Carburettor/Inlet Manifold Gauzes
 

I've recently installed these on my 2.8 cologne powered GTC, though plenty of owners have them on their 3.0 Essex's. The results are amazing, less or no use of the choke when cold starting, crisper throttle response and improved mpg.

But what on earth are they ? I hear you ask. Read on................

The following is the work of Alan Dean :-

Fitting

These instructions assume that the engine is set up properly and that the inlet tract/manifold is free from air leaks (this is most important). If you are satisfied about the inlet condition simply lift the carburettor and fit the gauzes as below. See note at end for Automatic choke units. 

If in doubt - Check that the base of the carburettor is flat using a steel straight-edge. If bowed, flatten with a broad flat file – carefully or seek professional help.

Check that the carb surface of the manifold is flat and correct (professionally) if necessary.

Before fitting the gauzes make sure that the butterfly valves in each choke are equal and closing identically. If not adjust the gear connection near the throttle cable fitment.

 You will need a vacuum gauge, an accurate rev counter (the one fitted will be OK but they are seldom accurate), and ideally a Colortune plug.

 1   Remove air cleaner, disconnect the throttle connection. Undo the four nuts holding the carb in place and lift       the carb off the manifold leaving the plastic heat shield in place.

2     Remove the W shaped metal flow barrier and its securing screws.

3     Leave the gasket in place that was on the heat shield.

4     Fit gauzes through the plastic heat shield ensuring that the flats (no flange) on the gauze flanges meet in the middle between the two chokes.

5      Put a small continuous run (3mm diam) of a petrol resistant silicone sealant around the outer edge of the gauze flanges.

6      Place a new thin gasket over the gauzes.

7      Refit carburettor, tighten up (not over-tighten) securing nuts and reconnect throttle cable system (Note – The carb flange nuts should have a plain washer and a spring washer beneath the nuts and tighten to partially compress the spring washer). Do not refit air filter yet.

8     Make sure the car is ready for the road.

When ready to start, pump accelerator twice and start engine. (normally this will be instantaneous but it is probable that fuel has drained from the carb in this operation), and drive away until the engine is at normal running temperature (NOTE – with a hot engine you do not need to pump the pedal). Fit a multimeter showing accurate RPM or the tacho in the car and fit a vacuum gauge to the brake servo vacuum line (unless there is another vacuum point on the manifold – Do not use the line to the plastic heat shield). Adjust the throttle adjustment screw to have engine revs not more than 900 rpm. Adjust the mixture screws to produce the highest rpm and/or the lowest vacuum. Using a length of clear PVC tubing it is possible to check that the `hiss’ level in each choke is identical when the tubing is held at the same point in each – with the other end to your ear. I always find this more accurate than all but the most expensive flow meters. If the `hiss’ is not equal then adjust the butterfly gear connection.  

Slow the tick over down to about 750 rpm (this varies between manual and auto boxes). Each choke should sound identical and engine should be running very smoothly. By slight movement both ways of the mixture screws check that the engine is still producing highest RPM – it should fall as you turn the mixture screw away from the previous setting. If it does not then adjust to highest RPM. A sniff at each tail pipe will indicate equal mixtures and the sound will indicate equal flow. A Colortune plug (ideally two plugs – one in the centre cylinder in each bank) will quickly tell you if you have the correct mixture at the flame. The flame should be a Bunsen blue without any bright yellow flashes – they are droplets burning and the gauzes get rid of those. You can adjust the flame colour to the level of blue that you choose. A rich blue is the richer mixture and the lighter blue is weaker and more economical. 

Please note that if you tune the carb to meet the book CO emission of 3% your engine will be grossly rich. Do not use this method of tuning regardless of what the garage guy says. The gauzes cause all the fuel to evaporate before separating to the inlet ports therefore there are no droplets of fuel entering the combustion chamber.  This enables a much better burn with the result that the exhaust contains much less CO, particulates and unburned fuel. You are using all of the fuel much more efficiently. The CO emission will be 1% or lower. 

To assist performance check that your plugs are cleaned and that your points are clean and properly gapped. Check the dynamic ignition timing setting it to 12 – 14 degrees advanced with the distributor vacuum line disconnected (Essex engine). This applies to both leaded and unleaded fuel (I always run unleaded). You will find after a few hundreds of miles that the middle spark plugs are the same colour as the end ones indicating that all cylinders are burning the same strength mixture. The plugs may burn whiter – better mixture – hotter flame – more torque. Colder plugs can be used. I have not changed mine and they burn almost white but do not overheat. 

When you refit the air cleaner it will probably make the mixture richer due to the pressure drop through the filter and slight adjustment may have to be made to the mixture screws to return to your original setting. It is always advisable to fit a new filter element. 

Your emissions will be cleaner. Particulates and UHC will be down (less smelly exhaust) and the CO level will be at 1% or lower instead of the book figure of around 3%. You will be burning more of your fuel correctly generating better torque and fuel consumption. 

After a full winter when you have confidence in the gauzes and you find you do not need the choke at all you can remove the choke flaps to improve (only slightly) the air flow through the carb. 

Automatic chokes

With autochokes back off the fast idle screw behind the water temperature sensor so that it is not opening the throttles – set throttle opening by using the screw adjust on the `gear’ connection. It is not advisable to use the autochoke fully because it will generate a very rich mixture – too rich probably for the engine to run at all. Until you are confident in the gauzes or if you live in very cold areas you can adjust the autochoke to just give a little assistance on cold starts by slackening the three screws that hold the water temperature sensor and rotating it to just open the choke a little when cold. This will also prevent any hesitation when the engine is cold which is a nuisance with an auto box. To completely immobilise the auto choke, slacken the three screws that hold the temperature sensor and rotate it  360 degrees to hold the choke flaps open all the time and you can reconnect the water hoses or put in a piece of copper pipe to bypass the sensor. As a precautionary measure it is advisable to mark the original position of the water sensor.

Why do the gauzes work? 

When the petrol leaves the nozzle above the butterfly valve it is almost all liquid. Liquid petrol cannot burn, nor can any other combustible liquid, it is only the vapour which when mixed with the correct amount of air that can burn. That correct ratio of petrol/air is 1/13.6 (Stoichiometric Ratio). A little either way will give slightly more power or slightly better economy. 

Ideally the mixture going into each combustion chamber should be at or close to the SR. If the proportion varies cylinder to cylinder the engine will be unbalanced with no one cylinder producing its full power. As the fuel passes the butterfly valve most of the liquid fuel is on the wall of the carburettor and it stays on the wall into the inlet manifold. The bulk of this liquid fuel would pass into the centre cylinder in each bank. The W shaped piece of metal put in below the carburettor by Weber is supposed to prevent excess fuel going into the centre cylinders on each side and it should diffuse the fuel. To some extent it does but much of the fuel is still liquid. Some does evaporate from the wall of the manifold which is heated by hot water from the engine. However, liquid fuel still enters the combustion chamber, moreso when the engine is cold thus washing oil off the cylinder walls. That liquid fuel is wasted fuel producing only particulates and carbon monoxide with some actually going out into the exhaust as unburned hydrocarbon. With a Colortune Plug it is possible to see the droplets trying to burn. The fact remains that each cylinder is receiving a different charge of fuel and so the engine cannot produce its full power, producing instead pollution. 

With the gauzes in place the fuel still runs down the walls of the carburettor until it comes to the bottom flange. Here it meets the gauze and it runs on to the wires of the gauze creating a very thin film of fuel all around each wire – the surface area of the wire is over 30 sq.ins. The airflow through the gauze is violently fast (can be supersonic) and a massive area of the surface of the fuel is exposed to this fast moving air causing total evaporation. Thus the fuel is fully evaporated before it splits to each cylinder and therefore each cylinder receives the same mixture. Additionally, since all of the fuel is evaporated a considerable amount of heat is removed from the airstream (cooling it by up to 25 Centigrade degrees) thus increasing its density and consequently increasing the weight of mixture going into each combustion chamber. From a thermodynamic point of view the colder the mixture the greater is the thermal efficiency.  

The fuel/air mixture is even across all cylinders giving greater torque and better fuel consumption. It burns the fuel more efficiently thus generating much lower levels of pollutants. You will find that CO levels are very low, well below the emission regulations. The same applies to unburned hydrocarbon (UHC). Overall your engine will generate lower levels of carbon dioxide per unit of power delivered – the aim of all engine manufacturers. 

During a cold start because the gauzes evaporate much more fuel it is unlikely that the choke will be needed at all. Pump the pedal once or twice thus placing petrol on to the gauze and start the engine. It will start instantly.

For ordering and more information please contact alanwdean@ntlworld.com