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Old 08-15-2011, 04:01 AM
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Converting a ported vacuum source to a manifold source

I am planning on converting a ported vacuum source to a manifold vacuum source on a Quickfuel SS750 mechanical secondaries carb. I have never done this kind of carb work and I want to do this so I don't have to have any ugly plastic vacuum "T's" on the engine. I plan to tap and plug the ported source within the throttle body and then either connect the milled channel with the existing manifold source or create a new channel beside the existing manifold source.

What type of sealant should I use on the set screw to lock/seal it in place within the ported passage within the throttle body? Is a small set screw the right method or do they make really small pipe taps and plugs I don't know about?

Should I tap and plug this port from the bottom or the top of the throttle body. Not sure which is best, I am mostly concerned about the screw ever coming loose if I tap the hole from the bottom but is this really an issue?

Does it matter if I connect the existing channels or create independent ones?

In the pic below the carb has been changed but the vacuum line to the distributor is basically run the same way except now there will be two lines with this modification. I will be getting a couple more brackets with 2 more holes to run the second line below the existing one.
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Last edited by Scrumgees; 08-15-2011 at 04:09 AM.
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Old 08-16-2011, 10:31 AM
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Looking at your photos, the full manifold source is the port to the left in the picture where the channel runs directly to the venturi opening. Connect your vacuum advance hose from the vacuum advance canister to that port. The port to the right of that one has the channel going up into the carb body. That port is for ported vacuum source. You can plug it off easily with a rubber cap (readily available from almost any auto parts store) .

No sealant or tapping involved. Very simple and quick changeover. Easily switched back at any time.

NOTE: Without changing anything else (timing or carb adjustment), when you hook up the vacuum advance hose from PORTED to a FULL manifold source, the idle speed will increase. Reduce the idle speed down to below 850 RPM (650-750 preferred) by adjust the throttle stop screw. The trans should be in DRIVE and the wheels blocked and the parking brake applied when performing this adjustment. DO NOT rev the engine up with the trans in gear. The idle will increase when the trans is placed in PARK after the above adjustment. Usually there is about a 250 RPM drop in idle when shifting from PARK to DRIVE with the brakes fully applied.

I modified your photo to show which port is which. See below. Left click for a larger view. They appear to have the rubber caps on them in your photo. Just remove the one from the port you wish to use and attach the vacuum hose to that port.
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Last edited by Frisco; 08-16-2011 at 10:43 AM.
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Old 08-17-2011, 02:09 AM
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Hey thanks for the reply. I totally understand what you are saying. My original post wasn't that clear now that I am reading it a day later, and the truck actually runs beautifully with all my accessories hooked up to full manifold vacuum. My query was regarding converting the ported vacuum source into an additional full manifold vacuum port.

My old Edelbrock carb had enough full manifold ports to run the PCV, Brake booster, Distributor Vacuum advance, and the tranny modulator valve on full manifold vacuum without having to "T" any vacuum lines. Just looking to see if there are any do's and dont's about making this little mod. On the new Quickfuel carb I am short one full manifold port and I am uninterested in using a cheap plastic "T", or numerous AN fittings to make it all match, or a vacuum log to solve the problem.

I'll post a picture when i'm done next week some time, both of the carb, and of the new vacuum line routing on the motor.

I have never tried to set the idle the way you described while it was in gear, but that doesn't sound like a bad idea at all, provided I do it safely like you described.

Last edited by Scrumgees; 08-17-2011 at 02:16 AM.
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