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, Sat, 20 Jan 2007 09:53:25
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I'm not sure my (limited) experience covers your specific model of Weber, but normally the fast idle cam is supposed to have a certain amount of float in it. The idea is that when you close the choke plate fully, the linkage from the choke pulls the cam up to the fast-idle position. Once it's in that position, it's held there by the pressure of the tang on the throttle shaft. That keeps the throttle at the fast-idle position as long as the choke plate is closed fully.
If you now open the choke plate partway, the linkage rod can move downward without moving the fast idle cam, because there's a slot in the cam; the end of the linkage rod just moves down freely in this slot. The fast idle cam stays at the fast-idle position as long as you don't touch the throttle, because the tang is still holding the cam in place. This allows you to open the choke slightly as soon as the engine starts, while still keeping it running at fast idle.
But if you blip the throttle, the tang releases the cam, and it's now free to rotate downward... until it's stopped again by the linkage rod reaching the end of the slot. When you release the throttle again, the tang will hit on a new part of the fast-idle cam (normally there are several steps cut into it) and again this holds the cam in place until you open the choke plate further and then blip the throttle again.
So, when the throttle is closed, the fast-idle cam always is held in place by the tang on the throttle linkage -- but when the throttle is open, the fast-idle cam is free to rotate somewhat, the degree of rotation being controlled by the position of the choke plate, which restricts the movement of the fast-idle cam via the linkage rod riding in the cam's slot. (Sorry, that sounds very academic, doesn't it? But when you look at the carb it should be clear.)
Now... the important thing to notice about this is that while the fast-idle cam is pulled UPWARD by the linkage rod from the choke plate, there's nothing that pulls it DOWNWARD. It just rotates downward under the force of gravity. If your fast-idle cam isn't rotating freely, gravity won't have enough force to pull it down when it should.
You should be able to see this by looking at the carb with the engine off. Open the choke plate fully by pushing the knob all the way in. Then press down on the throttle linkage so the tang moves away from the fast-idle cam. You SHOULD be able to take your finger and rotate the cam all the way up into the fast-idle position, and then when you take your finger away the cam should drop down freely to the slow-idle position. (In the slow-idle position, the tang doesn't even touch the cam, so it has no effect on the idle speed; instead, the idle speed is set by the idle-stop screw on the carb, the one that presses on the end of the throttle linkage.)
I'm guessing that your fast-idle cam might be binding instead of rotating freely, possibly because of dirt, a bent linkage rod from the choke plate, or whatever. Once you know how it SHOULD work, you should be able to look at it and see why it's NOT working that way, and then you'll know what to do to fix it.
But... if the fast-idle cam does turn smoothly and doesn't bear on the tang when the choke plate is open all the way, then it isn't the cause of your fast idle problem and you have to look elsewhere. Maybe the throttle linkage itself is binding somewhere... maybe the throttle-stop screw is turned in too far... maybe there's a vacuum leak or some other engine problem.
Sorry this was so long... if it still doesn't make sense, post again and maybe one of the real experts will step in...
posted by 68.227.170...
, Sat, 20 Jan 2007 09:53:25, Sat, 20 Jan 2007 16:24:57, Sat, 24 Mar 2007 19:09:17
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