Tips – Carby & Fuel

 

Carby & Fuel

Idle Problems with the Zenith Carburettor
Setting the Air Idle Screw
Choke Rod (Mixture Screw)
Jets
Carburettors – all you need to know
Fuel Blockages
Petrol Tank Rust
Leaking Fuel Fittings
Petrol Cap Air Breathing
Petrol Tank Long Term Storage
Petrol Tank Cleaning

 

Idle Problems with Zenith Carburettor

Members have expressed concerns regarding uneven idle or stopping with the Zenith carburettor in their Model A.  The most common cause is foreign matter in the idle jet. The opening in this jet is only 0.021 inch in diameter and hence it can become partially blocked even with a filter in the fuel line.

Another cause is an incorrect fuel level in the carburettor bowl. When braking hard when the fuel level is below the specified position the fuel will move resulting in insufficient feed to the idle jet.  The setting for the float to control the fuel level can be set at nominally 0.625 inch above the machine surface of the inverted upper body  (refer to Model A manuals).  A more accurate method is to remove the drain plug and position a clear plastic tube with fitting to the outlet and raise the tube above the upper body.  With the fuel on the petrol in the tube should be approximately 0 .625 inch below the gasket joining the two parts of the body.

Another, not so obvious, problem is the under cut section in the secondary well in the lower body of the carburettor, (P/No/ A-9545). In early production the secondary well had the undercut section positioned below the hole to the compensating jet thus affecting fuel supply.  To check your carburettor it is only necessary to remove the compensating jet and with a soft pencil mark the well via the jet hole and then remove the well to check.  The design of the secondary well was changed in 1930 providing a new location for the  undercut section.

John Moorehead, MARC WA Newsletter March 2017
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Setting the Air Idle Screw

(Knurled screw with short spring situated on the top section of the carburettor)

  1. Start your Model A and warm it up to operating temperature.
  2. Push the advance lever (left hand lever) all the way up.
  3. With the vehicle idling adjust the Air Idle screw as follows.  Turn it in (CW, which is richer) until the engine starts to run poorly.  Note the position from the slot.
  4. Now adjust it CCW (leaner) until the engine starts to run smoothly.  Continue turning it CCW until the engine starts to run poorly.  Note the position (how many turns).
  5. Turn the idle screw back (CW) to half way back between the positions found in step 4.

Somewhere a little back and /or forward you will find where your engine runs smoothest.

This adjustment may not have a great effect on some Model A carburettors.  This usually means there is an air leak into your carburettor.  Often this is wear in the throttle shaft or a gasket is damaged.

If you ever need to remove the Air adjusting jet, turn it in all the way in and turn it out one, to one and a half turns for a trial start then adjust as described above.

Alan JeffreeMARC WA Newsletter May 2012
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Choke rod (Mixture Screw)

We all know about the choke rod, but this is also the mixture screw which controls how much petrol goes into the engine.

With the engine stopped, turn the choke rod clockwise until it stops.  You will notice where the little raised marker is pointing.  Then turn it anti-clockwise about one turn if the engine is cold, for start up.  Only half a turn if the engine is warm.  After a few minutes running, try turning it clockwise, then anti-clockwise, until the engine runs smoothly and the engine revs without faltering.

Each carby has different amounts of wear, so experiment a little at a time so that your Model A runs and performs at its best.  Some will run closed when hot!  You should have it somewhere between closed (clockwise) and one turn open.

Alan JeffreeMARC WA Newsletter Nov 2011
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Jets

The jets used in the Zenith carburettors have been reproduced as replacement parts in many forms.  The jets must be designed to eject a fuel spray and not a stream of fuel.

The size of each jet orifice should be checked using number drills or similar to ensure the correct hole diameter.  Whilst over the years there were some minor variations the following sizes are generally accepted as satisfactory:

JetInchmmDrill
Compensator0.037"0.9463
Main0.035"0.8965
Cap0.036"0.9164
Idling0.021"0.5375

Another very important point is the design of the jet tip.  Some jet tips are raised and rounded or pointed.  These designs produce a poor quality air/fuel mixture.  The correct tip for the Main and Cap jets is concave with the orifice at the bottom of the concave tip.

John Moorehead, MARC WA Newsletter May 2014
mm drill sizes and drill numbers added by Ian Steer, Oct 2018
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Fuel Blockages

If your Model A staggers to the side of the road, starved of fuel, there are a few obvious points to check:

  • Dirt or water in the carburettor
  • Clogged vent in fuel cap (or wrong cap from radiator)
  • Clogged fuel filter
  • Clogged fuel line
  • Kinked fuel line
  • Loose fuel line fittings
  • Vapour lock
  • Main jet dirty
  • Is the tap fully on?
  • Is the outlet hole in the fuel tank clear?
  • Has the choke arm nut come adrift allowing the choke flap to close – causing flooding?
  • Is the carburettor float set correctly?

If you disconnect the fuel line from the carburettor and fuel does not run out, you have immediately isolated one problem.  Try blowing back down the fuel line to clear the blockage.  If fuel does start to run through, you may have cleared the system for a while, but probably have rust in the tank.

One way to clean out the tank is:

  • Disconnect battery
  • Remove floor boards
  • Drain fuel tank
  • Remove shut-off valve
  • Fill tank with water, then let it drain out where the shut-off valve was.
  • Repeat as many times as it takes to thoroughly clean out the tank.

If you are worried about a few drops of water, leave the car in the sun for a while before re-assembling the fuel lines and refilling the tank.

MARC WA Newsletter April 2016 (reprinted from a MARC WA 1995 Newsletter)
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Petrol Tank Rust

If you have a Model A which keeps having dirt in the carby problems, the following suggestions came from long time club member, Steve Read.  I have used it with great success.  Rust in your tank is usually the problem and if the vehicle is complete it is a huge task to take out your tank to work on it!!  Rust is metal which has disintegrated, but it is still steel which is attracted to a magnet.

Find a strong magnet which will fit into your tank through the filler.  I broke a magnet from a scrapped microwave until I had a piece fairly large, I then fitted a thin wire cage around it with a loop on top.  The loop is to position the magnet into the tank close to, but not too close to the internal outlet.  Use a torch to check where the internal outlet is and avoid damaging the filter which is attached to the shut off valve (part number A9193-T)  The loop does two things it allows you to position it and retrieve it if necessary.  The magnet needs to sit on the bottom of the tank where the rust accumulates.  Your magnet traps the rust particles instead of them going into the fuel outlet inside your tank.  The second part to this concept, is to position a small magnet inside the carby bowl close to the screw-in compensator jet.  It needs to be small so it doesn’t interfere with the float.  You can only do this if you have an original cast steel carburettor as it will move around in an aluminium bowl and could block the jet.

To break the microwave magnet I placed it in a strong cloth and hit it with a hammer.  The  magnet is very brittle and pieces will fly, which is why I wrapped it to protect eyes etc.  From the breaking, you get small pieces suitable for the carby.

When cleaning out the carby at any stage, take out the carby magnet and give it a blow with compressed air to remove the particles of rust which may have accumulated on it.  I have never removed the tank one.

You can buy all sizes of magnet so this would be a safer approach!

One person I passed this tip on to suggested that a strong magnet on the underside of the tank may do the same job!

Alan Jeffree,  MARC WA Newsletter Oct 2013
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Leaking Fuel Fittings

A little problem I have experienced over the years, and not only with the Model A, is that little fuel leak or weep from a pipe union or drain plug.

To solve the problem simply remove the offending part, clean thoroughly and paint with fingernail polish.  Allow the polish to dry thoroughly before letting the petrol to contact.

Once dry, the petrol will not dissolve the polish.

Ray Mahony,  MARC WA Newsletter Feb 2013
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Petrol Cap Air Breathing

If your Model A stops due to lack of fuel, the first check should be for a vacuum lock in the fuel tank.  As the Model A fuel system relies on gravity feed, it is essential that atmospheric pressure is maintained in the space between the fuel and the inside of the tank.  The fuel cap vent holes must be clean and free to allow the tank to breathe.  The fuel cap and radiator cap are similar but they are different in structure in that the fuel cap is vented.

This problem is very real and I know of three cases including our first run in our Model A.  After coming to a halt after driving for some time I received assistance from fellow Club members.  When the fuel cap was removed to check the fuel level I heard the rush of air into the tank as the atmospheric pressure was restored.  The cap was left loose for the remainder of the run until the vent holes could be cleaned.

John MooreheadMARC WA Newsletter March 2013
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Petrol Tank Long Term Storage

Taken from the Model A Ford Bulletins – If you are going to store your Model A
for some months it is a good idea to store it with a full or completely empty tank. If
your tank is partially full; in the case of a cold day turning warm or more humid ,
the cold petrol in the tank will cause condensation to form on the inside of the tank
resulting in rust on the exposed surfaces.

Located by Alan Jeffree,  MARC WA Newsletter July 2013
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Petrol Tank Cleaning

I have found on just about every restoration under-taken, there is corrosion inside the A Model fuel tank.  Here is my method of cleaning one out.

The tank must be out of the vehicle.  First all fixtures to the tank must be removed.  Next you have to seal all openings by using something substantial.  I use plastic lids off ice cream containers and cut a piece the size of the fuel gauge opening which you hold in place by using the screw-in ring of the old gauge.  You may need several thicknesses to fill the depth.  Use an old fuel cap to seal the top.  Block off the fuel outlet with an old tap or similar.

The procedure – hire or borrow a small cement mixer (electric is quieter).  Temporarily set the tank on the mouth of the mixer, some soft packing here is good.  Manufacture 4×1/4 “ holding bolts to attach the tank firmly to the rim of the mixer.  1928 and 29 tanks need 2 long and two shorter with spacers.  1930/31 tanks need 4 long bolts.  The bolts must have a very tight 180 degrees bend on one end which I put on the bench grinder to give a sharp edge.  Without going into too much detail here, manufacture a secure hold for the tank to the rim of your mixer with padding to protect it.  Next pour 8 to 10 kg of 1/4” blue metal into your tank.  Run the cement mixer for many hours (even a day or two if it is really rusty).  The blue metal is removed with a vacuum cleaner.  Tilt the tank in all directions and vacuum off the bottom, this can take some time.  You continue until you no longer hear blue metal in the tank.

Next I wash the tank out with water several times until no residue is left inside.

Finally I wash the tank out with 2 or 3 litres of methylated spirits.  This serves to dispel the last drops of water and will show any leaks that you will have to solder.

Allow the tank to dry completely. You can use a heat gun, hair dryer or just leave it out in the sun.

I then use a sloshing sealer (POR 15 is my choice).  To ensure a good seal and coverage inside the tank rotate several times in all directions, naturally again with all outlets sealed.

Allow the tank to drain via the tap opening, sitting the tank in a position as it would sit in the vehicle.  This tap thread is 18 TPI 3/8 NPT taper, it will need to be re-tapped to remove the sealer in the threads once the sealer is dry.  Do not allow the sealer to contact the fuel gauge threads, use a cork or cardboard gasket here otherwise you will be hours cleaning out that fine thread!

Hans Hurij, MARC WA Newsletter Nov/Dec 2013
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