4.6 - SU Carburetor Tuning ( Westneat/Smith,  July 27, 2005 )

Clark Westneat writes on July 27th, 2005:

The best thing to do is set the base line on the carbs.  Always
do this with a warm engine. Like the book says, we have to make some
assumptions that the timing, valves etc are all as they should be.  Even if they are not quite up to spec, they can be worked around.  The book says that for an 8:1 compression, the timing should be 8 degrees Before TDC. Two experienced mechanics have both told me 10 degrees is more realistic. I figure this gives me wiggle room and is not cast in stone. I set the timing at idle of 700RPM with a warm engine at 10 degrees BTDC.(Admins note: The information is in regards to a 420, other Jaguar Saloons have different timing settings but the timing concepts are the same).

Pull the air cleaner assembly (which probably hasn't been on for
a while now!!) then pull the domes and pistons off the carbs. Back
the mixture screws all the way out until the jet no longer moves (on both sides) Now screw the mixture screw back down until you just start to see movement. Do this on both sides. Turn on the ignition and see if the floats shut off the petrol with the petrol even with the top of the jets.  Make the necessary adjustments if they do not. A canted float lever will cause flooding at some point in the future. When you do this, you may get a small bit of flooding until the float bowls fill.  Wipe it out with a rag and check the petrol levels. Turn off the ignition.

Check to make sure both butterflies are closing. One of the
major problems I had was that the clamp that held the rods for the butterflies was not returning both sides equally. Every time I tuned the car it would go GREAT!! for about a mile, then it would lug down again. Come to find out, the rear butterfly was shutting and the front was not. It is hard to tell if both sides are clamped properly with the clamp Jag used on the 420. Mine was a brass configuration that would not hold. My S-Type had a steel device which works MUCH better and is now the same as is on the 420. Test the butterflies by sticking your finger in the carb when they are shut and see if you can move either one of them with out the other moving. If you can -this may be your problem. NO matter how much you balance them at idle -- it won't matter once one of the butterflies has shifted.

Now that you are sure the butterflies seat properly, make sure they shut all the way with the spring. I had to put on a new spring to make sure they shut all the way. The tension on the spring can not be too great however because that will cause a new set of problems. Just make sure that when you step off the accelerator, the butterflies return to full closed.

Now go back to your mixture screws and screw them down 3 ½ turns from the point they first start to move in the jets. Now screw in the fast idle screws all the way and back them both out equally 2 ½ turns. Replace the pistons and domes making sure the pistons have oil in the dampers.

You should now, because the car is warm, be able to start the
car with out the choke on and with out touching the pedal. Simply turn the key and hit the button and the car should fire up and run smoothly – albeit a bit fast probably.

The first thing to do is make sure they are sucking the same amount of air. I use both a Uni-sys and a piece of tubing. I actually find the tubing to be a bit more accurate in my case. Make sure the mechanical idle adjustment screw between the carbs is not affecting the idle. Run the fast idle screws in equally until the idle is at about 700 RPM. Now continue to adjust each side until they are sucking the same amount of air and they are somewhere between 600-800 RPM at idle. You will be able to tell if the engine is where it should be by the sound.

Check your timing to see where it is sitting. I discovered that the timing is a rather important step that was left out of the tuning – at least in my case.  It needs to remain at between 8-10 degrees BTDC at 600-700 RPM. Turning the distributor will change the degrees and the idle as you advance and retard the timing. This must all be in synch.

Once you feel comfortable, you need to start adjusting the mixture. At $2.39+ for petrol -- I like to run a bit lean but not too much. Continue to make adjustments on the mixture equally until the car starts to bog down and stall.  When it does this, you are near the right place. Adjust each side until they are at a point that when you lift the piston slightly, the engine speed rises slightly and then settles back down again to where it was at the lower piston level. Do this for both carbs.

Everything from here on is a matter of your own 420. Continue to adjust the idle screws equally checking to make sure the air flow is equal. Keep checking the timing to make sure you are in spec and adjust them all accordingly.  Once you feel that you have it to a point it sounds good and does not lug down when you snap the accelerator, road test it(after you have tightened everything down including the dizzy) and see what it is doing. You will need to make more adjustments until you are happy with it.

On July 28th, 2005 Merritt Smith adds:

To get technical, in an SU the fuel is drawn through the jet by a static pressure drop between the fuel in the float bowl and the air passing the jet. The fuel in the bowl is at atmospheric pressure, and not at pump pressure, because the needle valve manages the flow of fuel and the bowl is vented (those long silly tubes that piddle fuel on the ground when your float sinks or your needle valve sticks).

There are two components making up the pressure drop at the jet. The first is the air velocity itself (Ventura effect). Air in motion is at a lower static pressure than still air (if it starts at the same conditions). The faster the flow, the lower the pressure. The second component is from a total pressure drop that is caused by the filter.  The filter acts as a restriction to the air flow. At low engine speeds, the impact of the filter is minimal, so the pressure drop across the jet is about the same between the filter and no filter setups. But once you open the throttles, the pressure drop caused by the filter becomes significant. Since the carb was designed for the filter (needle selection), the higher static pressure at the jet for the unfiltered setup means that not enough fuel will be sucked up, and you have starvation.

It was the missing second component that killed your top end, and it's why people who change to a less restrictive intake system need to go to richer needles.