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Hello. Could anyone suggest where to buy a vacuum choke pulloff that is the proper calibration and fit for a 69 sb. Are the different vendors all the same? Thanks in advance.
From: At my Bar drinking and wrenching in Lafayette Colorado
Prototype test pulloff has been successfully fabricated, and is being used to create the "blueprint" for functional choke pulloffs for the '68-'69 Vette carbs.
Spring retaining end being cut off:
Spring and spring perch removed:
Difference between the springs being installed in the aftermarket pulloffs versus the stock, Rochester spring (right). You can see why the aftermarket pulloffs simply don't work:
Pulloff retrofitted with correct Rochester spring - fully retracted at 5 inches of vacuum instead of 15"
Fully extended at 1-1/2" of vacuum. Modified pulloff operates correctly as it should:
More testing now required to verify durability and reliability under operating temperature conditions.
From: At my Bar drinking and wrenching in Lafayette Colorado
There aren't any new '69 Q-Jet choke pulloffs being sold that meet spec - they are all pretty much crap. They have 2 major problems: They do not have the same "stroke" as the original pulloff, and they do not have the correct bleed-off rate calibration.
The stock pulloffs had a retraction stroke that was 0.39" long. All of the new pulloffs being sold have a stroke that is only 0.32" long. This means that the pulloff will not correctly crack the choke open upon cold-start unless you bend the heck out of the choke linkage tang that contacts the secondary airvalve rod.
Additionally, none of the pulloffs I have tested (and I've tested a lot of them) have the correct "bleed-off" rate. The choke pulloff, in order to provide a controlled opening rate of the secondary airvalves, cannot "instantly" extend upon loss of engine vacuum. The stock pulloffs provided full extension from the retracted position in about 1-1/2 seconds. All the aftermarket pulloffs have a diaphragm spring that is way too stiff, and the air bleed orifice in the vacuum nipple is way too big, resulting in the pulloff having "instant" extension upon loss of manifold vacuum. This will produce a bog going into the secondaries. You can work with this by machining a small brass restrictor to screw into the vacuum nipple. The hole in the restrictor needs to be about 0.016" diameter, but the stiff spring still causes probelms with the pulloff's ability to properly control the secondary airvalves: The aftermarket pulloffs require 15 inches of vacuum to be fully retracted, and start to extend at about 14 inches. They are fully extended at 4 inches of vacuum. The stock, original pulloffs only require about 5 inches of vacuum to stay fully retracted, and do not start to extend until manifold vacuum drops to numerically less than 4 inches. They are fully extended at 1 inch of vacuum. Many engines with performance cams will idle at 10-13 inches, which means the aftermarket pulloffs can't retract and are completely non-functional. Several of us well-known Q-Jet builders have complained about these problems to the manufacturers, but I'm afraid it's just falling on deaf ears - they seem to think we're the only ones having problems. The last batch I received in had the same stiff spring and big bleed-off, so they've done nothing yet, and they say it will "be a long time" before they can address it. Cliff and I used to change the springs out to make the units "right," but they have now started bonding the two halves together, which causes the housing to break when you try to pry them apart. I'm looking at doing some type of cutting operation to get the spring out to replace it, but no luck on a good technique yet.
From: At my Bar drinking and wrenching in Lafayette Colorado
Work and testing is progressing on creating a correct spec '68-'69 choke pulloff. Parts are on the machines right now being fabricated, and two configurations will be tested to create pulloffs that will be the only '68-69 parts that actually operate correctly. Stay tuned for updates and test results...
From: At my Bar drinking and wrenching in Lafayette Colorado
Prototype test pulloff has been successfully fabricated, and is being used to create the "blueprint" for functional choke pulloffs for the '68-'69 Vette carbs.
Spring retaining end being cut off:
Spring and spring perch removed:
Difference between the springs being installed in the aftermarket pulloffs versus the stock, Rochester spring (right). You can see why the aftermarket pulloffs simply don't work:
Pulloff retrofitted with correct Rochester spring - fully retracted at 5 inches of vacuum instead of 15"
Fully extended at 1-1/2" of vacuum. Modified pulloff operates correctly as it should:
More testing now required to verify durability and reliability under operating temperature conditions.
From: At my Bar drinking and wrenching in Lafayette Colorado
The development and testing of the '67-69 Q-Jet Choke Pulloffs has been successfully completed with excellent results. I now have in-spec pulloffs which retract with the right amount of vacuum, extend as they should with the correct "bleed rate," and with the correct stroke. To the best of my knowledge, these are the only 67-69 pulloffs available from any source with the correct performance specs. Anyone interested can contact me directly via e-mail - this is a unique product being offered just here on the CorvetteForum and which has been created based on the interest of you Forum members.
Only give-away that the unit is not "stock" is the custom calibrated brass restrictor insert that can be seen inside the vacuum nipple, and the round plastic bonding doubler disc on the end of the unit, which holds the entire modification together:
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