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 Qjet venturi vacuum port modification 
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Joined: Sun Mar 17, 2013 9:17 pm
Posts: 54
This 1980 Q-jet does not have a venturi vacuum port, all ports are full manifold vacuum. For a specific application it would be good to have a venturi vacuum port for the vacuum advance.

Is there a mod that can give a venturi vacuum port?


Fri Dec 18, 2015 8:49 pm
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
Baseplate with timed spark:
Image

Image

Baseplate without timed spark:
Image

Image

The castings should be the same, you just need to drill the holes and press in a tube for the vacuum hose.

If you don't want to do that, another alternative is to watch ebay for a cheap E4ME. Watch long enough and you can find one for around $25-30 shipped. Their baseplates fit all the other 1708- carbs and have the right fittings.

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Sat Dec 19, 2015 12:58 am
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Joined: Sun Mar 17, 2013 9:17 pm
Posts: 54
Thank You, I can do that. I appreciate the photos.

Second question;

With this carb it requires the throttle plates to be completely closed in order to have a slow idle. The throttle plates both secondary and primary are well centered and fully closed without out binding. The linkage does not interfere with the throttle closing. No external vacuum leaks, have even removed/plugged all vacuum port connections including PCV and brake booster.

The engine idles smoothly and slow enough with the idle set screw backed out and not touching. This is a completely stock 350 SBC in good condition.

The Q-jet is bypassing the throttle plates internally with some idle air. Where is it bypassing and can I plug/restrict the internal bypass?


Sat Dec 19, 2015 4:51 am
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
Path for the bypass air goes like this...

Inlet thru the two notches from the primary bores:
Image

Into the bowl casting:
Image

Exit the bowl casting:
Image

Enter the baseplate:
Image

Exit:
Image

The only easy place to resize it is the top surface of the baseplate, everywhere else is either too odd-shaped or too thin to hold a restrictor in place. Baseplate holes are too big for #10-32 but there's enough space for a 1/4-20 brass setscrew (McMaster #92991A533). Or, plug the holes with lead shot, then drill to size. Enlarge the upper ~1/4" of the hole by 1 or 2 drill sizes to create a little pocket, so the lead shot will be trapped between the step at the bottom & gasket at the top.

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Sat Dec 19, 2015 5:47 am
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Joined: Sun Mar 17, 2013 9:17 pm
Posts: 54
Thanks again, I can do that. If I completely plug the idle bypass holes will the primary throttle plates allow enough idle air without excessive opening?

Or should I drill the plugs to allow some idle air? If so, what size?

Again, Thank You, I appreciate the help.


Sat Dec 19, 2015 9:09 am
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
Completely blocked will probably take away too much, but it's easy to try it that way first and drill larger if it needs it.

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Sat Dec 19, 2015 9:29 am
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