Airbox flapper hookup

[Tommy]

Is the airbox flapper vacuum hose hooked up to the carb intake, or to the upper right throttle body of the front carb? On my vision it is currently connected to the latter, but I cannot feel a vacuum with the hose disconnected, and opening the throttle.

Thanks, Thomas

[QBS]

The carb top provides the vacume.  To test for correct flapper vacume motor operation, suck on its' vacume hose.  The flapper should be actuated.

[Tommy]

suck test worked fine - thx for confirming the hookup position.

[Night Vision]

with the seat up and the back of the tank propped up slightly, you should be able to slip your fingers in there and rev the gas to make sure it is working.... unless you're hamfisted

[Oz Vision]

Perhaps someone can clarify;
I now have two Visions and each has a different flapper set up. The Euro model has the flapper alone with no other fittings, while the oz model has the hose and rod set up that Thomas refers to. Did the rod and hose set up come later as a cure for the " vision stumble"?

[Lucky]

the original flapper had no vacuume dashpot.  that was an 82 midyear upgrade

[Rikugun]

When I was learning about the carbs and flapper I stumbled upon something unexpected. I'm sure the elders of the group are well aware but possibly some newcomers might find interesting.

I checked the vacuum powered flapper while running on a remote tank. The door could easily be seen opening when the engine was revved. Another time I had the airbox lid off and set aside but still connected by the vacuum hose. I was running the motor and observing the spray pattern of the accel pump nozzles while revving the engine. Quite a bit different compared to not running btw, but I digress...  Anywho, I noticed the flapper door was not opening. I figured the dashpot had crapped out and removed the hose to confirm there was vacuum present. I couldn't feel anything. Strange! I replaced the hose, replaced the lid and now it worked again.  ???

Although called a vacuum operated door, it doesn't operate on manifold pressure as would be realized south of the throttle plates. It works on a slight pressure you won't feel pulling against your finger at the hose end. However slight, when multiplied by the the diaphragm's area,  sufficient force is generated to move the linkage and door.

Removing the airbox lid upset the dynamic of air moving through the carb's horn where the dashpot line connects. Replacing the lid reestablished the flow characteristics past the orfice and the venturi effect resumed. I guess.  At least that's the best I could come up with that seemed reasonably satisfying such that I could move on to the spray pattern issue.  :)  :laugh:

[Tommy]

Rikugun, I noticed the same, and that is why I initially posted this topic. Thanks for the explanation!!!! Thomas

[QBS]

Rikugun, thank you for very much for the great explanation.  Definatley something I had rudimentary knowledge before.  Tommy thanks for asking the question.