MGB & GT Forum

There has been much discussion about problems and tuning the Zenith Stromberg. I have read all that I could find, searched the internet and had correspondence with members about this carb. My car has a 175 CD 5T and not a 175 CD 2. There is a difference. My carb does not have a temperature compensator or a bypass valve. Instead it has a fine idle adjustment screw and a coarse idle adjustment nut. John Twist does not address these. Moss shows the correct carb but the tuning info is silent on these. On page 82 and 83, Haynes has some confusing advice on adjusting them. Does anyone have experience with adjusting them. Please don't suggest replacing the ZS with something else as that is not my intention. The car runs well except for the fact that it does not want to idle below about 1000 rpm. It has a manual choke that seems to be performing well.

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1976 MGB, HS-4 Conversion, Lowered 1-1/2" in rear, Petronix ignition in 24D5 dizzy, 3/4" Front Sway Bar.

PM Sent.

That's exactly what I have along with manual choke.

I had to read the Haynes instructions a dozen times, I downloaded the tuning guide from the tech library here and read that a dozen times and finally found a JT video where he talks about tuning the ZS but it is on a Triumph - I found it still applicable. He has another video that talks about the choke and that should be absorbed as well as the choke is one of the top cuplrits for running rich.

Here is what I do...

Assume - valves set, timing set, no vacuum leaks, plugs rotor, cap and wires all good. Only then start messing with the carb. You need the ZS tool.

Assume the car runs if it does not let me know.

- Warm up the engine to normal temps
- Set idle as close to 800-900 as possible if it won't idle that low get it as low as you can
- Turn the coarse idle screw (yellow nut) CW - righty richy, left leanie. You are richening the mixture. Go until you think the car will stall becuase eventually it will. Then turn CCW (lean) to get max RPM.
- Then adjust idle (spring loaded) screw to try and get to 800-900 or lower - I find mine likes about 900 and I live with that. Wear, on various components means you may not get a perfect 700 rpm without carb overhaul.
- Every few minutes, run the RPM up around 1500+ and hold it there to clear the plugs.
- Now unscrew the dashpot and remove the damper assy.
- Carefully insert the ZS tool - if you "press" it while inserting you will note a change in RPM as you are pushing the needle down and "leaning" the car.
- Screw the allen screw CW (righty richie) and note the idle speed. It should change lower. Then turn it left to achieve max RPM again.
- The adjust the spring idle screw to lower the RPM.

I then do it all again a couple of times "dialing it" in. The fine idle screw can also be turned left and does the same thing as the coarse screw but slower. If you are lefting the fine idle screw and it feels like it is going to come out, unscrew the coarse about 1/2 to 1 turn and screw the fine screw back in a couple turns.

For reference and you can also do this as a start point before doing anything you could turn the coarse screw all the way in and then back it out 2 1/2 turns - this is "mid-range" for the coarse screw - It's nice to have a start point.

Finally - the Haynes book references doing all this with an air sensor tool - It was all about tuning for emmissions.

This carb and choke fails (rich) - I have found on mine I can't really get too lean. But I can smell rich - the fumes around the car will burn your eyes. Get it lean then road test. I was blowing a bit of smoke off stop signs. It was loading up at idle and was rich. I kept leaning until there was no smoke at any time.

Then I worked on off idle performance. Knowing the carb was pretty close I went back to timing and in my car ended up advanced pretty far. It was stumbling off idle until I get it fairly advnaced. After timing adjustments I would then tweak idle speed and mixture again.

I don't know what others do but I am darned happy right now acceleration, performance all good and I get 25mpg.

I am sure I could get the idle lower than about 800 where it is now but I like it where it is.

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Edited 1 time(s). Last edit at 2015-07-29 06:40 PM by Ex-Calif.