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Vacuum break assembly-carburetor-1968 Olds 442


Jrbrks
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Does anyone know a source for a vacuum break assembly for the 4MV carburetor on a 1968 Oldsmobile?

It has a stock (to my knowledge) 400 engine and stock carburetor.

Pictures are attached.

It seems the assembly just split apart

I pulled my car onto ramps to grease the chassis and smelled plastic burning. 

When I removed the air cleaner, I found the plastic vacuum break assembly had come apart.

 

i would appreciate any advice and/or source for a replacement part.

 

Also, it seems the car will run ok with this not functioning.  Is this correct?

 

Thanks

Jim

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Similar plastic housing as used on Corvairs and other Rochester carbs. The metal bracket changes with carburetor types. Sometimes the plastic part is available by itself, just reuse the metal bracket. 

 

Without a working choke pulloff, it will run very rich when it first starts.

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The vacuum diaphragm in question as a couple of functions.  It's primary function is to control the rate at which the secondary air valves on the Qjet open.  The rod that connects the diaphragm to the secondary air valves also has a specific bend in it that pushes on the choke linkage when the diaphram retracts, thus also helping with choke pull off.  Aftermarket vendor catalogs may call it either, as they are not particularly precise in their terminology.

 

I ASSUME your carb is a 7028251.  If so, here's one of a number of sources for the part.  FYI, the factory diaphragms used a specific orifice size in the nipple of the diaphragm housing to control the rate of secondary opening.  Most aftermarket replacement parts are generic and may not have exactly the same size orifice.  This will change the rate of secondary air valve opening and may cause a flat spot.  You might need to play with the air valve spring wrap adjustment to compensate.

 

http://www.carburetion.com/products/productdetails.aspx?part=PO-5131P

 

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A little more info.  The factory parts book calls this a "control".  Original GM part number was 7035607. This was superseded by part number 7038238. An NOS version of that is currently on ebay for the mere starting bid of $130. The aftermarket one I showed in my last post is $13.

 

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Thank you for the great advice and feedback.

 

I appreciate it.

 

It helps to call it by a proper name.  The Olds factory manual calls it a “vacuum break assembly”, the factory parts manual calls it a “control” (Thank you, Joe), and a “choke pull off” to everyone else.

 

Joe,

i purchased the following:

Standard Motor Products CPA 38

 

I do not know, if this is the proper part, as per the factory, but multiple sites listed it as a “correct” part.  I am clearly not a carburetor expert, so adjusting a “flat spot” may be beyond my current skills/experience.

 

i plugged the hose coming from the carburetor, while I am waiting for the replacement part.  While I am in south central California, where the temperature is rarely colder than 40 F, the car actually seems to start and run a little better on what we consider to be cold mornings.

 

Thanks, again, to all who weighed in with advice and help.

 

Jim

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CPA38 is one of those one-size-fits-all replacements. 

 

Once again, the primary purpose of this device is NOT to control the choke.  The primary purpose is to delay opening of the secondary throttle valves on the Qjet to avoid a bog if you snap the throttle open quickly.  Manifold vacuum holds those plates closed.  When you open the throttle, manifold vacuum goes to zero, but that orifice in the neck of the vacuum diaphragm controls how quickly the pressure against the diaphragm can return to atmospheric and thus allow the secondary throttle valves to open. One Qjet tuning trick to improve throttle response is to alter the size of that orifice to allow the secondary throttle valves to open more quickly.

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Joe

Is the CPA38 a poor choice?

 

This is a $15-$20 part plus shipping (EBay auction aside), so that is not the issue.

 

Is the Carbs Unlimited part listed above a more specific option?

 

I would like to do the repair properly.

 

Thank you, 

Jim

 

 

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I'm afraid the generic replacement part is your only choice unless you want to spend ten times as much for the NOS version on ebay. I'm willing to bet that the part sold by Carbs Unlimited is the same one, just repackaged.

 

Unless this is a 1000 point restoration and you need the RP logo on the diaphram for points, I'd go with the generic one.  As I said, you might need to adjust the air valve wrap spring a little differently than spec if this vacuum canister has a different response time. Unfortunately that will be a trial-and-error adjustment.

Edited by joe_padavano (see edit history)
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Thank you, Joe.

 

My 442 is restored and driven.  

 

I keep the 442 stock, but I realize original factory replacement parts may not always be a practical option.  I can do the adjustments that may be required, as you have suggested.

 

Thanks, again, for the help and advice.

 

Jim

 

 

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