Balance of power: twin carbs

How to tune your twin carburettors

SUs & CDs

Step 1

	Fig. 1: JetBetter
	Fig. 2

Having set the float levels equally at their correct settings, the engine should now be thoroughly warmed up. Ideally by going for a 10-15 minute drive. Do not do it by letting it idle for a long period with the bonnet closed, as this will artificially raise under-bonnet temperature and thus affect the accuracy of mixture settings.

We now want to get datum settings as starting points for the balancing operation. In order to get the mixture settings more or less equal, the jet or needle heights should be equalised. On SU's and Strombergs with adjustable jets, the jets should be wound up so they are flush with the bridge of the carburettor, or as near as possible, and at the same height. This can be done by holding the piston down on the bridge and winding up the jet until it is felt to contact. A more satisfactory method is to remove the dashpot and piston assemblies and do it by eye, or ideally, with a Jet-setter (photo 1) for greatest accuracy.
This done, back both mixture screws nuts off, (ie down/clockwise) by two complete turns. Again, the Jetsetter gives greatest accuracy.

For emissions Strombergs, it is the positions of the needles that have to be equalised. The amount of adjustment possible is limited. If you have only an Allen key, all adjustments must be made with the piston removed from the carb. If you have the special Stromberg tool, they can be adjusted in situ. To establish most accurately that the needle heights relative to the piston body are the same (photo 2), it is best to remove the piston from the carburettor body.

Step 2

Fig. 3

Since the jets will be higher than they were previously, the centralisation should be checked by the drop test. Lift each piston in turn and when released, it should fall freely to land on the bridge with a faint click (photo 3). 

If it jams, the jet will have to be centralised. 

Check also that both pistons fall at about the same rate. If there is a difference, it is possible that swopping round the pistons will sort it out. If it cannot be eliminated, acarb overhaul is needed at the very least, as it would indicate that the carbs are not matched and further tuning would be a waste of time. 

Step 3

	Fig. 4
	Fig. 5
	Fig. 6

We are now ready to set about the initial balancing of the airflows. First of all, the linkage joining the two throttle spindles should be disconnected, by slackening a nut in the zig-zag clamp (photo 4). Sometimes the clamps do not release even when the bolts are loose, in which case they should be marginally prised open with the point of a screwdriver.

Check that the spindles can be moved independently of each other. On carbs with the cold-start controls interconnected, those clamp screws should also be released.

Now the two slow-running screws (photo 5) should be unscrewed until they are well clear of the throttle stops with the throttle discs fully closed.

Then screw each one back in until they just make contact with the throttle stops.
Having reached this point, open each throttle by exactly two turns. This should give approximately equal throttle settings as a starting point for balancing the airflows.

Check that the fast-idel-screw (photo 6) on each throttle lever (some have only one) is clear of the cam that operates them, so that there is no danger of them interfering with the setting of the slow-running.
Where there is a fast-idle screw for each carb, use a feeler-gauge to equalise the clearances between the screws and the cams - as a starting point for when later you will be adjusting the fast-idle settings.

Now start the car, without attempting to use the throttle pedal to to get it started.
If it will not start as it is, open up each throttle by the same amount, till it will start and run at a reasonable speed.

The engine will have cooled off a bit by now, so let it run for a few minutes., with the bonnet open.

You should now be ready to balance the airflows of the two carburettors.

Step 4

Fig. 7

It should be realised that balancing the carbs makes precious little difference to power and general performance on wider throttle openings, but is vital to smooth and untemperamental slow-running.

By listening in this way (or by using a device) you will most probably find that one carb is drawing more air than the other. The idea is to adjust the slow-running screws to equalise the airflows.
if the idle speed is on the slow side, then open up the slower carb or vice versa if the engine is running on the fast side.

Step 5: achieving perfect balance

	Fig. 8
	Fig. 9
	Fig. 10

Getting the mixture correct and even is where the biggest gains can be made. However, it must be realised that getting the correct mixture at idle will only make the mixture correct at all other speeds if the car is completely standard and with standard needles, or already has the ideal needle setting to suit any modifications that have been made. This article is not about setting up the mixture for non-standard applications.

Another point is, even if you have the correct needle profile, the effect of mixture adjustments is progressively less, the wider the throttle opening. Thus, although two turns of the adjusting screw can have a substantial effect on idling mixture, the effect on full throttle mixture will be very maginal.

To get near to the correct mixture setting, you should now screw the two mixture adjusters (photo 8, SU's) up or down by equal amounts until the fastest idling speed, consisted with even running, is achieved. Then back off again till the speed is just about to fall, again by equal amounts fo each adjuster. When screwing up, make sure that the jets keep in contact with the adjuster nuts (on SU's).

If, as a results of these mixture adjustments, the engine speed has risen appreciably, then reduce it back to near normal by backing off each slow-running screw by equal amounts. Check to see that this does not upset the balance.

You now want to do the final seting of the mixture. The ideal way to do this, is with a Colortune see-through spark plug set., by which the correct mixture can be seen by the colour of combustion (photo 9).

Otherwise, use the lifting pin on the carburettor to lift the piston by 1/32th in. to see the effect this has on the running of the car. Not all Strombergs have a lifting pin. In which case use the tip of a small screwdriver to lift the piston by 1/32 in.
Do the forward carburettor first.

1. If the engine speed drops or if it stalls, then that carb is running too weak.
2. If the speed increases, it is too rich.
3. When the mixture is correct, the engine speed should either not change at all, or will increase momentarily and then settle at the same speed as before.

Do this for each carburettor in turn, but, as the carbs are to a degree interdependent, it may be necessay to go back and forth a few times.

The exhaust note should now be even. If the exhaust is colourless and the note is irregular with a 'splashy' misfire, you are still too weak. A rhythmical type of misfire with blackish exhaust denotes over-richness.

When the mixture is set to satisfaction, check the airflow balance one last time.

On emissions Stromberg, it is the needle height that is adjusted. This can only be done in situ with the tool as illustrated (photo 10).
With these carbs, it is a generally more tedious procedure, as the adjustments must be made in steps: using the tool puts a pressure on the piston which affects the mixture, so after each adjustment it is necessary to rev the engine up to clear it, then let it settle back to idle, before checking the mixture.

Step 6: clamping up the throttle interconnections

	Fig. 11
	Fig. 12

With perfect balance achieved, the throttle interconnections can now be clamped up again. On Strombergs they are directly connected end to end and it is a simple operation - though it helps to have someone to hold both throttle levers against the stops, while things are being tightened up.

On Su's it is a bit more complicated. Each throttle lever has a fork, in which sits a pin, attached to the central shaft. This opens the lever and it must have a clearance when the throttles are resting on their stops at idle.
There are two basic ways in which this is set.

1. The first is where the throttle shaft operating lever has a 'tail' which bears on the choke interconnecting shaft when the throttle is fully closed.
   A feeler gauge of the specified thickness, usually 70 thou., is inserted between the throttle shaft operating lever and the choke interconnecting shaft (photo 11). Then the throttle interconncetion clamps are tightened, with the pins resting on the lower arms of the forks. When the feeler gauage is then removed, there should then be a clearance between pin and fork.

2. On other set-ups, the clearance is set directly between the pin and lever fork arm (photo 12), again with a feeler gauge. The clearance varies, but 10 thou. is a fair average.

Step 7: fast-idle

	Fig. 13

Finally the fast-idle. With the fast-idel cams of each carb resting against their stops, clamp up the cold start interconnections, so the cams begin to move at the same time.
Check there is 1/16 in. free movement of the choke cable before the cams start to move.

Now pull out the choke cable to just short of the point where it begins to move the jet. |fire up and resort once more to your balancing device plus rev-counter to equalise the fast-idle airfloes (photo 13) at whatever the correct fast-idle for your car is.

Since the throttles are directly connected on the Strombergs, the operation is simpler and only one fast-idle screw is fitted (which obviously operates both throttles from the one fast-idle cam, by the same amount). You only have to set the speed, as the throttles should already be balanced.

	Fixed jets Although the Weber and Dellorto side-draught carburettors may be far more dlfficult to set up for a given application than SUs and Strombergs, conversely, the procedure for balancing the airflows and mixture, once properly jetted, is much simpler. You are concerned with three types of adjustment: the idle mixture screws for each carburettor the throttle stop screw and the balance screw. Let us assume that you are going to balance a pair of correctly jetted DCOE Webers or DHLA Dellortos. The first step is to screw the idle mixture screw for each barrel right in as far as it will go, but gently or you might damage the carburettor body. Now unscrew each one by an equal amount, say two turns. Unscrew the central balance screw until it is well clear of the tag on the one carburettor. Now turn to the throttle stop screw (B) on the other carburettor and unscrew this until it is quite clear of the throttle lever and thus that carburettor is fully closed. Turn back to the central balance screw and screw it back in till the other carb begins to close up as well. Eventually you will reach a point where the one carb is just fully closed and the other is just on the point of opening again. At this point, the throtles should be synchronised. Now use the throttle stop screw to open the throttles sufficiently to enable the engine to be fired up and idled at 1000 rpm. If there is no way that it will fire, re-adjust the mixture screws by equal amounts either way as necessary till it will fire. When the engine is running at 1000 rpm, use your balancing device or plastic tube (photo 7) to check the airflows of the carbs, and use just the balance screw to equalise. If this changes the idle speed appreciably, re-adjust to suit with the throttle stop screw. It now remains to finally set the mixture. Unlike with the SU/ Strombergs, the mixture to each barrel is independent of the other. By far the best way is with a Colortune (photo 9) plug - which is simply fitted to each cylinder in turn and the mixture optimised by altering the mixture screw on the barrel that feeds that cylinder. Failing the use of a Colortune, you must do it by the revs. Again, adjust each screw in turn until the highest revs with even running are achieved. This can be done with greater sensitivity if each carb is done in turn, with the other pair of cylinders put out of action by removing the spark plug leads. You will have to open up the throttle to get the engine to run fast enough on just two cylinders: be sure to do this with the throttle stop screw, not with the balance screw. This done, it is again an question of twiddling the screws for the fastest idle. When all is done, return to the throttle stop screw to get back to the correct idle speed, and re-check the balance for one last time.