The twin S U's
Not sure who made the comment when referring to the SU carburettor....."I honestly don't understand why people need to continuously fiddle with them" …. There are many who share this view.
The SU gives excellent all round performance and economy, and of course they look good … lifting the bonnet of our British classic there simply has to be at least two SU's ... It just cannot be any other.
This is not intended to be another “how to tune your twin SU's” story, for that I would strongly recommend you purchase "the SU carburettor high-performance manual" by Des Hammill ....
The SU uses a simple constant velocity principle, ie the piston (or dashpot) is lifted by a vacuum created by the drop in air pressure when the throttle is opened... the tapered needle attached to the dashpot allows a measured fuel supply to enter the air stream via a fixed jet, a simple operation and simple to tune … If only a single carb is used... not so simple when we have "Twin SU's"
So, why do so many enthusiast continually strip and find it necessary to tune there “twin SU’s”?... The most common reason is because multiple carbs just don’t perform to their full potential if they are not SYNCHRONIZED
The challenge of synchronization is often overlooked and is not a simple fix. …. Because both pistons need to rise at exactly the same rate of climb for any given throttle settings, allowing the air flow to get both carbs to feed exactly the same recommended air/fuel ratio to all the cylinders for any given throttle setting. (from idle to full throttle).
To achieve this they have to Synchronized) .... Easy, well no!....
To achieve synchronization between the twin SU carb setup it is important to check the basics first:
|Ensure that the piston and suction chamber are CLEAN (use spirits or petrol on a soft cloth).|
|Check that the needles are identical, not worn and fitted correctly.||Check that the piston springs are standard and the same (critical to get the pistons to ‘ride’ at the same height throughout the rev band)||Check spindle and linkage wear, (commonly overlooked) ... The mixture settings can not be set correctly if air is being leaked through the butterfly spindles.||Make absolutely sure that the fuel bowls (float chambers) volumes are identical, to do this accurately, run the fuel pump for a few seconds to make sure they are both full, remove the lid and measure the amount in each bowl by sucking it up with a syringe and measuring the volume. … Adjust accordingly.||Check the needle and seats in the float chamber, the slightest leak will cause flooding at idle, and upset the synchronization.||Then the piston and vacuum chamber of each carb have to be matched by doing "the drop test".... this is done by removing the vacuum chamber and the piston (do not mix them up), unscrew the dash pot and remove the needles. After cleaning, temporarily plug the drillings in the piston (normally two holes) with “presstick” … insert the piston (without the springs) into the upturned vacuum chamber, then turn right way up, allowing the piston to fall slowly from the chamber…. Measure the time it takes to drop out on both, (for the smaller 1 ½ , it should take about 5 to 7 seconds). The time taken should be the same in both drop tests … if not, try changing the pistons around. Let’s hope you are lucky. Unfortunately you cannot decrease the drop rate, but it is possible to increase the rate by lightly polishing the piston’s matching surface with Brasso… measure repeatedly and take care!||Ensure that the plastic stop is fitted into the piston base .. There should always be a minimum gap of about 3 to 5 mm between the base of the piston and the bridge of the carb body when the engine is not running.||Check the tension springs on all adjustment screws, the settings tend to change from vibration. (good idea to replace them)|
The biggest challenge trying to synchronize the twin’s is to finely balance them individually at idle (with the linkages loosened) The standard manifolds have a “balance tube” connecting the two carbs, which means that all cylinders are robbing from the tube. Essentially, if the front carb is running rich/or lean it will affect the rear cylinders as well (because of this action “colourtune” cannot be accurate) ....and the firing order of the BMC motor (1,3,4,2,) also does not help.
We have heard of people who can ‘balance’ SU’s by fitting a tube into their ears and listening to the hiss…, hmmm yes OK, but it is just not possible to get it absolutely right, the use of an inexpensive vacuum gauge is a must ….. Nothing less will do.
There are of course some bad practices often used, like ....”just take the springs out, or even worse just stretch them” ..... or just put thin or thicker oil in the dash pot even worse no oil at all, etc.etc. …..Please, it just does not work like that.
Having checked all the above …Set the mixture correctly, and ensure that both carbs are drawing the same volume at idle (trust the gauge)… we have a nice steady idle, tighten the linkages, but before putting sae 20 oil into the dashpot, check the synchronization …by removing the dash pot and insert a piece of rolled paper or milk shake straw (8 mm dia) approximately 100mm long into the piston (dashpot hole) ….draw a simple scale, number 1 starting at the top ridge of the vacuum chamber (engine not running) and working down wards 5mm apart, for approximately 50 mm (ie 10 lines marked 1 to 10). …start the engine and both pitons should rise to the same level on your scale at idle, increase the RPM to 2,000 and again they have to be at the same, and so to for the entire rev range .. this will also allow you to check that both carbs are lifting at the same rate under acceleration by slowly increasing the RPM ….
Your SU’s are now synchronized, delivering the same fuel/air ratio over the full rev range, to all four cylinders, and you are guaranteed an increase of performance and economy.
The only thing that still needs attention is to determine which of the many needles will work on your engine, all needles measure the same at idle, but the trick is to get the mixture correct throughout the rev range …. That is “Another Story”