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texson
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Joined: June 13, 2009
Posts: 180
Location: Nacogdoches, Tx
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 Posted: Fri Nov 08, 2013 1:00 pm Post subject: Dashpot?? |
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This came with my 71 in a box of parts the PO had removed. ebay calls it a dashpot. does this belong on my 71 Super with a 34 pict3 carbbie? If so, could someone provide a pic of how it is hooked up?
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Luft kühl
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Joined: January 21, 2009
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Location: Allentown, PA
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Fri Nov 08, 2013 7:09 pm Post subject: |
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Like Luft said, it's a throttle positioner. It must hook up to an altittude adjuster to work. (You can see the altittude adjuster on the left wall.) It slows the closing of the throttle so you don't get the sudden enrichment that usually occures when the throttle slaps shut. This is for emmisions but it also does help a stock (jets, dvda, etc) engine not suddenly bog down and even die whenever you let off the accelerator pedal.
Later VWs started coming out with dashpots to help do the same thing. The difference is that the throttle positioner uses engine vacuum to work whereas the dashpot is a diaphram that fills with air and has a tiny hole that makes it compress slowly.
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Luft kühl
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Joined: January 21, 2009
Posts: 1179
Location: Allentown, PA
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| Posted: Fri Nov 08, 2013 7:33 pm Post subject: |
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| Juanito84 wrote: |
Later VWs started coming out with dashpots to help do the same thing. The difference is that the throttle positioner uses engine vacuum to work whereas the dashpot is a diaphram that fills with air and has a tiny hole that makes it compress slowly. |
They don't do the same thing.
A dashpot only slows the return rate of the throttle as it closes.
The throttle positioner controls both the return rate and the position of the throttle under different load/altitude conditions. Altitude (ambient air pressure) and manifold vacuum are used to determine the position of the throttle. |
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Sun Nov 10, 2013 7:08 pm Post subject: |
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| Luft kühl wrote: |
| Juanito84 wrote: |
Later VWs started coming out with dashpots to help do the same thing. The difference is that the throttle positioner uses engine vacuum to work whereas the dashpot is a diaphram that fills with air and has a tiny hole that makes it compress slowly. |
They don't do the same thing.
A dashpot only slows the return rate of the throttle as it closes.
The throttle positioner controls both the return rate and the position of the throttle under different load/altitude conditions. Altitude (ambient air pressure) and manifold vacuum are used to determine the position of the throttle. |
Interesting. But what are the actual effects? I would guess more vacuum in the positioner itself would pull more on the throttle, right? Or is it the other way around? I know that less load would be more vacuum and more load is less vacuum. Naturally you get less vacuum at higher altittude, right? So I would guess the altittude adjuster limits vacuum at lower altittudes and allows more at higher altittudes. Right? And how does all that "posittion" the throttle? It still seems to me it would only have an effect as the throttle closes otherwise you would have a high idle or perhaps you would even loose control of your of your own throttling. I'm not mocking you, I sincerely want to know. (PS I have a 34 PICT-4 and it doesn't have the arm for the throttle positioner, only the dashpot.)
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glutamodo 
The Android
Joined: July 13, 2004
Posts: 26527
Location: Douglas, WY
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| Posted: Sun Nov 10, 2013 7:46 pm Post subject: |
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My opinion is that by the time of 34PICT-4, VW had given up on the positioner. I guess they decided that just a delay on the throttle closing was enough. I think that may be part of whey they added the radial spring on those later carbs, for more oomph against the dashpot.
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Juanito84
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Joined: March 17, 2012
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Location: Colorado Mountains
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| Posted: Sun Nov 10, 2013 7:50 pm Post subject: |
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Yep! That's my carb! I understand how the dashpots work. But still, how does the positioner actually work?
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glutamodo
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Joined: July 13, 2004
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Location: Douglas, WY
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| Posted: Sun Nov 10, 2013 8:02 pm Post subject: |
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| when you coast along in gear, foot off the gas pedal, with the clutch engaged, "engine braking" - enough vacuum is delivered through the correct port on the carburetor to cause the throttle positioner to activate and open the throttle, proventing the throttle from totally closing, allowing some air into the manifold ensuring a good burn of what fuel does trickle out of the carb at that point. It also kind of did the same thing as a dashpot, and delayed the closing of the throttle . |
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Juanito84
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Joined: March 17, 2012
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Location: Colorado Mountains
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| Posted: Sun Nov 10, 2013 8:20 pm Post subject: |
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| glutamodo wrote: |
| when you coast along in gear, foot off the gas pedal, with the clutch engaged, "engine braking" - enough vacuum is delivered through the correct port on the carburetor to cause the throttle positioner to activate and open the throttle, proventing the throttle from totally closing, allowing some air into the manifold ensuring a good burn of what fuel does trickle out of the carb at that point. It also kind of did the same thing as a dashpot, and delayed the closing of the throttle . |
Ah! Yes! That makes perfect sense! Thanks! I know carbs can lean out in that condition. On 4cycles it only affects emissions but on two cycles it can sieze them. By holding the throttle open it must be getting more fuel through the off idle discharge ports. That would keep the ratio closer to stoic.
Thanks again!
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