Zenith Carb Rebuild part 3

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Page 1: Remove the carb

Page 2: Disassemble the carb

Page 3: Reassemble the carb

Page 4: Reinstall the carb

Note: I bought the 'full' carb kit, which meant it included the secondary vacuum diaphragm (according to the guys at the shop). It was ~$46 instead of ~$22. Per carb. I also had the diaphragm for the choke, which (rumor has it) is also not included in the base kit. My kit did not have the correct idle mixture screw, threads were too coarse and it was too big. It also didn't have the rubber gaskets for either the mixture screw or the other mixture screw. It did have 2-3 gaskets for for each of the main gaskets I needed (between major parts of the carb body). Choose the right one for your app, they have minor variations. Finally, despite my commitment to making good use of my rebuild kit, the great majority of the parts in the kit remained unused after the rebuild.

Things you should have before you start
  • Tiny allen wrench (so small I can't see a size marked on mine)
  • Full set of metric combo wrenches
  • Couple of good screwdrivers, medium and small
  • Access to all the parts you removed during disassembly
  • Access to the full carb rebuild kit
  • An exploded view of the carb, ideally with numbered parts
Things to do before you start reassembly
  • Clean everything. Use lots of xylene and acetone. Scrub surfaces, especially mating surfaces. Get grime away from moving parts, linkages, etc.
  • Identify which parts from the rebuild kit you will be able to use. There are 2-3 gasket types for each major gasket location. Find the correct one by matching with what you removed or with the carb itself (after it is clean, of course).
  • Blow air through as many passages as you can find. Make sure you remove any lint from parts you have cleaned.

 

 

Reassembling the Secondary Diaphragm
1
   Screw together the diaphragm and the "standoff". Use the standoff that has the fine-pitch threaded hole for the external piece with the spring captive in the tube. I was able to use the washer but not the lockwasher that came with the kit (not enough threads on the male end of the standoff).
2
   Attach the external part of the linkage and use the nut to set the length to that which was measured before disassembly of the old pieces.
3
   Set the diaphragm in the bottom of the metal housing. Wrap the edge of the diaphragm around the internal ridge, flush with the bottom since the diameter is slightly less there.
4
   Press the cover over the diaphragm being careful not to pinch the diaphragm between the two metal pieces. I used some vaseline on the diaphragm to make the process smoother!
5
   Attach the four screws and lockwashers to the outside of the case. Screw the two halves of the case together.

 

Reassembling the Fuel Inlet Diaphragm
6
   Place a gasket down first (you should have two identical gaskets in the rebuild kit)
7
   Replace the old diaphragm with the one from the rebuild kit.
8
   Place the load cup over the diaphragm, curved part contacting the diaphragm.
9
   Place the gasket over the diaphragm
10
   Place the spring onto the load cup, align it so it is centered.
11
   Place the cover on top of the spring and align with the bolt holes.
12
   Use the bolts and lockwashers to secure the cover.

 

Reassembling the Choke Diaphragm
13
   Install the new gasket. Install the new diaphragm and shaft (a bit of jigling to get the shaft down into the choke housing). Make sure the shaft is correctly oriented to catch the rest of the mechanism in the choke housing.
14
   Install the other new gasket (you should have two identical gaskets in the rebuild kit).
15
   Install the spring and top cover.
16
   Install and tighten the three bolts and lockwashers.

 

Reassembling the carb
17
   Install the insulating spacer between the throttle butterfly section and float housing.
18
   Install a gasket above and below this spacer. We don't want any air leaks here.
19
   Reattach the linkage with the turnbuckle-like piece.
20
   Reattach the throttle butterfly section to the float housing. Install and tighten the 4 allen-head bolts and lockwashers. Make sure that the one shorter bolt is in the correct location.
21
  

Reinstall the mixture screw with rubber O-ring. Seat and turn out two full turns.

22
   Reinstall the mixture bypass screw with rubber O-ring. Seat and turn out two full turns.
23
   Reinstall the fuel shutoff valve.
24
   Connect the short vacuum line to the throttle butterfly section. The other end attaches to the float housing section later.
25
   Reconnect the linkage for the secondary diaphragm.
26
   Install the gasket for the secondary diaphragm to float housing mating surface.
27
   Install the three bolts and lockwashers to attach the secondary diaphragm to the float housing.
28
   If you followed my directions and removed the jet and washer at the bottom of the accel pump, this would be a good time to put them back.
29
   Reinstall the choke plate connecting rod at some point. Later than this might be OK, I don't recall exactly when I did it.
30
  

Reinstall the choke cover plate. Make sure the bimetallic spring engages the end of the operating arm. Make sure to align the score marks you made when you removed it. Tighten the three screws in the ring which holds the cover captive.

31   Reinstall the emulsion tube ("4N") and air correction jet ("120") for the secondary side
32   Reinstall the emulsion tube ("6S", with notch) and air correction jet ("100") for the primary side
33   Install a new check valve and spacer for the float chamber in the bottom of the bedplate.
34   Reinstall the float mounting pieces. These are the float, rod stock, mount plate, and bolt and lockwasher.
35   Install a new gaset between the float housing and the bedplate.
36   Attach the bedplate and the float housing. You'll need to hold the float up out of the way when you bring the two pieces together.
37   Reconnect the wire from the connector to the shutoff valve.
38   Reinstall the perimeter bolt which held (maybe) either a BMW tag or a bracket for the connector and wire to the shutoff valve.
39   Reinstall the idle air jet labeled "125" aove the primary bowl.
40   Reinstall the bolt near the jet you just installed.
41   Install the new gasket between the bedplate and the carb cover assembly.
42   Install the carb cover assembly. Make sure the choke linkage rod is in the right place here. Fasten the captive center bolt in this assembly.
43   Reinstall the "cover" that attaches with 3 bolts. One is small, the other two are part of the set of nine which hold the carb cover to the bedplate - the longest two.
44   Reinstall the other 7 bolts which hold the cover assembly to the bedplate.
45   Attach the choke linkage rod to the choke plate linkage with the setscrew and the tiny allen wrench.
46   Reinstall the air filter housing mounting stud

 

Adjustments
NOTE: This section closely follows the descriptions given in the Haynes E3 manual
    Secondary butterfly
    Throttle butterfly
    Accelerator pump volume

 

Fuel inlet diaphragm
There's a very similar diaphragm at the fuel inlet block. The diaphragms don't seem quite as fragile, but you should certainly replace any rubber pieces you can while the carbs are apart.
43   Remove the 3 screws on the top of the cover plate.
44   Remove the cover and gasket.
45   Remove the spring and the load cap that it sits in (so it doesn't load the diaphragm directly)
46   Remove the diaphragm.

 

You are done reassembling the carb.

 

Done with the cleaning? Go to the reinstall page
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