Zenith Carb Rebuild part 2

By ??

Page 1: Remove the carb

Page 2: Disassemble the carb

Page 3: Reassemble the carb

Page 4: Reinstall the carb

NOTE: This removal is specifically for the carb nearest the firewall. I don't have a similar list for the forward carb, since it was much simpler to get at. All the items to remove for the carb are the same, and there were fewer ancillary things to move around to get good access.

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
  • A good way to store the small parts you are removing. I have a clean workspace with a long strip of masking tape at the back edge. Then I remove the parts in order, and number them along with my written notes. I have also used egg cartons and yogurt containers to store and separate parts.
  • Lots of evil volatiles, like xylene (commercial carb cleaner is great) and acetone so you can clean everything.
  • Cleaning equipment like Q-tips, pipe cleaners, paper towels, etc.
  • Access to medium-pressure air (a bicycle pump)
  • An exploded view of the carb, ideally with numbered parts
Terminology

What I called the four main parts of the carb from top to bottom (this is what they are called in the Haynes E3 manual):

    • Carb cover assembly - this thin and topmost section has the choke plate over the primary.
    • Bedplate - this is where all the jets live, the float attaches to the bottom, the fuel inlet block is at this level.
    • Float housing - the float bowl is here, linkage and secondary diaphragm attach at this level.
    • Throttle butterfly housing - base with secondary linkage stuff, idle mixture screws, studs through manifold, and the butterflies.

 

Let's take apart the carb
1
   Remove the stud for in the center of the carb for the air filter housing.
2
   Loosen the tiny set screw for the choke linkage (use the tiny allen wrench).
3
   Remove all the screws around the perimeter and top of the carb which hold the top plate to the rest of the carb. There are a total of nine screws as follows:
  • 5 of the same length on the perimeter of the carb
  • 2 of the same length as the 5 above "inboard" from the edge of the carb; one is between the primary and the secondary, the other on the outboard side of the secondary.
  • 2 longer screws (of the three screws holding on the "cover" piece with the right-angled tube; #134 in Haynes exploded view).
4
   Remove the third, smaller screw from the "cover" (#134), set it and the "cover" aside.
5
   Slide the choke linkage apart so the long rod is free from the mount to the choke plate.
6
   Loosen the captive bolt from the center of the carb (accessible through the hole from which the stud was removed above).
7
   Lift off the carb cover assembly.
8
   Remove the gasket between the carb cover assembly and the bedplate
9
   Position the carb with the primary/secondary toward you; then the primary will be on your right and the secondary on your left.
10
   Remove the bolt at "1 o'clock" above the primary.
11
   Remove the idle air jet at "12 o'clock" above the primary. Mine was labeled "125"
12
   Remove the bolt at "12 o'clock" on the perimeter of the carb. It may have a BMW tag or hold a bracket for the connector which goes to the electrical shutoff valve.
13
   Remove the wire from the terminal of the electrical shut-off valve.
14
   Remove the bedplate of the carb from the float housing. NOTE: There's likely a bunch of gasoline in the float bowl, no need to tip over the base of the carb just yet .
15
   Remove the gasket between the bedplate and the float housing.
16
   Remove the primary air correction jet (which unscrews) and emulsion tube (which falls out). My primary air correction jet was labeled "100", the emulsion tube was labeled "6S". The emulsion tube has a notch in it to make sure it is oriented correctly and looks different from the secondary emulsion tube.
17
   Remove the secondary air correction jet (which unscrews) and emulsion tube (which falls out). My secondary air correction jet was labeled "120", the emulsion tube was labeled "4N".
18
   Remove the screw for the linkage to the accelerator pump. Remove the part of the linkage which passes through the carb body.
19
   Flip over the bedplate so you can remove the float assembly.
20
   Remove the bolt which holds the float in place. Remove the lockwasher and U-bracket. Remove the float.
21
  

Remove the check valve for the float bowl and the washer. This takes a 14mm deep socket.

NOTE: I never managed to remove the accelerator pump when I did this, though I did remove the jet and washer at the bottom of the pump to see why I couldn't get the pump out.

 

The secondary diaphragm
22
   Remove the three bolts and lockwashers for the mount of the diaphragm to the carb body.
23
   Pull the sping/linkage as far from the diaphragm housing and possible and rotate to free the linkage from the rest of the carb (more obvious than it sounds)
24
   Remove the 4 bolts and lockwashers for the cover of the secondary diaphragm.
25
   Remove the cover, note that there's a spring on the inside - not under much tension.
26
   Look for the number stamped on the outside of the diaphragm cover (should be "4"). Also count the number of turns in the spring inside the diaphragm housing (should be 10).
Back to the main part of the carb
27
   Mark (score in the metal) the orientation of the automatic choke housing. Remove the automatic choke housing by removing the 3 screws in the ring that holds the choke cover captive. The tab which holds the wound spring-coil will slip off and the coil will relax. The coil should stay with the carb side of the auto choke, not with the cover.
28
   Screw the bypass mixture screw (the large one) all the way in. Count how many turns out it was when you started.
29
   Remove the bypass mixture screw (and rubber gasket).
30
  

Screw the mixture adjustment screw (the small one) all the way in. Count how many turns out it was when you started.

 
31   Remove the mixture adjust screw (and rubber gasket, though mine was completely disintegrated).
32   Unscrew the fuel shutoff valve
33   Remove the vacuum line between the throttle butterfly section and the float housing. It connects at the bottom of the fuel inlet block and (looking at the bowl end) the right side of the carb base.
34   Remove the linkage from the choke.
35   Remove the old bottom gasket (between the carb base and the manifold). You may have done this earlier, but I didn't. I found that the gasket was quite attached to the base and I spent a fair amount of time scraping it off and cleaning up this surface.
36   Remove the turnbuckle-like linkage section to allow you to separate the two sections of carb that remain.
37   Remove the 4 allen bolts and lockwashers from the throttle butterfly section. Note which one is shorter for reassembly; it's recessed further into the throttle butterfly section. These hold the throttle butterfly section to the float housing.
38   Separate the throttle butterfly section from the float housing. Remove the thick insulating gasket/spacer from the parts. NOTE: Most of you should also have a gasket above and below this spacer. Remove the gaskets and clean all the surfaces. I didn't have one of these, which made my carbs sad.

 

Choke Diaphragm
There's a triangular plate on top of the choke area. It's got a diaphragm in it that is famous for failing. Mine were bad on both carbs.
39   Remove the 3 screws on the top of the cover plate.
40   Remove the top cover, be careful to grap the small spring when you do this.
41   Remove the diaphragm (working the shaft clear of the linkage in the choke housing as needed).
42   Remove the gasket.

 

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.

 

What now?

That's all the disassembly I did (or all I captured in my notes along the way). I'll be happy to include any corrections from other folks who have done this. I'm gonna try to go through the drawings I have and add some more useful names to some of the parts.

I spent a fair amount of time (more than was needed from a strictly utilitarian point of view) cleaning all the pieces before reassembly. That included external pieces, de-sooting the venturis, etc. Lots of volatiles were released. Don't do this in a closed space. I had the big garage door open, and the side door to get a breeze moving through.

 

Done with the cleaning? Go to the reassembly page
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