Yet another choke inquery...

[Nunyas]

I'm now sure how closely related my choke problem is to the one that Jaybird is having, but it seems like my choke isn't working correctly. I have a Z-S carburetor with a manual choke conversion on it. When I start my 'B with the choke fully applied it's extremely difficult to do, if even possible. I normally have to use roughly 1/2 choke to start my car. Once the car is started, the engine seems to run overly rich (rhythmic exhaust note) with any amount of the choke applied. Disengaging the choke before the engine has a minute to warm up tends to want to stall unless I hold the accelorater down to maintain 1kRPM until the engine is warm enough to run without it.

Does this mean that my normal fuel mixture is too rich? If not is there away to adjust the enriching properties of the choke? I should note that all of the adjustment screws that I've seen on my "spare carb" have been covered with what looks to be a plastic or possibly JB weld.

Thanks again oh wise masters of the LBC domain!

 

[Dave Russell]

It sounds like the "fast idle" on the carb is not engaging. The first part of the choke knob pull should engage the fast idle. Further pull should engage the choke itself. There is a fast idle adjustment screw to set this. Idle should probably be set 1200 to 1500 rpm when the knob is pulled part way, & return to normal when the knob is fully in the fully returned position.
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[Nunyas]

Well, let's see here. I've made some adjustments to the fast idle screw, and there seems to be a minor difference. On a cold morning, I start the car with the choke fully pulled out, and I watch the tach and listen to the idle quality. On initial startup the engine idles at about 1500 RPMs, but after a few seconds (easily less than 30 seconds) the idle speed begins to drop and the quality becomes cyclicly lumpy (Haynes Manual decription of over rich to the 'T'). If I don't start cutting back on the amount of choke applied when the idle changes, the car will stall. Around this time frame, there is also the distinct strong odor of rotten eggs coming from the exhaust (I have to have the Cat. for Cali emissions).

My float is currently set to approx. 15mm-15.5mm, which I'm beginning to think may be a smidgeon too high for various reasons.

 

[Dave Russell]

I think that you are on the right track. After a few seconds of full choke, you should be able to reduce the choke enough to clear the running but still have the fast idle partially engaged. (The fast idle should engage before the choke enrichment itself starts to engage) I would turn up the fast idle a little more & try it. Then, as the engine runs lumpy, reduce the amount of choke but the linkage will still keep fast idle partially engaged until the choke is fully released.
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[John S Farrington]

All previous responses are very germain and should be checked out. The "rotten egg smell" makes me think that it could be a catalytic converter that is about to be totaly clogged. This would also induce the rich-like symptoms. Try unhooking the catalytic an see if that makes a difference.

 

[Nunyas]

the cat is fresh, it was put on the car less than a month ago. Since its installation, I haven't experienced any of the conditions that cause my last one to go bad.

 

[Dave Russell]

In my "limited" experience, the rotten egg (hydrogen sulfide) smell is nearly always the result of an over rich mixture going into the converter. In this case, too much choke. If it were to continue for a long period of time it "could" cause the converter to overheat. As mentioned before, back off the choke a bit when the engine starts running rough.
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[Banjo]

Rotten egg smell comes from sulfur oxide emissions (SOX)and it is far more related to the fuel that you bought than any adjustment you can make to the car. Some fuels have more sulfur in them than others. Toyota, for instance has a current TSB dealing with the odor. Basically the only thing they can do is tell you to try a different brand of fuel. From what I gather the west coast isen't having near as many problems as the northeast. We have seen a lot of bad batches of fuel this winter, causing anything from just the smell to drivability problems.
I don't think that adjusting the carb is going to have much of an effect on the "egg" smell. Also, a new cat tends to have some potent odors until it has broken in. So I woulden't take the smells alone as any indication that you have done anything wrong, or have a problem that needs to be taken care of immediatly. With that said, if the mixture is rich, like has been posted above, that can lead to failure of the new cat. I would just try to base that determination on other indicators than SOX emissions alone.

 

[Dave Russell]

Hi BJ,
I think you may be confusing hydrogen sulfide (H2S) with sulfur dioxide (SO2). H2S has a distinctively rotten egg smell as Nunyas described. SO2 has a much more "penetrating" smell like a struck match. They can both be present in the exhaust, but H2S is much more prevalent with an over rich mixture. SO2, is as you say, is a result of high sulfur fuels but is not affected as much by a rich mixture.

If you have ever opened an old refrigeration system which used SO2 as a refrigerant, you would immediately detect the difference in smell between the two.

Here is the Toyota tech:
https://www.autoshop101.com/forms/h56.pdf
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