71 Type II 34pict3 question

[71 Type II 34pict3 question] v Top left external to the carb, there is a small can with a plunger pressing down against the throttle lever. What is that? There seems to be a worn rubber boot or bushing which lets the plunger slop off the throttle lever defeating its function. v The mystery can isn't shown in any of the 34pict pictures I've seen. Can this item be replace or repaired?

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Reply to
Grape Daddy
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Grape Daddy

It is sloppy even with the rubber intact. It's a throttle return control device, it's job is to slow down the closing speed of teh throttle when you take your foot off the pedal. WHY, I don't really know. I have taken these out and I've noticed ZERO effect. I.e. they don't seem to serve any purpose. Still, they must have bene installed (On buses) for some reason. Could it be to provide enough vacuum for the brake booster

*just* a hair longer after you have taken your foot off the gas, getting ready to hit the brakes? That's all I can think of. Then again, the brake booster vacuum signal is picked up from the manifold under the carb, not above it.

I don't know.

Jan

Reply to
Jan Andersson

Grape Daddy

Clearly shown in Fig 5-19 of your Bentley Manual.

It is a "dashpot" (not to be confused with dashboard) and was used on both Type 1 and Type 2 cars.

"A device consisting of a piston that moves within a cylinder containing oil, used to dampen and control motion."

In the Solex case, they used air rather than oil and a rubber diaphragm instead of piston.

The dashpot was added to slow the throttle closing just above idle. This made enough of a difference that the engine could meet emission requirements that year regarding NOx (I believe) when the driver suddenly takes foot off. It has no effect on performance.

Speedy Jim

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Reply to
Speedy Jim

It is there to cut down high HC, not NOx. When the throttle closes, high manifold vacuum pulls extra fuel thru the idle circuit but because the throttle is closed no oxygen is present to burn the extra fuel = high HC. The dashpot slows the engine speed down to idle speed to lower manifold vacuum and allows a little extra oxygen in just before the throttle closes.

NOx is oxides of nitrogen. NOx is formed when combustion temps exceed 2500 F. EGR is used to lower combustion temps.

CO is burned fuel.

HC is unburned fuel.

Reply to
Karl

manifold vacuum pulls extra

present to burn the extra

manifold vacuum and

EGR is used to lower

Excellent! Thank you, Karl.

Jim

Reply to
Speedy Jim

71 buses also had a throttle positioner, which was operated by vacuum from a little device on the left side of the engine compartment. I'm not sure which device he means, but the throttle positioner has a nipple for a vacuum line, while the dashpot doesn't.

I believe the Bentley manual discusses both.

The throttle positioner holds the throttle open a bit between shifts so that the throttle plate doesn't close far enough to activate the vacuum retard. If this device stops working, the engine will become very hard to get past the shifts. You can either fix it, or disconnect the vacuum retard, too.

I think I even still have a repair kit for the throttle positioner in stock.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

Thanks. I got the dashpot working by tightening up its boot with an addional plastic sleave slipped over the dashpot rod and glued inside the remaining boot base. As I found it, the thin rubber end of the boot had separated and wasn't holding the rod straight. v This engine doesn't have a throttle positioner. v I don't remember if the retard vacuum hose was properly disconnected back when its original distributer was still in place. I just remember problems with the distributer after the last rebuild.

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Reply to
Grape Daddy

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