carb problems

[Chas Henderson #28127]

On my 71 small block,I have a Q-jet carb, part no. 7041212. The vacuum pot on the passengers side that works the secondaries wont hold vacuum. So the secondaries wont work and when you step on it, it makes a lot of noise but has no umph. I have it off the carb and I am getting ready to try and find a new one but the books I have looked at do not have a part number listed, and I need to know if there is any type of a replacement that will fit properly sence it has a shaft coming out of the rear that hooks up to the rod. Also, would this affect the over all operation of the carb?

Chasman

[Clem Z.]

CPA 75 is the hygrade number for the pull off

this controls the speed at which the secondary air valve opens

[Mike M.]

Re: carb problems

As Clem states, the vacuum pot you are refering to is called the "choke pull off diaphram" and is readily available at most auto parts stores.
The choke pull off serves two purposes, first is to "pull" the choke open to a "running" position after the engine starts. If the pull off function is not working and the choke thermostat is properly set up, the car will run very rough and rich for the first few minutes of operation. The choke thermostat closes the choke plate completely when the engine is cold and you release the choke by stepping on the accelerator pedal. The complete closure assures the engine is getting a very rich mixture to aid in cold starting. However this mixture is much to rich for a running engine, even a cold one and the choke pull off opens the choke plate to where the engine will run smoothly (proper mixture). After a few minutes, the choke thermostat opens enough to begin to open the choke plate and it completes the opening operation.
The second purpose of the choke pull off is to act as a damper on the secondary air valves to prevent them from opening prematurely and creating a bog. They only allow the air valve to open when the intake is sensing a low vacuum condition (WOT). They do not open the secondary air valves, but keep them from opening until the engine is ready for them to open. If you are hearing a lot of noise but with no umph, the secondaries are opening too soon. The system works the opposite of what you indicate in your posting.
Mike

[Jack H.]

Re: carb problems

There were two sources for QJets then, Rochester and Carter. I can't swear this is true but it seems the Carter built QJets have a pull-off assy that's half plastic/half metal while the Rochester units came with an all-plastic pull off assy. The hybird (metal/plastic) pull off assys are the ones I've seen prone to leaking vac as time goes by.

Plus, the leak is typically at the crimped edge line where metal meets plastic. As, others have said, fresh replacement pull-off assy's are off-the-shelf items, but I haven't seen a replacement that's like the factory original half plastic/half metal ones...

Last, I've seen these sucessfully repaired pretty easily by mixing epoxy and flowing it around the crimped metal edge to 'plug' the leak...

[Chas Henderson #28127]

Re: carb problems

Mine is all plastic. Its good to know that they are still a common item. I will try a few stores tomorrow with part in hand. As to my car, when I start it up, it fires right up and the usually dies. On restart, it will run and keep on running and then drive just fine. I guess I have been driving on only two barrels. But if the part I need is leaking - not holding any vacuum, then wouldn't that effect the total operation of the engine? If so, does it affect my tune up operations?
Thanks for the responces.

Chasman