Zenith Stromberg Water Choke. This information comes from a web page that is no longer available. I saved this some time ago. Credit for the content and images is to Rick Jaskowiak of Bemidji MN. It was well written and the images are very helpful so I wanted to preserve this information and make it available to the MG community, even if the Zenith Stromberg carburetor is almost universally hated. So as a reminder the “I” referred to below is Rick Jaskowiak
Introduction and Context #
When a recent Midget project ran poorly after an engine rebuild due to an over rich mixture, I decided it was finally time to find out what makes the water choke mechanism so mysterious and widely despised by many owners of later model MGs. I scoured the MG archives (http://www.mgcars.org.uk), got my manuals together and ripped into it. After studying for a short time, I thought some pictures would help and grabbed my digital camera. By the time I was done, I had a pile of pictures and notes, and maybe even a rudimentary understanding of how this thing was supposed to work. And to top it off, my Midget ran great.
I didn’t start out planning to write this “tech article”, but as long as I had the photos and notes, I put them together and posted them in the hopes other may benefit.
Disclaimer #
I have a disclaimer: All of the following is only my understanding. It’s not necessarily correct. It only covers the choke mechanism. I tried to use the right terminology, but I also made up some names. There are enough pictures so there should be no confusion of what I’m talking about. I’m not a carb/choke specialist….not even a mechanic. I just like MGs and this choke was getting in the way of enjoying them even more. So, consider what you are paying for this information, and take it for what it’s worth.
The carb and choke shown are off a 78 Midget and has a water choke, as original. However, most everything should apply to the ZS on the MGB.
Main Parts of the Water Choke #
Refer to the picture and numbers as follows:
- Heat mass/ water jacket
- Bimetallic spring
- Insulator (black plastic)
Continuing the main parts of the water choke refer to the picture and numbers as follows
- Kick Piston Mechanism
- Enrichment Needle Mechanism
- Fast Idle Pin
- Fast Idle Cam
- Lever
How the Choke Works (I believe) #
With a cold engine, before you start the car, the kick piston spring is pulling the kick piston rod in. The rod is hooked to the lever, and the other side of the lever is hooked to the enrichment needle, which is pulled out. In this mode, the path is open from the carb float bowl to the carb throat. I’ll show that later. The fast idle cam is also in position to engage the fast idle pin.
When you start the car, the carb creates a vacuum in the kick piston and the rod is pulled out. Since the kick piston rod has a long notch, the enrichment needle and the fast idle cam are now controlled by the lever, which is hooked to the bimetallic spring on the heat mass. The lever and spring are not necessarily in the correct position in the pictures because the heat mass and bimetallic spring are not attached.
As the engine warms, it warms the heat mass, which in turn heats the bimetallic spring. The bimetallic spring will then rotate counter clockwise. Since the spring is connected to the lever, the counter clockwise motion closes the enrichment needle and steps down the idle. When the engine is fully warm, the enrichment needle is pushed all the way in and seated in the tube, closing the gas flow between the carb float bowl and the carb throat. The fast idle cam is also rotated fully counter clockwise until the fast idle pin no longer engages high lobe of the fast idle cam.
Digging Into The Choke & Carb Connections #
The images above show the water choke on the left and the corresponding ports on the body of the carburetor. The numbers are explained as follows.
- Number 1 on the left (choke) goes to the kick piston. Number 1a is the port that creates the vacuum to work the kick piston
- Number 2 is the top of the enrichment tube. It matches with number 2a on the carb
The hole number 3 is the bottom of the enrichment tube which mates with number 3a and this is a port to the float bowl. I didn’t get a picture where that ends up. Number 4 is just a hole in the housing and it doesn’t have a purpose as far as I can tell.
The images above show where 2 of these ports are on the main body of the carburetor.
- On the left image 1a is where the kick piston on the choke and it’s connection to the carburetor body (that creates the vacuum to work the kick piston) comes out on the engine side of the throttle disc.
- On the right image 2a is where the enrichment tube hole from the choke ends up here on the intake/air filter side of the carburetor and throttle disc.
Disassembly #
- Take the E clip off the center peg, lift off the fast idle cam, spring and lever (better seen in the prior picture)
- Take out the three screws from the top of the kick piston mechanism (as shown in the picture)
- Remove the top, gasket, plunger and rod, and spring
- Unscrew the brass breather plug from the housing. The enrichment needle can be removed through this hole
- Brass breather
- Enrichment needle
- Enrichment tube
- You probably don’t have to remove the enrichment tube, but if you really want to, put something on top of the enrichment needle tube, like a plastic pan head screw, so you don’t damage the brass. With a punch, lightly tap out the tube. The plug on the end will also come out
- This is the plug that will pop out when you tap out the enrichment tube
- Enrichment assembly
Reassembly #
Obviously, maybe not, but all parts should be properly and thoroughly cleaned before reassembly.
- Renew the o-rings on the enrichment tube
- Make sure the brass screw is in the bottom of the enrichment tube and push the tube back into the housing.
- Notice that the bottom holes on the tube is in a groove around the circumference of the tube. This hole does not have to align exactly with the hole in the housing. However, the top hole should align with the housing, as shown below. The tube goes in until it comes up against the rise inside the casing
- Tap the bottom plug back in
- Renew the tiny o-ring on the enrichment needle
- Lubricate it with some light oil and slide it through the breather hole back into the tube
- Face the notch at the end of the needle up
- As shown in the image there is also a vent hole in the cap make sure that’s clear
- Also, make sure the vent hole from the kick piston cylinder to the back of the housing is clear – refer back to hole number 1 in the Digging In section above
- Clean and lubricate the kick piston cylinder, but don’t get carried away sanding or honing. If you enlarge the cylinder or reduce the diameter of plunger, it may not provide enough seal to produce the vacuum necessary to suck the rod out. Been there, done that!
- Reinstall the spring, rod and plunger, gasket and top. Put the three screws back in. Make sure the notch in the rod faces up.
You can check the operation of the kick piston by proper lip placement over hole #1 (Picture 11). Sucking on this hole should cause the kick piston to activate, forcing the kick piston rod out.
- Drop the lever back onto the center peg. One side of the lever fits into the kick piston rod, and the other side fits into the enrichment needle
- Drop the spring back onto the center peg with the bottom bend between the arm that engages the kick piston and the upright that will engage the bimetallic spring
- The next part can be a little tricky. You have to get the top bend of the spring into the small hole of the fast idle cam, and place the cam back onto the center peg as shown
- Put the e-clip back on the center peg.
- Make sure everything works freely with no binding.
- Reinstall the choke back onto the carb, using a new gasket. Put a little thread sealer on the three screws that hold the choke to the carb.
- Set the idle screws
- Fast idle screw (not a factory screw shown)
- Idle screw
- Space should be about 3/32 inch
- 035 (.025 for MGB, 175 carb) between fast idle pin and the cam at slow idle location
- Put the insulator in position, and then the heat mass, making sure the loop in the bimetallic spring engages the lever
- Reinstall the three screws that hold the heat mass, but do not tighten yet.
- Rotate the heat mass until the index marks on the heat mass and the choke body align, then tighten the three screws. (Not too tight…they strip easily)
- Reconnect the hoses to the heat mass, making sure you don’t rotate the heat mass while doing so
- It should be ready to go.
You will probably have to fool with the fast idle and the slow idle screws to get them set correctly. Also, you may find that lining up the index marks may result in a mixture too lean or too rich for starting and cold running. To adjust, loosen the three screws that hold the heat mass, and rotate the heat mass counter clockwise to lean the mixture, and clockwise to enrich it. Don’t turn too much as a little goes a long way. You may want to refer to the John Twist Videos about calibration of the heat mass.
If everything is working right, the mixture should lean as the car warms. The idle should also drop in steps as it warms, but you have to tap the accelerator to move the high idle pin off the high idle cam, allowing the cam to rotate counter clockwise. Note that on the MGB the Driver’s Manual says to fully depress the accelerator peddle and let it go in order to set the choke on.
Other Materials #
I’ve built this website and spent time on the knowledge base to bring together a lot of information I have collected from around the internet. The intent is making it easy to find rather than having to know about and have skills with search engine query terms to get what you were looking for. Please make use of the knowledge base.
