QJet Experts: Need pictures or help with idle vent valve - Team Camaro Tech
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post #1 of 6 (permalink) Old Sep 2nd, 14, 07:55 AM Thread Starter
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QJet Experts: Need pictures or help with idle vent valve

I am almost done rebuilding the original Quadrajet for my L35 396 car, and I am confused about the fuel bowl vent assembly. This carb has a little rubber flapper that is attached to a small metal strip. There is also another metal piece that hooks into a small metal rod that wraps around the accelerator pump lever. Fromm what I have read, the rod is supposed to be actuated by the pump lever to open it in the idle position and allow it to close otherwise. To add to the confusion, I read that some models have a thermostatic bi-metal strip that is supposed to open the valve at temperatures above 75*, and the rod is supposed to close the valve for off-idle operation.

I'm pretty sure the small metal strip that the rubber seal is attached to is the aforementioned bi-metal strip. I think I understand how it is supposed to be installed, but I am not sure about adjustment and operation of the other metal lever and actuating rod that attaches to the accelerator pump lever. Can anyone shed some light on this? Some pictures of the vent correctly installed on a Q-jet would be very helpful. This part is usually under a small square metal "doghouse" on top of the carb's fuel bowl.

I'll try to get some pictures of my carb and the parts in question later today when I get home from work.

Thanks for any help!

1967 RS/SS 350 Conv - triple black, 4sp, tach & gauges

1969 VN-built RS/SS 396 Conv - Garnet Red/Black, 4-sp, tach & gauges
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post #2 of 6 (permalink) Old Sep 2nd, 14, 11:27 AM
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Re: QJet Experts: Need pictures or help with idle vent valve

In the meantime there used to be a great rebuilding video on YouTube - it was for a marine version but the video was very well done and did a full rebuild. Several of the ones out there are just poorly executed and only refresh. The one I'm taking about did full breakdown and soak then full rebuild using a quality kit.


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post #3 of 6 (permalink) Old Sep 2nd, 14, 11:38 AM Thread Starter
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Re: QJet Experts: Need pictures or help with idle vent valve

Thanks Brian - I might look for the video. The marine Q-jets do not use the idle vent - they are sealed up because you can't have gas fumes venting into a closed engine compartment.

I have rebuilt dozens of Q-jets over the years, so I am pretty familiar with them for the most part. However, this one has been sitting apart for so long that I can't remember specifically how this thermostatic idle vent system is supposed to operate. I have a general idea, but want to make sure I'm putting it back together correctly.

1967 RS/SS 350 Conv - triple black, 4sp, tach & gauges

1969 VN-built RS/SS 396 Conv - Garnet Red/Black, 4-sp, tach & gauges
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post #4 of 6 (permalink) Old Sep 2nd, 14, 02:04 PM
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Re: QJet Experts: Need pictures or help with idle vent valve

Quote:
Originally Posted by RSSSfanatic View Post
.....To add to the confusion, I read that some models have a thermostatic bi-metal strip that is supposed to open the valve at temperatures above 75*, and the rod is supposed to close the valve for off-idle operation.
The bi-metal strip on my 67 327/275 was on the back (rear) side of the carb, facing the distributor. It was covered by a stamped metal piece that screwed to the rear of the carb. Overall, the stamped piece was a rectangle and was mounted long side up and down. You can see where I'm describing in the picture below. I believe the wire rod you are describing is used on the bowl vent on the top front of the carb and has nothing to do with the temperature sensitive vent.

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Last edited by dyno jonn; Oct 13th, 14 at 02:31 PM.
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post #5 of 6 (permalink) Old Sep 2nd, 14, 02:18 PM Thread Starter
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Re: QJet Experts: Need pictures or help with idle vent valve

Quote:
Originally Posted by dyno jonn View Post
The bi-metal strip on my 67 327/275 was on the back (rear) side of the carb, facing the distributor. It was covered by a stamped metal piece that screwed to the rear of the carb. Overall, the stamped piece was a rectangle and was mounted long side up and down. You can see where I'm describing in the picture below. I believe the wire rod you are describing is used on the bowl vent on the top front of the carb and has nothing to do with the temperature sensitive vent.
Hi Jon,

I think the earlier Q-jets like you have use a different venting system. Mine is mounted on top. Here's a quote from an article that describes the operation of the later vents:
"A temperature controlled idle vent valve is used on some models (Figure 3). In place of the standard vent valve; a heat sensitive bi-metal strip is used as the valve holder. This is mounted beneath the idle vent valve arm.
The bi-metal strip holds the vent valve on its seat (closed) at temperatures below 75. When underhood temperatures are above 75 to 85 the bi-metal strip bends upward moving the vent valve off its seat. This lets fuel vapors, caused during hot engine operation, escape from the float chamber. This results in improved hot engine idle and hot starting. At temperatures below 75, the vent valve remains closed and retains fuel vapors internally to supply extra fuel for good cold engine starting.
During hot engine operation, when the thermostatic vent valve is open, it is necessary to close the valve except at idle to maintain an internally balanced carburetor. This is accomplished through the spring steel vent valve arm which operates off the wire lever on the end of the pump lever. As the throttle valves are opened from the idle position, the vent arm exerts pressure on the bi-metal strip and forces the valve closed. The thermostatic vent valve is adjustable to make sure it closes at the proper time during throttle valve opening from the idle position."


The part I'm grappling with is how the wire lever is supposed to actuate the vent arm to force the valve closed when not at idle. I've assembled it a couple of different ways, and cannot see how this would work.

1967 RS/SS 350 Conv - triple black, 4sp, tach & gauges

1969 VN-built RS/SS 396 Conv - Garnet Red/Black, 4-sp, tach & gauges
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post #6 of 6 (permalink) Old Sep 2nd, 14, 05:46 PM
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Re: QJet Experts: Need pictures or help with idle vent valve

Here is a 66 carb with the "doghouse" removed. Look at the top down picture.

http://www.ebay.com/itm/GM-Rochester...bde066&vxp=mtr

That is how they are supposed to be assembled, but the temperature compensating valve is still in the rear. The front vent is not temperature compensating.

Jon N

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