What does a dashpot do?

[Oregon72]

Maybe a stupid question, but I don't really understand what a dashpot is for. My bus has never had one, and I found one at the local yard and figured I might need it for who knows what so I picked it. I have stock PDSIT Solex duals.

Thanks - --

[glasseye]

In general, a dashpot is much like a shock absorber. It averages out excursions of a variable from a set point.

Wikipedia:

A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring (which acts to resist displacement).

How it's supposed to work on a dual carb Volkswagen I have no friggin' idea. , even though I own one.

[dhoch14]

it was developed for emissions and is used to slow down acceleration between shifts. Ratwell has a write up on it. Colin says "no" on them.

I have one solely for the purpose of synch the carbs at high RPMs. It's great to put it in place and have it hold the throttle open so you can synch the left and right.

One of these days at the lab I'll show you....or we can use it when trying to dial in your carbs after your rebuild.

[Oregon72]

In general, a dashpot is much like a shock absorber. It averages out excursions of a variable from a set point.

Wikipedia:

A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly used in conjunction with a spring (which acts to resist displacement).

How it's supposed to work on a dual carb Volkswagen I have no friggin' idea. , even though I own one.

That's funny Glasseye - It makes total sense and is totally clear now.

[Oregon72]

it was developed for emissions and is used to slow down acceleration between shifts. Ratwell has a write up on it. Colin says "no" on them.

I have one solely for the purpose of synch the carbs at high RPMs. It's great to put it in place and have it hold the throttle open so you can synch the left and right.

One of these days at the lab I'll show you....or we can use it when trying to dial in your carbs after your rebuild.

I need to start looking up ratwell a bit more - that would be very helpful instead of the old saftey wire method. I'll use my new dashpot for the same thing then.

Yes, I would really like your help with the dialing in of the carbs once my rebuild it complete. You must have been actually paying attention on your itinerant visit.

[satchmo]

The dashpot is there to prevent the throttle plates from quickly snapping shut when you let off the accelerator suddenly. I think it is worth having for smooth shifting and driving tractability, but it isn't necessary by any means.

Tim

[Amskeptic]

Here's the lowdown. The VW's dashpot was just the last iteration of a serious effort to moderate the spike in HCs when you release the throttle. This issue was first deigned an emission-control necessity back in 1968, but anti-stall dashpots go back further into the late 50's where your big honking lazy V8s had buckets of gasoline dumped into the carburetor throats by the accelerator pumps. If you changed your mind at the railroad tracks after punching the gas pedal, and you suddenly lifted off, you could stall the engine outright and flood it.

Also, when you release the throttle suddenly with an engine that is further up the rev range, you get the following:
all the fuel inside the intake tract is "rev ready" but the air in the intake tract has been restricted because of the now-closed throttle plate, this causes an emissions spike if not the danger of an engine stall.

VW first used a precision throttle positioner with barometric compensation from 1968-1970. This was because the idle air flow volume was very little back then when you adjusted the idle speed by cracking open the throttle plate. After the air bypass carburetors were introduced with vastly more air flow and retarded timing, a simple "shock absorber" dashpot did fine.

They allow a calibrated extra shot of air into the engine as it is decelerating back to idle. Normally, just a couple of seconds is all that is required to keep the HCs lower. I only like throttle positioners and dashpots when the engine returns too quickly to idle. This botches the nice timing of the 1-2 shift. I will sometimes recommend a dashpot (or the decel valve in the FI buses) to help you time your upshifts.
Colin

[hambone]

I've noticed that the Solexes seem to maintain a higher idle soon after stopping, without the dashpot. It quickly drops to normal idle RPMs in a couple seconds.

[Amskeptic]

I've noticed that the Solexes seem to maintain a higher idle soon after stopping, without the dashpot. It quickly drops to normal idle RPMs in a couple seconds.

That can be a function of some centrifugal advance units. Some like to "hang". They serve almost as useful a function for smooth upshifts as a happy dashpot or decel valve.
Colin