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Choke closes on its own as I drive

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skucera

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Ever since I bought my "new" '76 Spitfire, its choke closes from the fully open position as I drive. I discovered when I pulled to a stop that the engine idle was 1500-2000 RPM, and at first I thought I was being forgetful about pushing the choke knob in all the way as the engine warmed up. As I started watching it I confirmed a few weeks ago that it really does close slightly every few minutes. I've gotten in the habit of pushing the choke knob every few minutes to reclose it, and my fuel economy has increased noticeably.

This week I've ruled out any sort of springy interference with the cable or the choke linkage on the carb. (I'm still running the original Zenith-Stromberg.) The choke seems to just close slightly from engine vibration. My choke cable doesn't have any friction built into it to hold the choke wide open when I push the knob in.

I dimly remember (from almost 20 years ago) that my Midget 1500 had a detent at both ends of the choke knob travel. Does anyone feel a little detent action when they push their choke knob in? If so, I'll be in the market for a new choke knob and cable. /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Scott
 
Scott - on my TR3 the choke cable has detents (a couple notches) in the knob end of the cable. When I pull the choke cable all the way out, I then turn the knob about 1/4 turn and it "locks" into place.

I gradually remove choke as the engine warms, by turning the knob back, letting the cable go in, and turn again to lock at the next "notch".

Is that what happens on yours? I know that if I don't "lock" the cable knob, the cable moves in on its own.

Tom
 
Scott - The same thing happens with my 1971 TR6 if I do not turn it the 1/4 turn to lock it in place. I am pretty sure this is how it is on most all of our cars.

Hope this helps.
 
I believe on the late ZS carbs, there was supposed to be a detent built into the cold start valve (aka choke) itself. Sounds like maybe your detent ball has gone missing.

One of Nelson's articles talks about problems with this design, and shows how to convert to the earlier, spring-loaded setup.
 
I may be wrong, but I am interpreting Scott's problem as the choke closing, i.e. coming on, rather than opening or the knob going in by itself.
If that is the case should there not be a spring on the carb to help pull the choke off. Certainly there is on my TR3's SUs. I am not sure what carb the American market Spitfire was fitted with.
 
NickMorgan said:
I may be wrong, but I am interpreting Scott's problem as the choke closing, i.e. coming on, rather than opening or the knob going in by itself.
Click to expand...
Me too ... Hey Scott, how about a clarification ? Or maybe a report on what you found ?
 
Yes, as I wrote the problem was that the choke closes (meaning that it is applied) as the engine runs. Randall's picture was a huge help, and I found that the my spring-loaded ball is working just fine, as it was originally designed to. That, combined with some research in my manual and on the internet led me to greater understanding. What I found is really interesting.

The choke lever attaches to the cold start mechanism shaft. It has a cam cut into its back which pushes on a screw head that opens the throttle on the same lever that the idle set screw affects. This was the simple cause of why my idle was too high. The subtler cause is on another side of the cold start lever... there is a vacuum plunger that is supposed to be connected to the EGR valve and to the intake manifold by vacuum lines. (I think that the choke lever presses in this spring-loaded valve, which blocks a vacuum opening, and allows vacuum to move the EGR valve. I have a hunch that this is only supposed to happen once the cold start process is done. Once pushed in a little, vacuum might hold it closed so the choke lever can be pushed fully open.) My spring plunger has no vacuum line on it, so this spring plunger is always extended, preventing the choke from closing fully. I've turned in the screw that cams open the throttle by one revolution as a hack, but I really need to reconnect the EGR valve and this cold start vacuum plunger, then turn that screw head back out to its proper place to make this car run the way it was designed to.

(Yeah, some folks may be aghast that I want to get my EGR working again. I learned on my old Midget 1500 that the Z-S carb really works best with a little EGR, like a carburetted Toyota or GM car in the days before they went to fuel injection.)

I'm pretty sure I can get the little rubber vacuum elbows and T connections and the plastic vacuum pipes by mail order. The PO who removed so many vacuum lines also removed the fuel vapor traps that protect the carb from the very hot EGR valve. I've checked around town and the auto parts stores don't stock them anymore. Does anyone here have any extra fuel vapor traps in a drawer? There are no more British sports cars in any junk yards in Oregon... they were all scrapped years ago. I have a feeling that I'll have to wander through junk yards looking at older American and Japanese carburetted, emission controlled cars to find a couple of vapor traps before I hook everything back up the way it once was.

Scott
 
Wow, that _is_ a surprising price. Well, there is Plan B. Let's hope I can find a used one somewhere instead.

Thank you for the link.

Scott
 
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