WEBER 40 DCNF COLD START/FAST IDLE MECHANISM

Steve,

It is the cold fast idle mechanism. At the bottom right of your circle, the red line passes through the adjustment screw for the cold fast idle. There is a thermostatic spring in the housing that rotates the cam as the coolant warms that in turn moves the lever and pushes on the throttle linkage.

From your picture, it looks like you snapped it when the engine was warm as the flat side of the cam is at the lever. Also the fast idle adjustment screw at the throttle linkage is backed all the way out, so even when cold, the fast idle mechanism would not come into play.

The way it is set now is the way it should be when synchronizing the carb throttle linkage. Once your linkage and 2 main idle stops are set correctly, you COULD crank down on the cold fast idle screw to have the function operational. Most people however disable it the way your picture shows because the cam lobe can stick and if it does, you are stuck with a fast idle.

 

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Steve,
I'm no expert here, but I do happen to be in the middle of overhauling my throttle linkage at the moment. I had horrible stiction that would delay the throttle closing for a good 2 - 3 seconds and finally traced most of the problem to (what is probably) 46 year old grease in the throttle linkage pivot's needle bearing.

But in the course of stripping the linkage, I too had the opportunity to ponder the fast idle device. Even though I had no idea it held cold idle at 3,100ish RPM until I read Dockboy's post, I knew just by looking at it that I was going to disable it. It just seemed like something else ready to fail - like the choke mechanism - and not do my engine any favors when it did. Best of luck with your project.
- Dave

 

Hello,

For me first thing to do is to be sure that the spring inside is well installed in the Pin, and when your engine is cold (20 degree Celsius) the Lever C (in the Picture) is correctly aligned with the +20. To do that you can remove the 3 nuts holding the mechanism and have a look it is easy.

Then when this is done, you can do the procedure when the engine HOT, put the cam at 20 degree and adjust speed.

Me i was using 2000 rpm, but i think it is a bit to high for me, i will reduce it a bit to 1600 rpm, maybe other can comment on that