Gents
I approach SU tuning from the point of view of an engineering understanding of what the carb is trying to achieve together with highly successful practical experience on an MGC (3ltr staight six).
SU's work on the principle that fuel is let into the engine in response to the level of suction in the inlet manifold. This pulls up the piston pulling the tapered fuel needle out of a hole (the main jet) containing a petrol feed, the more suction the bigger the effective hole to spill petrol into the inlet manifold. The amount of petrol entering the engine is then a function of the suction (aka size of fuel hole) and the amount of airflow. The airflow is a function of engine speed and throttle opening.
So, starting at the beginning, make sure the carbs are in good condition and there are no defects influencing what goes on. You'll need to get fast of a carb overhaul kit - check out
http://www.burlen.co.uk/default.aspx for lots of usefull info and spares.
The really important thing to do first is make sure the float chambers are in A1 nick and that the petrol level is correct. You'll want to change the floats and needle cut off valves while doing this. If these are wrong, in worst case the fuel pump is able to just pump straight through into the engine without restriction, or the fuel level in the float chamber will be wrong which will affect the amount of fuel that comes out of the hole (or jet) as the needle is raised. The web site above has all the info you need.
Next check out the throttle butterfly spindles and operation of the various lever mechanisms. Repair any obvious defects, you might finish up needing new spindles or at worst a carburettor body if there is slop or clearance - that would let unplanned air flow into the carb.
Next match the dashpots. I do this by removing the dashpots and pistons from the carbs and removing the oil damper from the top of the dashpot. With everything cleaned up check that the fall time, ie how long it takes the piston to fall out of the dashpot, is the same for both carbs. I'd lay a very large amount of money that it won't be and by quite a big margin. At that point it helps to have a few spare carbs lying around. Simply mix and match dashpots and pistons until you can achieve an identical fall time. The significance of this is that if the fall times are not matched then a given manifold vacuum will cause a diffeerent response from each carb and you'll never be able to set the engine up! I found this to be by far the most critical action with twin SU's and yet I've never seen it written up anywhere.
Next check out the needles, if there is any marking at all on the needles, change them.
Then you are on to the throttle linkage. Everything is unclampable and adjustable, which is great for inveterate fiddlers (like me) but also increases the chances of things having moved! So set up the throttle linkage so that you achieve the best compromise between 1, both throttles crack simultaneously and 2, both throttles reach fully open simultaneously. In theory you should be able to do both but I bet you can't! Sounds basic but you could have lost some of the bushes in the linkage, got a bent rod, things "moved over time" etc etc.
A natural progression at this point is to set the chokes up so both chokes operate together (but getting the idle speed etc set up is for later when the rest of the carbs are set).
Very rarely on the V8 you could also be upset by a defective joint between the carbs and the engine (carburettor to manifold or manifold to head) drawing air in after the carbs.
All carburettor tuning must be done with the ignition system operating as it should - so satisfy yourself on condition of points, plugs and static ignition timing. Its worth checking also that the distributor inertia advance is working (you should be able to twist the plate holding the points against a spring) and the vacuum advance is working (suck down the tube, if nothing happens your ok, if your lucky you might be able to see the baseplate twist a little as well; if you can draw air the diaphragm at the distributor is split).
Now you're on to the exciting bit! Hopefully the engine still runs at this point, so warm it up. You can worry about cold start performance and final setting the chokes up later!
Now you need to be able to asses whether the engine is running weak (too little petrol) mixture or rich (too much petrol). The easiest way of doing this is to use a "colourtune" - a replacement spark plug with a quartz body that lets you see the colour of the flame inside the cylinder. Otherwise an exhaust gas analyser will do the job (but likely to be garage equipment rather than home workshop) but is more difficult because it averages out the two carbs rather than showing you exactly what is happening in one cylinder.
Next set a fastish idle speed (1200rpm?) and use the idle adjustment screws to get the same amount of air going down each carb. With the air cleaner off just use a piece of old heater hose to listen to the airflow into each carb - a bit like a doctor's stethoscope - and get the wheezing the same. Then use the colourtune to set the first carb's mixture so that the flame is just on the point of turning from blue to orange. Swap the colourtune into a cylinder fed by the other carb and repeat. The airflow will now be wrong so you need to reset that and then go on and check the mixture again. Repeat until everything is correct at the same time, then reset the idle speed back to normal, again re-balancing the airflows.
That process is often referred to as balancing or tuning the carbs, but note that it only looks after low throttle running and response from idling. The work you did earlier with the throttle linkage and piston fall time is what counts when you're pedal to the metal!
Finally it is time to set up the choke response and there I would simply follow the manual.
This process is identical for the 4 cylinder engine, but there you could also be upset by a defective or missing heat shield between the carbs and the exhaust manifold.
Once you understand the principle here, its all very simple, if a little time consuming.
Note I haven't mentioned the dashpot lift pin once! So what is it there for? Well if all the peceding had been completed (or was 100% on a new car set ex factory) you would still expect the mixture setting of the carbs to drift slightly, especially during the running in period. So the lift pin test allows you to asses the state of the mixture setting alone on an otherwise perfectly set up engine. In other words it's a quick dodge to use at the first service after running in!
Chris
