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Rochester HV duckbill vent.
Posted by: Wagon Master ()
Date: May 07, 2022 02:58PM

Putting a 140 together for our 69 vert. Since they're no longer functional, I was considering removing the duckbill vents in the secondaries. Who doesn't want a little more flow? Then I thought ???? Has anyone ever removed the duckbill vents from the primaries along with the choke pull-offs? Just letting the now vacant slot in the choke blade be your 3/16" gap you usually set the pull-offs to.
It'll be a bit, but I'll let you know how it works out.


Been aircooled since 1973
Northwest Ohio 45840

Re: Rochester HV duckbill vent.
Posted by: Phil Dally ()
Date: May 07, 2022 03:07PM

We took all that stuff out of the 500 Holly on the racecar.

Attachments:
Re: Rochester HV duckbill vent.
Posted by: REM654DPG ()
Date: May 07, 2022 03:18PM

To answer your question Wagonmaster here's and old post with some good conversation on the subject.

[corvaircenter.com]

Regan Metcalf
Portland, OR

Re: Rochester HV duckbill vent.
Posted by: Wagon Master ()
Date: May 07, 2022 03:24PM

That's great guy's....but I'm not considering the going chokeless. Just duckbill vent and choke pull-off less.

Re: Rochester HV duckbill vent.
Posted by: Frank DuVal ()
Date: May 07, 2022 07:52PM

Why remove the choke pull off? It opens the choke on start up to lean the mixture. When it doesn't work, the mixture will be waaay rich until the engine warms up the choke coils.

Frank DuVal

Fredericksburg, VA

Re: Rochester HV duckbill vent.
Posted by: Wagon Master ()
Date: May 07, 2022 08:29PM

Frank DuVal Wrote:
-------------------------------------------------------
> Why remove the choke pull off? It opens the choke on start up to lean the mixture. When it doesn't work, the mixture will be waaay rich until the engine warms up the choke coils.

Because as you state, they sometimes fail. You will be left with the open slot in the choke blade where the vent used to be.

Been aircooled since 1973
Northwest Ohio 45840

Re: Rochester HV duckbill vent.
Posted by: vairmech ()
Date: May 08, 2022 05:13AM

Well, think about the way the system works and what it does then what the engine needs!

Yes, the choke closes off the opening of the carb but what does it do? First off the engine when cold needs a really rich mixture to fire off but it won't run well this way. With the plate closed the fuel is literally sucked out of the venturi when you crank. The choke pulloff is also starting to open the plate the required amount even as you crank. (Get a remote starter and watch it sometime)
The accelerator pump helps this situation and you can even start the car without the chokes using only the accelerator pumps but the you must also keep tickling the throttle to keep the engine running.
So the engine starts and the choke pulloff is now cracked open have you ever looked down at the choke when it is that way then reved the engine some? The choke plate actually will open slightly more because of the airflow but keep in mind that just because the engine is running it is still cold and needs a rich mixture.
What can happen if you don't have that rich mixture with a cold engine? Have any of you ever played with the old outboard engines and the idle mixture? What happens when you lean it out to much? It sort of sneezes through the intake system, a backfire through the intake system.
Do I need to keep going? Now that you know sort of what happens experiment first with your choke settings before you actually do changes just to see what will happen.

BTW, just so you know, the duckbill does disrupt the flow but that is all it does. Removing it will not increase the air flow, just straighten it out. The biggest restriction in the intake system is the venturi, why do you think the racers make it bigger?

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

Re: Rochester HV duckbill vent.
Posted by: chris ()
Date: May 08, 2022 06:44AM

Not sure how relevant it is, but Mike Dawson in our club removes the duck bill from the carbs. To cover the slot on the choke plate, he just uses a sacrificial piece of an extra choke plate (he's got a few!). My carbs have them and I still use the regular pull-offs.

'66 Monza coupe
4 speed
140hp
Gardner, KS

Heart of America Corvair Owners Association

Re: Rochester HV duckbill vent.
Posted by: dryenko ()
Date: May 08, 2022 03:52PM

The area of the choke plate at the top of the carb is so much bigger than the stock venturi that little if any benefit is gained from removing it.
The later carbs have a off set leg on the venturi arms to compensate for it's presence regarding distortion of the airflow cause by it and the "duckbill" at high stock carbs air and fuel flows.
Thus providing equal mixture strength to all cylinders.
Yes we racers remove the choke plate and the duckbill to gain any advantage we can on Corvair racing engines using more than twice the airflow for the 200 + plus HP we develop.
In addition we enlarge the venturi to flow almost twice the airflow at the same pressure drop.
And jets in the #70 range
Finally, and of note , modified Corvair HV racing carbs use a venturi cluster with equally spaced extended arms to deliver a homogeneous mixture to the intake ports.
As developed by Yenko Sports cars and Rochester for the Yenko D Prod race cars for Don Yenko.
They really did know what they were doing...

Bob C aka Dryenko
Dobson, NC 27017

Re: Rochester HV duckbill vent.
Posted by: Early Guy ()
Date: May 11, 2022 07:14PM

There is a guy east coast area somewhere that makes/sells a nice stainless choke plate without the duckbill opening. I have removed the duckbill from several primary carbs using the solid choke plate. I don't have any way to test for any improvement but it sure looks a lot cooler.


Ray Johnson Villa Park IL 63 Spyder Conv. 140 EFI

Re: Rochester HV duckbill vent.
Posted by: REM654DPG ()
Date: May 12, 2022 09:28AM

I have run the chokes (for other people's cars) with the duckbill removed and it does not seem to make any difference with the open slot in the plate. Everything worked as normal so there really is no need to fill that slot.

Regan Metcalf
Portland, OR

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