SU Carburetor Tuning

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Some time ago I wrote a HowTo guide on the dolomite club forum. Sadly I cannot now find it, so this seems like as good a place as any to replace it and give it some permanence. There is already loads of good stuff no the net about SU carbs and how to set them up. This guide is by no means meant to be authoritative, but is a distillation of the experiences of an owner who has tuned his own twin SU's that have graced virtually every car he has owned over some 40 years.

But I'm still learning.....

Ok so lets start with the basics because an SU is an essentially simple device. What an SU does is much like any other carburetor: mix petrol vapour with air to produce an explosive mixture such that the internal combustion engine to which it is attached can run at its best in all conditions. So there in one simple statement we have the ideal that continues to this day to occupy the minds of the world's best and worst mechanical engineers, not to mention a few amateurs like us. Of course the other side of the story is that no carburetor can work on its own. Like love and marriage and the horse and carriage, any carburetor is not much use without an ignition system.

This guide will say nothing about ignition systems except this... Just like a large percentage of all quoted statistics are made up on the spot, so it is that a large number of faults attributed to carburetors or fuel systems are ignition related. If I had taken note of my own advice on this over the years, I would have saved myself a lot of time and frustration, and you can do the same. So if your car isn't running as well as you think it should, and some how you have ended up at this guide looking for a silver bullet fix, the you've come to the right place. Sort your ignition out first. Change only one thing at a time and substitute it wherever possible with known working components, and not necessarily brand new ones either.

When you have done that your car will almost certainly be running well enough such that fiddling with the carburetors should be an enjoyable optimisation process that will make it go even better. So your car is running on all cylinders, you have set the ignition variables like points gap and timing as per the book and you can see a semblance of ignition advance with your timing light. Your car is up to running temperature and its a sunny are ready to start. Well sort get the air filters off and lets play.

You can skip this bit if you already know how an SU works.... An SU is a single variable jet carb. It uses just the one hole, of varying size, to let petrol be sucked into the engine. Lots of other carbs have a variety of jets but the SU has just one. Therein lies its beauty in simplicity. The hole size varies as a tapered needle moves up and down within it. The position of the needle is governed by the piston to which its connected, sliding up and down in a partial vacuum chamber above the hole, and this also works as a variable size air inlet too. It is brilliant as it kills two birds with one stone. The size of this air hole is demand related. As the engine sucks in more air, so the piston is sucked up higher and the air passage gets bigger. So how does the engine get told how to suck more air by you when you are driving? Between the air valve and the engine is another crucial part of the carburetor, the throttle valve. This in the case of an SU is a circular disc in a hole that opens by sitting flat relative to the airflow and closes by going at right angles to the flow and closing the hole off. It does this by rotating on a spindle through the carb body, and you move the spindle because its attached to the thottle linkage and via cable or rods to your accelerator or gas peddle.

So understanding this will help you see where the possible adjustments that we can make come in. First up is the throttle stop screw. This screw controls the degree to which the throttle disc stays slightly open and is the major influence of the engines idle or minimum speed. When trying to adjust or tune your SU in any way this is the first thing you need to identify. On all SU's fitted to dolomites, which are of the HS design type, the screw sits beside the vacuum dashpot, towards the engine side and is cross headed and sits in a small recessed opening. If you screw it downwards....right hand thread into the carb body, it will open the throttle disc more and the engine will speed up. If you unscrew it upwards, the converse will be true and the engine should slow down. The throttle disc closes under the action of a spring. SU's fitted to dollies come in 2 flavours spring wise....early and late. On early SU's there is a conventional spring that hooks onto a fork on the thottle spindle and the other end hooks onto something solid on the inlet manifold. The spring is under tension and the weakness of the design is evident in that its always pulling the throttle spindle in one plane and causes it to wear out the spindle bushes after anything from 40k miles or even less. The later design uses a coil spring wound around the spindle and these last a lot longer. Some on my cars have now done 130k miles with no perceptible spindle bush wear.

If your throttle stop screws and spindles are all working as they should you can set the idle easily at anything from 600 - 1200 rpm just by twiddling with this screw. So what can go wrong with such a basic mechanism and what do yo do if the throttle adjustment makes no apparent difference? Well there are a number of no particular order. If your idle speed is too high and twiddling with the screw just won't bring it any lower than say 1000 rpm what can you do? The first thing to do identify the cause. Is there just general dirt on the linkage stopping the spindle returning to rest? Is there an air leak on the spindle bushes caused by wear? Is the return spring not working as it should? If you are unlucky enough and have spindle bush wear than you need to replace the carb or have the spindles rebushed. This is sadly quite expensive and I usually find that a replacement with the later spring design is a cheaper way to go. Cleaning the dirt and replacement springs will usually cure the other faults. Failing this you may well have an air leak on the gasket or the breather tubes. Again its just a matter of identifying and fixing. New gaskets and replacement breather tubes are the fixes you will need. Sometimes the air leak can be located with a bit of fine bore tube and listening for the hiss. The most likely cause for an excessive idle speed is a badly set up throttle linkage. What happens in this case is that the fast idle screw is set to give too high a speed when the choke cable is pulled half way. When the choke is pushed in the fast idle screw is still bearing on the throttle fork and so it needs to be unscrewed so that it has complete clearance with the choke off. The fast idle screw is a tad obscure in that it sits under the side of the carb and can be masked from above by the spindles of the choke and throttle linkage on twin carb cars. But its there, and there is one for each carb. Diagrams in the work shop or Haynes manual or some shots on the internet will help you find them. I'll dig around for some url's when I have time. If anyone else wants to add references please feel free.

So with the throttle stop screws covered and the fast idle alluded to we now have our third and final SU adjustment and that is the idle mixture setting. This is a big brass nut at the base of the carburetor body and it controls the position of the jet hole relative to the needle valve. Dropping the jet down the taper makes the hole bigger and lets more fuel in and the mixture is richer. Conversely raising the jet reduces the hole size and the mixture is leaner. The jet adjusting nut has a range of about 5 full turns and lets the jet move over a range of only a few millimeters, so its quite a fine adjustment. If your fingers are strong enough you should be able to turn the jet adjusting nut without a spanner, but failing this I think its 11/16. If its all clean it should be quite easy to turn with fingers. If its very stiff it needs a good clean up. WD40 and an old toothbrush will work wonders. The books all say that as a starting point the jet adjusting nut should be screwed fully up into the body and then unwound 12 flats or 2 full turns. That isn't bad advice. Before fiddling with the idle mixture too much its important that our expectations are realistic. It is only possible to set the mixture off load and at idle speed. The mixture on load and at higher rpm is influenced far more by the needle profile and these profiles are worked out as a set of best guess compromises by the manufacturer. Over the years I have found that 1850 dolomites are needled too weak at higher rpm and would go better with different needles. The same is true to a lesser extent of 1500's. To get the best results though requires a rolling road session and a rolling road with a good range of needles is now sadly a tough thing to find. I run my spit 1500 on AAQ's and these are if anything too rich at higher rpm.

Dolly Sprint owners are a more experimental bunch and we ought to gather here a list of what needles go best with the range of cams that are fitted.

Anyway I want to know how to tune it as best you can. To do this you will need decent hearing, a length of fine bore tube and a small screwdriver, and whatever spanners you find that will loosen the clamps on the throttle linkage that makes your carbs work together. You need to do this so you can make each carb work on its own. Having done this you need to slow one carb down on its idle screw so that its doing virtually nothing and make the engine run at about 1000rpm on the other carb. You can stick the tube in your ear and listen to the hiss at the air inlet and you should be able to hear one carb doing all the work and the other doing nothing. Now use the small screwdriver to just lift the air valve piston by 1/32 inch...that isnt very much. If the mixture is right this will make no difference. If its too rich then the speed will rise and stay higher. If its too weak then the speed will drop and the engine may even stutter or cut out if excessively weak. Then its just a matter of raising or lowering the jet a couple of flats at a time to get the mixture right. On later cars like Sprints the workshop manual advises adjusting for the smoothest idle. With practice you can tell when your engine is happy...too weak and it will have lots of splashy sounding misfiring, too rich and it will sound rythmically lumpy. On cars with longer inlet manifolds I find the need to give them a good rev every minute or so. On ohv dolomites and spitfires this isn't usually necessary.

So having got a happy idle on one carb its a repeat process on the other. Turn your first carb right down to nothing and bring the other one up to 1000 rpm and repeat the piston lifting and listening to the speed change. I find the sweet spot is about 3 flats wide, either side of which you can tell if its too weak or too rich quite easily. When its spot on you may get a momentary rise in speed before it drops back again. Its hard to notice this as we are only talking about 50 in 1000 rpm. On 1850 Dolomites and Sprints its very hard to get this as its masked by the long inlet manifold. On ohv cars like 1500 dolomites and spitfires with the short inlet tracts its much easier to get them obviously spot on. You can also use fancy stuff like the Gunsons colour tune and C-O meters. I'm not a colour tune fan myself as I find they burn my fingers, but whatever works for you.

The next phase is to get the air flow matched on both carbs. To do this bring the idle speed up again on the "turned off" carb and reduce it on the last tuned one. Now listen to the hiss in each inlet until they are both the same as you make fine adjustments to the idle speed. Set them both so that the engine is still running at around 1000 rpm and then try lifting the air valve piston on each carb. You should see very little effect from each carb and if all went well before, then maybe adjusting each carb by 1 or 2 flats will be all that's needed.

Now reduce your idle speed to your desired figure of about 800 rpm. Again make sure the each carb is sucking by the same amount, by listening to the hiss down the tube. Then retighten the clamps on the throttle linkage and you are almost done. Move the throttle linkage by hand to open the throttles and you should see the throttle forks moving together. The final step is to make sure the choke linkage works so that you get a fast idle speed of about 1100 rpm with the jet still in the normal running position. You will see that the choke is actually misnamed on an SU carb. Its more a coarse mixture adjustment and it works by moving the jet tube down by about a quarter inch. The linkage is designed to bring the fast idle screw into action before the jet drops and you can turn it so that it ups the idle speed by the required amount. Make sure it has clearance from the throttle fork when the choke control is pushed in.

It will also be worth while to check that the air valve pistons rise and fall smoothly with the same amount of resistance on each carb. Its also a good time to refill the dashpots. Lately Ive taken to using ATF to do that, but 20-50 will do.

So now you are done bar refitting the air filter and that all important and hopefully satisfying test drive and enjoying the massive increase in fuel economy and performance you have achieved.

Happy tuning....