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Lotus Seven S4 (Type 60) Restoration: Recondition a Weber 32DFM Twin Choke Downdraught Carburetor

Module by: Andrew R. Barron. E-mail the author

As used in the Seven the 1600 cc engine was essentially that from the Ford Cortina GT and used a standard Ford intake manifold with a single Weber 32DFM twin choke downdraught carburetor. The original downdraft Weber 32DFM twin choke downdraught carburetor had melted during the fire (Figure 1) and become fused to the intake manifold (Figure 2).

Figure 1: The remains of the Weber 32DFM twin choke downdraught carburetor and the intake manifold in place on the engine.
Figure 1 (graphics1.jpg)
Figure 2: The melted Weber 32DFM twin choke downdraught carburetor fused to the intake manifold showing the damage to the latter at cylinder 2 and 3 ports in particular.
Figure 2 (graphics2.jpg)

As a replacement it is still possible to purchase new Weber carburetors (Southern Carburetters or Weber Carbs Direct), however, the 1969 Lotus Seven S3 that I had been previously restored then used on the Millennium Monte in 2000 and the Winter Challenge in 2001 (Figure 3), had been converted to a race car with twin side draft Weber 40 DCOE carburetors (Figure 4). Therefore, it was decided to recondition the old unused Weber 32DFM carburetor (Figure 5).

Figure 3: The 1969 Lotus Seven S3 while on the 2001 Winter Challenge.
Figure 3 (monte7.jpg)
Figure 4: The 1969 Lotus Seven S3 racing at Sebring after converting to twin Weber DCOEs. Copyright by SVRA (2009).
Figure 4 (HSR7.jpg)
Figure 5: View of the replacement Weber 32DFM carburetor.
Figure 5 (graphics3.jpg)

Even though the carburetor had been working up to the point it was removed, it had not been used in 12 years. It was decided therefore to replace all the gaskets and clean all the jets, etc. The rebuild kit for Weber 32/36 DGV/DFM carburetors is available from Pegasus Auto Racing Supplies (Part No. 1586-18).

The first step was to remove the Formula Ford style carburetor spacer with gaskets (Figure 6). Both new replacement and the standard carburetor-to-manifold gasket (0.063” thick) are available from Pegasus Auto Racing Supplies. The final decision may rest with the clearance of the air filter with the bonnet.

Figure 6: The typical carburetor spacer with gaskets as used for downdraft Weber 32DFM carburetors in Formula Ford applications.
Figure 6 (graphics4.jpg)

Removal of the air filter housing is accomplished by first undoing the retaining nuts for the top cover (Figure 7), followed by removal of the air filter cover (Figure 8). With the cover of the filter housing removed, the filter is lifted out exposing the four bolts that hold the bottom plate of the air filter housing to the carburetor (Figure 9). Undoing these bolts allows for removal of the bottom plate of the air filter housing from the carburetor (Figure 10). Underneath is a card gasket, which is lifted of (Figure 11) and labeled for reuse (Figure 12).

Figure 7: View of the air filter cover after the retaining nuts are removed.
Figure 7 (graphics5.jpg)
Figure 8: View of the air filter and chokes after removal of the air filter cover showing the threaded pins that fit through the holes in the cover.
Figure 8 (graphics6.jpg)
Figure 9: View after removal of the air filter showing two of the four bolts that hold the bottom plate of the air filter housing.
Figure 9 (graphics7.jpg)
Figure 10: View after removal of the bottom plate of the air filter housing.
Figure 10 (graphics8.jpg)
Figure 11: View of the top of the Weber 32DFM carburetor after removal of the air filter housing.
Figure 11 (graphics9.jpg)
Figure 12: The card gasket which fits between the carburetor and the air filter housing (see Figure 11).
Figure 12 (graphics10.jpg)

Once the air filter housing is removed, the various components of the carburetor can be cleaned and/or replaced. The fuel inlet was unscrewed (Figure 13) to allow for air to be blown through. Turning the carburetor upside-down allows access to the strainer (fuel filter) inspection plug (Figure 14), which is unscrewed to reveal the strainer assembly (Figure 15). The strainer assembly (filter) is carefully removed by hand or using tweezers and replaced with a new assembly that is included in the rebuild kit (Figure 16). The fuel channels are blown out with air prior to inserting the new strainer assembly and refitting the inspection plug.

Figure 13: Removal of the fuel inlet.
Figure 13 (graphics11.jpg)
Figure 14: A view of the strainer inspection plug (A).
Figure 14 (graphics12.jpg)
Figure 15: View after removal of the strainer inspection plug (A) showing the cylindrical strainer assembly (B).
Figure 15 (graphics13.jpg)
Figure 16: View of the old strainer assembly (A) and a new replacement strainer assembly (B).
Figure 16 (graphics14.jpg)

The top of the carburetor is removed by removing six (Figure 17) cover fixing screws with a flat head screwdriver (Figure 18). This allows access to the float assembly and jets. Taking the cover of the carburetor first, the cover gasket is removed. The cover is then turned upside down so the float assembly is on top (Figure 19). The float fixing pin that holds the float assembly in place is removed by pushing from one side (Figure 19A). Once the float assembly is free, the needle of the needle valve can be unhooked (Figure 20 and Figure 21). Using a 10 mm wrench or socket undo the needle valve seating and remove the gasket below. Replacement seating and gasket are provided in the rebuild kit (Figure 22). The new gasket is fitted along with the new needle valve seating. The hook on the new needle valve is hooked onto the float, and the float assembly positioned into the carburetor cover. The fulcrum pin is inserted into the two posts to hold the float assembly in place. It is always worth checking that the height of the floats has not changed during this procedure.

Figure 17: View of top of the carburetor showing the six cover fixing screws (arrowed).
Figure 17 (graphics15.jpg)
Figure 18: Unscrewing one of the cover fixing screws that hold the carburetor cover in place.
Figure 18 (graphics16.jpg)
Figure 19: View of the inside of the top of the carburetor showing the float assembly and the fixing (fulcrum) pin partially removed (A).
Figure 19 (graphics17.jpg)
Figure 20: Side view of the float assembly showing the needle of the needle valve.
Figure 20 (graphics18.jpg)
Figure 21: The original float assembly (A), needle valve (B), and float assembly fixing pin (C).
Figure 21 (graphics19.jpg)
Figure 22: The original (upper) and replacement (lower) needle valve seating (A), needle valve (B), and needle valve gasket (C).
Figure 22 (graphics20.jpg)

Before the carburetor cover is reattached to the body, each the main and air corrector jets (Figure 23) were removed and cleaned with break fluid and compressed air. In addition, the accelerator pump (Figure 24A) is removed by unscrewing the pump delivery valve (Figure 24D). The old gaskets are replaced by new ones from the rebuild kit (Figure 24B and C). The accelerator pump is reattached and screwed in place.

Figure 23: Inside of the Weber 32DFM carburetor showing the accelerator pump (A), the primary and secondary air corrector jets (B and C, respectively), and the primary and secondary main jets (D and E, respectively).
Figure 23 (graphics21.jpg)
Figure 24: View of the accelerator valve (A), the old pump jet gasket (B), the new pump jet gasket (C) and the pump delivery valve assembly (D).
Figure 24 (graphics22.jpg)

The accelerator pump cover is removed by undoing the four screws that hold it in place (Figure 25). With the cover of the accelerator pump diaphragm assembly will fall out along with the pump loading spring (Figure 26). The spring is refitted into the cup in the body of the carburetor (Figure 27). The new accelerator pump diaphragm assembly (Figure 28B) is held in the cover assembly and the whole unit reattached, and screwed in place (Figure 29).

Figure 25: View of the accelerator pump cover assembly after removing the four cover fixing screws.
Figure 25 (graphics23.jpg)
Figure 26: View of the accelerator pump housing after removal of the accelerator pump diaphragm assembly and the pump loading spring.
Figure 26 (graphics24.jpg)
Figure 27: View of the accelerator pump housing and the pump loading spring.
Figure 27 (graphics25.jpg)
Figure 28: The old accelerator pump diaphragm assembly (A) and the new accelerator pump diaphragm assembly (B).
Figure 28 (graphics26.jpg)
Figure 29: View of the replaced accelerator pump cover assembly.
Figure 29 (graphics27.jpg)

The idling jet holder on the same side of the carburetor body as the accelerator pump (Figure 30) is unscrewed and the primary idle jet removed and cleaned (Figure 31) before replacing back into the carburetor body with a new washer. This process is repeated for the secondary idling jet on the other side of the carburetor.

Figure 30: View of the idling jet holder (A).
Figure 30 (graphics28.jpg)
Figure 31: Idling jet in its holder with a new gasket. The old gasket is shown beside.
Figure 31 (graphics29.jpg)

The cover gasket is replaced on the cover of the carburetor ensuring that the floats are not hindered from movement. The cover is then carefully replaced and the cover fixing screws. The air filter housing is replaced along with the fuel inlet. The filter will be replaced with a new unit once the clearance with the bonnet is checked upon instillation.

Resources

  • Southern Carburetters, Unit 7 Priestley Way, Crawley, West Sussex. RH10 9NT, UK. Tel: +44 (0) 1293 533843. www.sciperformance.co.uk.
  • Pegasus Auto Racing Supplies, 2475 S 179th Street, New Berlin WI 53146, USA. Tel: +1 262 317 1234. www.pegasusautoracing.com.
  • Weber Carbs Direct, 20 Constance Court, Hauppauge, NY 11788, USA. Tel: +1 866 679 3237. www.webercarbsdirect.com.

Bibliography

  • A. K. Legg, D. Peers, R. Maddox, and J. Haynes, The Haynes Weber Carburetor Manual, Haynes Publishing Group, Sparkford, UK (1995).

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