vac ports on q jet
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Melting Slicks
Joined: May 2000
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From: Orlando,Fl,USA
vac ports on q jet
I had an idle problem with very rich smelling and poor vacuum reading until I pulled the vac line hose from the front drivers side of the carb. It was connected to the T off of the top of the water outlet from where the thermostat is. I had a strong vac pull from this outlet at idle. What was this vac outlet used for. I originally had my vac advance connected to it but later found the correct one on the pass side front.
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From: St. Charles IL
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Re: vac ports on q jet (dogboy)
Drivers side front port to vacuum advance. Passenger side vacuum port to EGR temperature switch on water neck. (at least on my 75) :D
Thread Starter
Melting Slicks
Joined: May 2000
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From: Orlando,Fl,USA
Re: vac ports on q jet (Big Fish)
That was the original set up. I was getting constant vac all the time even at the lowest idle speed . I needed a timed vac port that would kick in at about 1,000 rpm and increase with speed so I could set the timing correctly. When I did the cam swap I discovered that my advance was rust seized at 10 degrees advance. I removed the dist. and cleaned up the surfaces;now it works as it should. That is when I discovered the problem with the vac port on the carb. I switched it around and it worked well for the distributor but my idle suffered and it ran rich. Now that I capped it off it idles fine.
Do I even need that hook up .I have an egr setup but I'm not concerned with emmisions inspections.
Do I even need that hook up .I have an egr setup but I'm not concerned with emmisions inspections.
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From: St. Charles IL
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Re: vac ports on q jet (dogboy)
The original setup described above has the advance going to the ported vacuum connection, so there should be no vacuum at that port until you are off idle. You may want to curve your distributor and change the vacuum servo to allow for better performance. Lars has a great article on this in the http://www.corvettefaq.com site, a sister site to this one. Essentially you want to adjust to get full advance (@32 degrees) at 2500 rpm. The distributors leave the factory adjusted more for emissions than performance. There may be something out of focus with your carb if you get vacuum from the forward driver's side port at idle.
[Modified by Big Fish, 4:47 PM 12/5/2002]
[Modified by Big Fish, 4:47 PM 12/5/2002]
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From: NorCal
Re: vac ports on q jet (dogboy)
GM, Ford and many others tried many things to inprove emmisions and mileage in the early '70's, not all condusive to performance. Hook it up ported vacuum to the distributor, plug the EGR, set the timing up a couple degrees from stock and performance WILL return. Your mileage may vary, but it works for most folks. Better breathing (Aftermarket air cleaner - I like the Moroso's) helps as well, but may not fit under the hood...
Hans
Hans
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Joined: Aug 1999
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From: Ontario
Re: vac ports on q jet (dogboy)
There are many variations on the emissions set-up. Here is a start.
http://temp.corvetteforum.net/c3/pau...ssions_001.jpg
http://temp.corvetteforum.net/c3/pau...ssions_001.jpg
Thread Starter
Melting Slicks
Joined: May 2000
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From: Orlando,Fl,USA
Re: vac ports on q jet (paul79)
Paul 79
Excellent diagrams, Much appreciated. I didn,t realize how many set ups there were. I'm plugging off the other vac ports as it seems to run 100X better.
Excellent diagrams, Much appreciated. I didn,t realize how many set ups there were. I'm plugging off the other vac ports as it seems to run 100X better.
