FORD CARBURETTOR

The Ford type dual carburettor (formerly marked Chandler-Groves) was first used on the 1938 cars. It is a plain tube dual down-draught type and any mechanic understanding plain tube carburettors should have no difficulty with this model. For jet and venturi sizes for the various models, see Specifications above.

In this type all the main channels are carried in a removable nozzle bar (see insert, Fig 8) which carries the idle tube and an aspiration nozzle. The central portion of the nozzle bar forms the discharge nozzle. In this construction it is possible to locate the discharge nozzle in the centre of the air stream without having attaching brackets or bosses which interfere with the flow of air into the venturis.

The discharge nozzle itself is located in the smallest part of the venturi (see Fig 7) is circular and of such diameter as to create high suction at the end of the nozzle. This suction, in addition to the atomising holes in the nozzle, helps to vaporise the fuel completely.

This dual carburettor can be considered as two carburettors built into one unit. There is a separate set of venturi, idle tubes, nozzle bars, main metering system, idle system and throttle plates, one for each side. There is one accelerating pump with fuel being divided at the pump discharge nozzle (see insert, Fig 10), one air chamber and one fuel chamber. There is one power valve which takes the fuel from the fuel chamber through one passage and divides the fuel evenly for each side.

In the following explanations one barrel is generally referred to unless mentioned otherwise.

Choke

The choke valve is mounted on a shaft located off-center in the air passage (see Fig 7). A torsion spring S tends to close the choke valve when the choke lever is moved to the choke position. There is a certain amount of free movement at the mechanism at part choke position so that if the choke is partially closed to operate at a relatively low speed, the air rushing in at higher speed will force the valve open and compensate for the increased speed. This, how-ever, does not mean that the car can or should be continuously operated with the choke control in part choke position.

With full choke the valve is held in a locked position by the control lever. If the choke is held in a full closed position after the engine fires, a poppet valve or air bleeder T in the choke will open. This supplies enough air to keep the engine running and eliminates choke sensitivity. The opening of this poppet valve and the rush of air flowing though it makes considerable noise, which should attract the owner’s attention to the fact that the choke button is out and will continue to make this noise until the choke button is pushed in, either all the way or to a part choke position.

When the carburettor is choked the throttle valve is automatically open to the correct position for starting. For this reason it is neither necessary nor desirable for the operator to pull out the throttle button or pump the accelerator, when starting.

In full choke position, everything below the choke valve is subjected to inlet manifold vacuum and the bulk of the fuel is supplied by the main discharge nozzles (see Fig 7).

Idle Fuel Supply

The fuel from the carburettor bowel passes through the main metering jet into the idle tube F (see arrows, Fig 7). Air is introduced into the fuel system by the idle air bleed A and a small additional amount of air is bled in by a small hole B in the aspirating nozzle (see insert, Fig 7). The idle mixture goes around the aspirating nozzle by means of an undercut around it’s outside diameter’ as shown. The mixture the travels down the idle passages C to the idle discharge holes D and E.

When the engine is to a speed of 350rpm; the mixture is discharged out of the lower hole E only. As the throttle plate opens and the speed is increased, the upper D starts discharging. In this carburettor the lower holes only discharge from idle to about 450rpm. The upper holes very gradually start discharging, in addition to the lower holes, from about 450rpm to 1250rpm. The action and timing are such that the upper discharge holes gradually start to feed, reach a maximum about 750rpm, and then gradually become less effective as the main nozzle starts.

The lower discharge holes are provided with an idle mixture adjustment. Turning the needle out gives a richer mixture and in, a leaner mixture. The idle adjustments should be set for the highest and steadiest vacuum reading. The idle adjustment should not be jammed against the seat hard enough to groove the point. If this occurs the adjusting screws will have to be replaced in order to obtain a satisfactory idle adjustment.

Main Fuel Supply

As the idle system becomes less effective, the main nozzle G starts to deliver fuel. This occurs at about 900rpm. Between 900 rpm and 1250rpm there is a definite blend of the idle system and the main metering system. The power valve remains closed in this range, and up to approximately 3800rpm except under load, which causes the manifold vacuum to drop. In this range all the fuel passes through the main jets (see fig *), up through the main vertical well, then up and around the idle tube. The main fuel is emulsified by air entering at the main fuel supply air bleed H which lightens the fuel and makes the mixture more responsive to throttle changes. The mixture is again aspirated by the aspiration nozzle as it starts down the main nozzle G.

The nozzle bars are held in place by clamps and the channels sealed against leaks by the nozzle bar gaskets. In dismantling and assembly these nozzle bars, care should be taken to see that the gaskets are in place and are in good condition and that the clamp screws are tight. When removing jets, be sure a screwdriver which fits the slot is used; this will eliminate the danger of slipping and damaging the metering orifice.

The power valve J (see Fig 9) is operated by the vacuum below the throttle plate through passage L, and the power valve spring K. At idle, the vacuum is the highest and decreases ass the load increases. The diaphragm (actuated by vacuum) holds the power valve on it’s seat until the vacuum drops to from 81/2" to 9" of mercury where it is not high enough to resist the action of the spring. This point, at level road running at a constant speed is approximately 3800rpm.

Under load as in climbing hills, etc., the vacuum drops as it becomes necessary to open the throttle wider in order to maintain speed. When the vacuum drops to from 81/2" to 9" of mercury, the power valve is opened by the spring, as when the engine speed exceeds 3800rpm on level road, and the fuel then flows into the power valve and the channels and through the high speed gas restrictions into the centre or main vertical well M (see arrows, Fig 9); this gives the additional fuel required for high speeds and for heavy loads at full throttle and low speeds.

ACCELERATING PUMP

The accelerator pump is directly connected to the throttle and it’s function is to enrich slightly the mixture for rapid acceleration. Fuel is drawn into the pump chamber through the pump inlet check valve N (see Fig 10) on the upstroke of the pump piston (closing the throttle). When the throttle is opened the piston O moves down, closing the pump inlet check valve and overcoming the weight of the pump discharge valve needle. The accelerating fuel then goes around the pump discharge valve P and out of the pump discharge nozzle (see insert, Fig 10). Free movement against a spring load is provided in the pump piston stem discharge and the pump operating rod to give a prolonged discharge when the throttle is opened suddenly.

* 1941 carburettors have a countersink at one bleeder hole. 1942 carburettors have a countersink at both bleeder holes. How ever they are interchangeable.

** Replaced by 91A-9922-23.

The accelerator pump is provided with an adjustment for varying the quantity of the accelerating charge. This adjustment is made by changing the position of the link R. The positions are marked 1, 2 and 3. No 2 is the average setting; No 1 the summer or hot weather setting; and No 3 the extremely cold weather setting.

Failure of the accelerating pump is mostly due to dirt in the pump inlet check ball seat. This can be checked by removing the carburettor air horn and operating the pump with just a small amount of fuel in the bowl. If the check valve is leaking, air or fuel will bubble back into the fuel bowl from the inlet hole. When cleaning this seat care should be used in re-installing the pump piston to be sure the leather is not damaged.

CARBURETTOR PARTS IDENTIFICATION

Many carburettor parts are similar in appearance, yet they are not interchangeable for the various carburettors.

The following illustrations indicate the difference in parts which are similar, and the locations of their identification marks.

Note: To see a bigger version of the above picture Click here.

Nozzle Bars

1. Bars having a large main fuel passage (.160") stocked under part No 78-9922-23 (See Fig 11).
2. Bars having a small bottom skirt (.375") are stocked under part No 26H-9922-23.
3. Bars having no extension on the side are stocked under part No 1GA-9922.
4. Bars having two countersinks at the air bleeder holes and a large bottom skirt (.437") are stocked under part No 06H-9922-23.
5. Bars having no countersink at the air bleeder hole and having an extension on the side of the bars are stocked under part No. 91A-9922-23.
6. Bars having an indentation between the bleeder holes are stocked under part No 922a-9922-23.

Air Bleeder Plug for Nozzle Bar

Two types of nozzle bar bleeder plug are used (see Fig 12). The bleeder plugs having a skirt are used in all nozzle bars.

The Pump Discharge Nozzle

The carburettor pump discharge nozzle has an identification number stamped on it at the location (see Fig 13).

The correct nozzles for the various carburettor models are as follows:

Idle Tubes

Jets with no identification mark are used on all carburettors.

Pump Springs

The carburettor pump spring for the various carburettors can be identified by the diameter of the wire in the spring. The wire diameter can be measured with a micrometer.

The diameter of the pump spring wire for the various carburettors is as follows:

Economiser Valve

The economiser valve, part No 78-9904, is the same on all carburettors and has no identification mark.

Idle Adjusting Screw

The idle adjusting screws used in the Stromberg are not interchangeable with the screws used in other carburettors. These screws can be identified by the slot extending approximately half- way across the knurled end of the screws (see Fig 16). The idling adjusting screw for the Ford carburettors can be identified by a slot extending across the knurled end of the screws (see Fig 17).

Main Metering Jets

The main metering jets have their size number stamped on the jets.

The Pump Links

The carburettor pump links are of different lengths for the various carburettors and can be identified by a mark stamped on the link (see Fig 18).

The correct links for the various carburettors are as follows:

Note: The information above was taken from the "FORD V8 & MERCURY 1932 – 48" workshop manual by Scientific Magazines Publication Co. PTY LTD, Sydney. Australia. (Printed 1956). It is reprinted in the interests of those who are interested in the use of and preservation of V8 Ford Sidevalve motors.

Redned.

 

From: rodnut:

 

The answer to the leaking fuel line fitting problem is as close as your local NAPA store. Ask for p/n 2-93, it is a repair fitting that will cut it's own thread and convert's to a AN type seat, the line nut that screws into it is p/n 105-5 for 5/16 fuel line.

The fitting cost $6.63 and the nut was 13 cents.

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They call them a 'Copper Crush Washer' with a part number of AN 900-8 (1/2" I.D.) and they'll sell you one, or more at $0.40 cent each, no minimum! They will take a credit card, or COD and will ship ground carrier (UPS, etc.) or mail them to you. Their phone number is 253-848-9349. I just ordered a dozen via US Mail. You might also try an aircraft parts house in your area for these. But Spencer's got them! I've mentioned this here before, but the late-style Holley power valve found in just about every new 94 rebuild kit is not designed to correctly fit the 94 carburetor! The correct 'early' power valve has a slightly different gasket base and ports compared to the late valve. The late valve has a radius at the inner corner and the ports are low and square shaped. The early valve has a flat gasket base and round ports set slightly higher in the stem. Using the late valve and kit gasket in a 94 is where all of the trouble comes from. They don't line up, or seal properly. The new valve will work fine with the 1/2" AN crush washer IF you tighten it down properly. You need to 'crush' the washer in order to properly seal due to the radius, obtain the clearance AND to make sure the ports are exposed. With the late valve, install and tighten the valve and AN crush washer, then carefully remove it and check the port openings. You want the top of the ports exposed and showing above the gasket surface. That's all you need. If the valve is tightened sufficiently to clear the well, the ports should be exposed enough to function correctly. Don't be bashful- tighten it up, but don't go overboard. Next check the clearance w/o the base-to-bowl gasket in place. The valve should clear the well. With the early valve, the ports are high enough to where their exposure is not a problem, but you still need to make sure it clears the well..Because the actual machined gasket seat on these late valves is outside the seat area of the 94 bowl section, you now need to seat the stepped, unmachined inner area of the valve (with the radius) to the carb body instead. It's this stem radius and unmachined area inside of the actual outer seat that must now seat to the 94 body. This is why flat washers will not work here. You need to allow for the radius and the unmachined area being used for a seat. A rolled copper crush washer is ideal for this. Place the copper crush washer, with the open/split side toward this new seat of the valve, against the radius at the bottom of the threaded stem. This will allow the stem's radius to fit into the rolled I.D. of the crush washer and it will form nicely when crushed. Now I'm speaking of a decent body with good, solid threads because you need to tighten the valve sufficiantly to crush the washer. If your carb body has marginal threads, then please beware. You can also use a drop of oil on the valve's threads to help prevent galling when tightening the valve.

 

Remember also that the late p/valve will still stand out farther from the body than the correct early valve because of the improper/taller radiused seat area now being used on the late valve. This holds the late valve about .050" closer to the floor of the vacuum well in the base than the early valve. With this washer properly crushed, the bottom of a late p/valve will still be very close to the well floor. The base-to-bowl gasket then allows enough clearance for the valve to function properly. I also use the extra thick base-to-bowl gaskets available from Vintage Speed..

 

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