slackened off slightly)
- Screw the adjusting nut as far up as it can go (HS type only)
- the jet can be moved side to side slightly
- remove th hydraulic damper and press the piston down onto the jet bridge with a soft pencil, hold the jet at the highest
- the needle will now line up with the jet
- tighten the jet locating nut and check if the piston drops freely
- replace the coil spring and reassemble the cold start mechanism/linkage
Sometimes the piston itself can be stuck into the bore.
Or if you want to blueprint
your SU's, check this following routine.
- remove damper rod and the dashpot with the piston
don't swap the pistons between dashpots. They're matched to one another so that the clearance between the piston and the
wall of the dashpot makes a tight seal but permits easy rising and falling.
- clean the dashpot and piston and slightly oil the inside chamber
- block the air holes around the face of the piston (Plasticine will do nicely)
- install the piston in the chamber and turn it upside down.
- time how long it takes to drop out (don't let the piston drop on the floor!)
- it should take between 5 and 7 seconds to drop out
- make sure BOTH pistons have about the same drop out time (max. difference 1 sec.)
- if the difference is more than 1 sec. take the slowest piston and dashpot
and polish the surfaces where they touch slightly with metal polish
- measure the drop out time again and repeat if necessary
- If the piston takes much more time to drop, check for dirt in the assembly
Another point is to check the butterfly shaft for wear. A leak around a worn butterfly shaft will
result in erratic running at low throttle openings and low rpm. No effect will be noticed
at wider openings. Especially on our Rover Twin SU setup it can be difficult to balance the idle airflow if
the carbs have worn butterfly shafts. To test this you can spray some carburettor cleaner on the outside of the
throttle shaft. Carburettor cleaner is non-combustible, and if the engine speed drops, it means some of this is
getting into the air stream from outside the carburettor. You may also have leaks from the manifolds, from tubing such
as the vacuum advance line to the distributor (if fitted), or from other places; the carb cleaner trick works well for
locating those leaks as well. EFI owners......also take note!
Setting the mixture
- Bring the engine at operating temperature
- Disconnect the cold start cable and linkage or make sure that the switch
for the auxiliary carburettor is in the "off" position.
- Screw the jet adjusting nuts or screws on all carburettors so that the jet is flush with the carburettor bridge or,
if this cannot be attained, to its highest position.
- Turn down the jet nuts or screws on all carburettors two full turns.
- Slacken the clamp bolt on the throttle inter-connection shaft. Start the engine and adjust the throttle
adjusting screw on each carburettor so the engine idling speed is set at the manufacturer's specifications (800 rpm).
- Check all the throttle valves are open an equal amount. The simplest method to carry out this test is with a short
length of rubber tube, one end held to the ear, the other placed in turn at the corresponding place in mouth of each
carburettor. Adjust the appropriate throttle valve screw until the amount of hiss is equal on all carburettors.
It sounds primitive, and it is, better use a Gunson carb balancer or Unisys.
- Turn the jet adjusting nut or screw either up or down on each carburettor until the fastest and most even
running of the engine is obtained. Turn the nut or screw upwards to weaken the mixture and vice versa.
On HIF carburettors the mixture adjustment is by a cross head screw situated in a recess in the base of the body
- Check the mixture strength by lifting the piston approx. 1/32". If the mixture is correct the rpm
will rise momentarily and drop back more or less to the original rpm. If the mixture is too rich the
engine speeds up considerably and rpm stays high. If the speed decreases as soon as the piston rises and continues to drop
the mixture is too lean. Repeat this operation on both carburettors.
- Tighten the clamp on the throttle interconnecting shaft.
- After the carburettors have been tuned, connect the cold start cable so that it has approx. 1/16" free travel before it starts to move the jet lever. Pull out the control knob until the free travel is taken up and any further movement would move the jet lever. In this position adjust the fast idle screw to give an engine speed of approx. 1,000 R.P.M. Push the control cable right in and ensure that there is clearance between the fast idle screw and the throttle stop. If no clearance, the free travel of the cable must be reset.
- The final step is to fill the hydraulic damper to the correct level with Oil.
If the damper cap has a vent hole drilled through it, the correct oil level is ½" above the hollow piston rod. If there is no vent hole in the
damper cap the level is ½" below the piston rod.
SU needle centring
One of the problems with SU's after the H type's is the way the centring is achieved.
The early types could be set properly without the needle touching the jet. This resulted
in very little needle wear. However with the HS carburettors the needles started to wear fast
because of the bias needle design.
The needle is always touching the jet a little bit. So if you have an old
carb. changing the jet will restore performance. The reason the jet wears more is because
the needle always touches the jet at a single point, but the jet touches the needle
over its entire length. It is recommended to change the jets every 20,000 miles.....
Guess you didn't do that now didn't you ?
Which oil for your SU?
You can fine tune your carburettor setup under acceleration by experimenting
with thicker or thinner oil. Thicker oil will slow down the reaction of the piston so more fuel will
be added under acceleration. Thinner oil will result in a leaner mixture (reducing performance)
Also take note that the viscosity of the oil changes with temperature. For that reason it is not a bad
idea to use synthetic oil in the dashpot because this will give a more constant level of viscosity over
a wide range of operating temperatures.
The colortune plug
The colortune plug
The mixture can also be set by using a transparant spark plug called the Colortune.It involves replacing the spark plug
in one of the cylinders with the Colortune plug. This plug has a transparent window that allows you to see the flame color in the
cylinder as it fires. An overly rich mixture will have a yellow flame and will be accompanied by black smoke
(and noxious fumes) from the tailpipes. As you lean it out to the optimum mixture,
the flame color will change to Bunsen blue. A mixture that is too lean will still have a blue flame, but the
brightness will decrease as the mixture is weakened. The optimum setting is if the flame is going from rich orange to just blue.
Check not one cylinder but at least one inner and one outer cylinder for each carburettor. (Yes it will cost time but
your car will run baby bottom smooth)
If the piston dampers in each carb are working correctly,
you should see the flame color change from blue to yellow when you open the throttle, and then change back to
blue as the engine comes up to speed. Read the Colortune instruction sheet for full details on how the system works.
When setting the mixture this way you might have to reset the idle speed. This means you also have to rebalance the carbs. !!
When all is done set up the pick-up arm so that it picks up the throttle linkage of each carb at the identical moment
otherwise at anything other than idle the engine will be running more on one carb than the other.