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#1
05-01-2021, 05:54 PM
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1971 Quadrajet vacuum issue
We just started the fresh 400 in my LeMans yesterday and after a few minor issues it runs VERY nice.
My issue is with the carb, it seems to have a massive vacuum leak at the rear top of the carb. On initial starts the leak did not seem to be there but after some adjustments and a restart vacuum was being drawn thru a tube behind the secondarys on the top of the carb. It sticks up about a half inch or so. I stuck my thumb on it and the engine immediately ran smooth. I am not a guru on Q jets so I am not sure what happened, so for now i stuck a plug in it and sealed it off so we can make sure the car runs good. At the moment the vacuum for the brake booster is also sealed off. The vacuum advance is not sealed off and is working fine. Can someone educate me on what this is and possibly what happened? Is there a permeant cure for this leak? Other than this the motor is great. S cam in a 1971 400, 18 inches of vacuum, 65 lbs of oil pressure. Long time coming! Thanks everyone, Mark |
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#2
05-01-2021, 07:44 PM
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That's part of the emissions system.
Probably due to the 'restart' letting the heat build up more than normal at the rear of the carb.
There's a bimetal 'valve' inside the rectangular recess on the back of the main body that is designed to open when the engine gets too hot in order to lean out the mixture. Pontiac called it a 'hot idle compensator'. It's only supposed to open when the engine is a little too hot (like after you turn it off while breaking it in and it's HOT), and leans out the mixture so the engine can clear the fumes from the gas in the fuel bowl boiling. Then, when the engine gets back to normal operating temp, it's supposed to close. If you don't want it to work, you can cap the tube that sticks up, or plug it. Pontiac used to note in their service manuals to cap it or plug it when adjusting the idle mixture. We've disabled a few by filling the recess on the back of the carb with epoxy while rebuilding. Here is a little explanation: https://www.gmcmhphotos.com/photos/3...r-casting.html Last edited by Joe's Garage; 05-01-2021 at 07:58 PM. |
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#3
05-02-2021, 02:04 PM
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I believe we will be plugging it up. But before we do, further test and tune will be going on, maybe it will not be a problem. Only time will tell.
Thanks again, Mark |
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