Understanding the Zenith Stromberg 175 CD2 carburetor

[David Bowyer]

I thought it might be useful to gather together advice on understanding the various features, mainly lesser known ones in the first place, of the Zenith 175 CD2 carburetors and their adjustment and possibly post them to the Wiki page on the Forum. I have one immediate query as follows:

On the arm that holds the throttle stop screw, there is a small lever with an adjustment screw in it. When the throttle is closed, the lever strikes against a pin which is located in line through the body of the carb with the tube onto which the vent pipe to the air filter is connected. When the throttle is opened the lever retracts and lets the pin drop presumably opening a channel from the air filter into the carburetor.
What is the purpose of this ? On the carbs I have, the lever mechanism closes the pin on one carb but not the other. I presume that is wrong. Furthermore what is the purpose of the small adjusting screw? I thought it might be to adjust the lever so that it does contact the pin when the throttle is closed, but it seems to be set in the wrong orientation to the pin to do that. Any thoughts?

[redstag]

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Hi David,

It's all clearly explained in the ROM.

Pete

[StagnJag]

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This might help if you want more info

http://www.mv.com/ipusers/last/emissions/booklet.pdf

Rgds

Dave

[martin]

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The adjustment screw is factory set, don't touch, although if you do, you may find it's locked anyway. Therods you mention alter the carb breathing, running and idleing, you're right, they should both work, needle nosed pliars or a long fingernail and WD 40 are all you need, don't bend it, there is a screw adjustment butI wouldn't bother with it. The coffin shaped plastic housing on the side of each carb,theoretically opens and closes an air valve according to temp, but they never work and make no difference. On each carb, there is a large horizontal brass spring loaded screw, this was fitted for fine tuning on US cars, screw it fully in, then back off 3/4 of a turn. Apart from Mixture, idle speed and fast idle that's it. Easier than injection. Martin.

[David Bowyer]

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Many thanks for the above replies. All very useful and I am most grateful. The information in the training guide is so valuable and I hope that a link to it can be included in any Carburetor section in due course.
Martin re the "don't adjust the vent valve", unfortunately or otherwise it had been altered anyway, so I am really glad to find the instructions for setting it up in the training guide.
Have completed the work on the spindle seals, new spindles and throttle butterfly plate and despite the need for further fine tuning (careful balance of air flow, mixture settings, by-pass valve adjustment, plus of course the vent valve, which should be done before the by-pass valve) there is a good improvement in performance so far. I begin to think it all worth the time invested.
I will try to keep a list of hints and tips from the experience for wiki
Best David

[martin]

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Good project David. Very few Stags have their carbs set up properly, even by specialists, as you've found, people in desperation tend to twiddle everything trying to get them right, when in truth you need to go back to first principals as you have done. Perhaps it's worth a mention that unleaded petrol wears the needles faster than leaded, another point worth checking is the height of the jets, if they're different the carbs are virtually untunable, also check that they are IDENTICAL, I've come across quite a few Stags running with odd carbs, Triumph 2000 etc. Love to know how you get on. Martin.

[David Bowyer]

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A quick update on the adjustment of the vent valve(s). No problem following the instructions in the training guide and the adjustment on one carb was fine, ie the screw adjusted so as to raise the vent pin enough so that there was no restriction of blowing down the vent tube on idle. However, I was at first unable to adjust the other carbs' vent valve and in the idle position there was restriction to blowing down the tube. The adjustment screw would not push the pin up far enough to remove the restriction. Now the range of adjustment of the adjustment screw is small so I sought another reason, which turned out to be that the lever holding the screw was bent. On straightning the lever the adjustment screw had sufficient range of adjustment.
Why all the bother I hear you say. Well I saw in the training notes that for correct adjustment of the by-pass valve (effects engine braking on decelleration) it is important that the vent valve is properly adjusted. Adjustment of the by-pass valve is a job for the morrow.
Thank you all for helping me to do these jobs correctly.
David
PS In passing, I have seen recon carbs in which the vent valve pins were seized in the up position. In any event they should be lubricated with WD40 from time to time.

[David Bowyer]

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Hi Martin,
Thanks for your further post about unleaded petrol causing wear of the jets and needles and also the point about the jets needing to be pressed equal distances into the bridge of the carbs.
I renewed my jets some while ago and had a very good engineer press the new ones in for me. He turned a piece of aluminium so that it had a projection of the right diameter to press the jet and the correct depth. Of course, using the same device on each carb ensured that the jets were pressed into the same depth. Interestingly we had to have two goes at it, because the first time the jets were not pressed in quite far enough and I was unable to get the proper range of adjustment of the needles (ie. could not get them quite rich enough). A futher ten thou did the trick. In passing I checked with Burlen fuels what depth the surface of the jet should be below the bridge for my carbs (Mk1s 3425 although fitted to a Mk2 car), but this could obviously also depend on whether the appropriate needles were fitted. Indeed, if one is changing jets, it might be best to follow advice which I saw somewhere, but forget where, to simply press the replacement jets in exactly as far as the old ones had been.

[David Bowyer]

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H Martin,
Me again. I wanted to take you up on the issue of the temperature compensators. The do work if they are in proper working order, making the mixture leaner above their operating temperature which is about 120 F which is when they first open and progressively weaker at hight temperatures. That does make sense since a hot engine should need a leaner mixture. Despite what is said in various manuals about not touching the adjustment, it can be done quite easily using a dish of hot water and a thermometer. I discovered how to do it in a very comprehensive account at

http://www.vtr.org/maintain/temp-compensators.shtml

Some people say just fit replacement ones if in doubt, but in my experience the remakes (like so many remakes) are not very good. In one of the remakes I tried (sourced from Rimmers) the neoprene cone which is supposed to close the air flow at cold did not seat so the compensator was always allowing air flow even on cold. No amount of bending the supporting bi-metal strip etc. fixed the problem, so I got some old ones and adjusted them as per the article cited above. The key thing is, as with all the other carb tuning, to make sure they are balanced. It is more important the then exact temperature at which they start to open.
Best David

[martin]

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Thanks for that David, when they get old, the bi-metal strips seem to go to sleep, mine have never worked despite my best attentions. I'll have to look again. I would be interested in Kryten's opinion ? Martin.

[David Bowyer]

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HI Martin,

I don't think they go to sleep. Who is Kryten?
Best David

[Gresley]

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David Bowyer wrote:

Many thanks for the above replies. All very useful and I am most grateful. The information in the training guide is so valuable and I hope that a link to it can be included in any Carburetor section in due course.
Martin re the "don't adjust the vent valve", unfortunately or otherwise it had been altered anyway, so I am really glad to find the instructions for setting it up in the training guide.
Have completed the work on the spindle seals, new spindles and throttle butterfly plate and despite the need for further fine tuning (careful balance of air flow, mixture settings, by-pass valve adjustment, plus of course the vent valve, which should be done before the by-pass valve) there is a good improvement in performance so far. I begin to think it all worth the time invested.
I will try to keep a list of hints and tips from the experience for wiki
Best David

'scuse my ignorance, but what and where is this by pass valve?:?

Brian

[martin]

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All right then, they DON'T go to sleep - they just stop doing anything. Martin.

[stustag]

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Gresley wrote:

David Bowyer wrote:

Many thanks for the above replies. All very useful and I am most grateful. The information in the training guide is so valuable and I hope that a link to it can be included in any Carburetor section in due course.
Martin re the "don't adjust the vent valve", unfortunately or otherwise it had been altered anyway, so I am really glad to find the instructions for setting it up in the training guide.
Have completed the work on the spindle seals, new spindles and throttle butterfly plate and despite the need for further fine tuning (careful balance of air flow, mixture settings, by-pass valve adjustment, plus of course the vent valve, which should be done before the by-pass valve) there is a good improvement in performance so far. I begin to think it all worth the time invested.
I will try to keep a list of hints and tips from the experience for wiki
Best David

'scuse my ignorance, but what and where is this by pass valve?:?

Brian

I have to ask the same Brian what are you on about David/Martin what bypass valve?

Do you mean the little spring valve on the butterfly valve?

[martin]

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DON'T TELL 'EM DAVID !