Zenith Stromberg Tech Tips

by Scott Suhring

Based on personal experience and many discussions with fellow Triumph owners, there are two fairly common problems which arise with Zenith Stromberg Carburetors that I would like to offer technical advise on how to correct (remember the personal experience aspect).

Persistent Loss of Damper Oil

For those members whose engine has ZS carbs with the adjustable metering needles, you will at some time experience the persistent lose of damper oil. When it gets to the point that you need to fill up the guide rod in the damper assembly every time you check the damper, it is time to replace the O-ring on the jet adjusting screw. This is an easy job and one that will resolve the disappearing oil phenomenon you are experiencing by completing the following instructions:

  1. With the carbs attached to the manifold (no removal required), remove the suction chamber top cover (you can leave the damper cap attached or remove first).
  2. By grabbing the exposed guide rod, lift out the air valve straight up so as not to damage the metering needle.
  3. After draining out any remaining damper oil, proceed to your workbench or kitchen table (if so permitted in your household).
  4. Using your metering needle adjustment tool, insert the Allen key end into the guide rod and turn counter-clockwise until the metering needle emerges from the base of the air valve to its furthest position (you will not be able to turn the key any further).
  5. Using a small head screwdriver, remove the spring loaded locking screw found on the side of the air valve.
  6. Grasp the needle and gently pull it out. You cannot remove the needle without first performing the prior step.
  7. Looking into the metering needle hole, you will see the base of the metering adjusting screw. Position the air valve so it is standing on the air valve guide rod (upside down). Using a 1/4″ metal rod (8-12″ in length), insert the metal rod into the bottom end so it rests flush against the base of the needle adjusting screw. Gentle tap down on the end of the rod with a hammer so that the adjusting screw is driven down and out through the air valve guide. Note: there is a retaining clip at the top of the adjusting screw that will also come out.
  8. With the adjusting screw and retaining clip removed, the O-ring will be visible on the adjusting screw. Remove the O-ring and replace with a new one (TRF & Moss carry these as part of the carb rebuild kit or you can purchase separately).
  9. Assembly is the reverse of above. The retaining clip, which goes in the air valve guide after the adjusting screw, may take numerous tries if you drop it down the shaft. It will not land level, so you need to get it slightly tilted and then using the rod tap it down level against the top of the adjusting screw. Or, you can place it on the top of the adjusting screw when it is just below flush with the top of the air valve guide and tap both down the shaft together.
  10. With the adjusting screw in place, insert the metering needle so that the grove on the side of the base of the needle lines up with the retaining screw shaft. Insert the spring-loaded retaining screw and screw in. You can visually see if the end “springs” into the grove of the metering needle by looking down at the base of the air valve with the needle pointing up. If the spring-loaded screw is not extended into the grove (this must occur for the adjusting screw to properly engage and hold the angle of the needle in the proper position when adjustment of needle height is performed), you will need to turn the base of the needle to get it to engage. Once engaged, push down on the key and turn clock-wise to engage the adjusting screw with the base of the needle and draw the metering needle back up into the shaft. Appropriate height will need to be adjusted by following mixture adjustment procedures.
  11. Reassemble air valve and suction chamber top cover. Fill up air valve guide with appropriate oil to the point that when you insert the damper cap, you will feel resistance when the base of the damper cap thread is 1/4″ from the top of the cover cap opening. Adjust carbs for proper mixture.

The Bypass Valve Headache, (a.k.a. can’t get normal idle below 1500 to 2200 RPMs)

The inability to maintain a correct slow idle setting can be caused by numerous problems, including: vacuum leaks, timing, slow idle screw setting, vacuum retard, stuck temperature compensator, incorrectly fitted or lose throttle disk, stuck choke to name some of the more common areas to be investigated. One problem when the idle will not seem to settle in below 1500 to 2500 RPMs, even when the slow idle screw is fully retracted and the throttle disk is closed, is almost assuredly caused by the bypass valve either leaking or being stuck open.

The bypass valve, in essence and quoted from the Haynes Techbook Weber Carburetor Manual 1 “The bypass valve provides a means of limiting the very high manifold depression which occurs when the engine is in the over-run condition.” Thus, this allows the air and fuel mixture to be sucked into the manifold around (bypassing) the throttle disk when it is shut (let off of the gas peddle when running the engine, lets say at 60 mph) when the engine is running, allowing for engine breaking without relying only on the idle set of the throttle disk.

If you experience the symptom of a high idle that won’t drop to the normal 800 RPMs, an easy way to test if it is in fact the bypass valve is to remove the bypass valve from both carburetors and plug the bypass holes. To do this, the following steps should be taken:

  1. Warm the engine up so that normal slow idle should be achieved (taking into account the existing problem of high idle reading).
  2. Turn off the engine and remove the bypass valve from both carbs. There are three standard head screws and three Phillips head screws on the bypass valve. The standard head screws secure the bypass valve to the carb and the three Phillips head screws hold the two faces of the bypass valve together. Remove the three standard head screws.
  3. Using a piece of duct tape, seal off both the orifice and the bypass valve openings on both carbs.
  4. Before starting the engine, be sure that you have reset any adjustments you might have made in an attempt to bring the slow idle speed down (slow idle screw and/or timing).
  5. Start the engine and determine if the high idle problem goes away (I’d bet on it).

There are some Triumph owners who just block off the bypass valve to avoid having this problem. I would not recommend this since the bypass valve does assist with engine breaking and emissions control. Fixing the bypass valve is not difficult and entails the following steps:

  1. Purchase 2 new bypass valve gaskets and 2 new bypass valve diaphragms.
  2. Separate the two halves of the bypass valve by removing the three Phillips head screws. Be careful, there is a spring inside of the bypass valve and you don’t want to end up on your hands and knees searching the floor for them!
  3. Pay particular attention to the position of the gasket, which goes between the bypass valve and the carb. It looks funny (kinda like a Salvidor Dali rendition of Mickey Mouse’s head) that has an odd fit to it.
  4. Remove the old diaphragm (it is probably brittle and torn) and clean the inner faces of the bypass valve.
  5. Put on the new diaphragm. The cupped side faces toward the spring, which fits into the cup.
  6. Screw back the two halves of the bypass valve, install the new gasket and reinstall the bypass valve to the carb.

Early ZS carbs have an internal adjustable spring while later models have an external screw at the tip of the bypass valve, which allows for the adjustment of the tension of the spring on the diaphragm. To properly adjust the spring tension, follow the instructions provided in the Bentley Manual as follows (I am presenting these instructions as for the external adjustment, but the concept is the same for the internal adjustment).

Turning the screw clockwise reduces the spring pressure and at some point allows the diaphragm to “float”, resulting in a sudden increase in idle (symptom we started with). To adjust:

  1. Disconnect the vacuum retard line and plug the line. This should bring the idle speed (RPMs) up to about 1300 RPMs.
  2. Turn the bypass valve adjusting screw clockwise (starting with the screw fully engaged with full tension of the spring) until the engine speed increases abruptly to approximately 2000-2500 RPMs (indicating the valve is floating).
  3. Turn the adjusting screw counterclockwise just far enough to bring the RPMs back to 1300. Using the throttle, suddenly increase engine speed and let it return to normal. Engine speed should drop back to 1300. If it doesn’t, the valve is still floating and should be adjusted counterclockwise a half turn at a time until you can rev the engine and have it return to normal (1300). When you find the point at which this happens, back the screw out counterclockwise one half turn more to properly seat the valve.
  4. Reconnect the vacuum retard line and adjust slow idle speed screw as necessary.

Your high idle problem is now resolved and you have properly set the bypass valve to specifications. Now, what was that next thing I wanted to fix…

Scott Suhring, suhring@lancnews.infi.net
1 A K Legg LAE MIMI, Don Peers, Robert Maddox and John H Haynes; Haynes Publishing Group; Haynes North America, Inc. 1995.

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