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Holley 600 CFM & Vapor Canister Carb Hose

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  • Holley 600 CFM & Vapor Canister Carb Hose

    Hi all. I bought a '74 Corvette with a Holley 600 CFM a few weeks ago and have been upgrading various systems. I installed a new vapor canister and noticed the old one didn't have the hose from the canister to the carb connected. I'd like to fix that and believe the proper place to connect it is the smaller of the two vacuum inputs at the bottom of the carb on the left as seen in my photo. Can anyone verify that is the proper place for me to attach the vapor canister -> carb hose? Thanks!
    Click image for larger version

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  • #2
    Yes, the smaller fitting should work, depending on your hose size. Both are manifold vacuum. Gary
    Regards, Gary

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    • #3
      It sounds like you're missing a intake temperature check valve. And it goes to a ported vacuum line, not a full time vacuum port.
      On behalf of every manufacturer, delivery drivers, supply chain representatives, and workers of the world, "We're Sorry."

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      • #4
        Thanks for both of your replies. I read on the Corvette forum the vapor canister -> carb had to go to a ported line which was why I was double checking here and unsure if that small port underneath was the correct place to connect. I put a vacuum test gauge on both of those front ports and they appeared to be pulling the same regardless of RPM. Does this mean there are no ported lines on the 600 CFM to connect this to?

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        • #5
          The bottom two are manifold vacuum. The single fitting on the side of the primary metering block where you have your distributor vacuum advance connected to, is ported vacuum.
          Regards, Gary

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          • #6
            Got it, makes sense. Many on the Corvette forum (and other sources I'm finding) are recommending I move my distributor vacuum advance to manifold vacuum rather than ported vacuum. Besides the advantages of that, it would free up the ported vacuum connector for my vapor canister. Thoughts?
            Last edited by Dennis.1960; 09-23-2021, 01:42 PM.

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            • #7
              Yes, you could do that. But when checking your ignition timing, you must disconnect & plug the distributor vacuum line. Your idle would also be higher from more timing at idle, so your carburetor throttle plates can be backed off a bit. Smoother running engine is generally the result.
              Regards, Gary

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              • #8
                So is it accurate to say that if I move the distributor advance hose from ported to manifold, that I'll need to readjust the timing and other carb settings? Thanks.

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                • #9
                  You don't need to adjust your base timing. You should only need to trim the idle speed down. Try adding manifold vacuum to the distributor, and you'll see the engine idle pickup by 200 RPM or so. Gary
                  Last edited by Gaz64; 09-23-2021, 05:37 PM.
                  Regards, Gary

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                  • #10
                    Sounds good. I can handle adjusting the idle speed screw. Thanks!

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                    • #11
                      You're welcome. Should be a good package once you have it all hooked up. Gary
                      Regards, Gary

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                      • #12
                        @Dennis.1960: Do you still have all the factory, water intake sensors? Lots of GMs used a full time/ported, water temperature sensor. Basically the distributor would normally use ported vacuum. Then as the engine got hot in stop & go traffic. The temperature switch would switch the distributor to full time vacuum. This would raise the idle speed. So you would get faster water, and fan speed to help cool down the engine.
                        On behalf of every manufacturer, delivery drivers, supply chain representatives, and workers of the world, "We're Sorry."

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                        • #13
                          Quick follow up: Moved the vacuum advance vacuum from ported to manifold and connected the vapor canister to ported. Had to drop the idle about 200 RPM. Took a test drive and it runs great! Thanks everyone for your help!

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                          • #14
                            Originally posted by Dennis.1960 View Post
                            Moved the vacuum advance vacuum from ported to manifold and connected the vapor canister to ported. Took a test drive and it runs great!
                            Of course it runs great. Vacuum advance should be connected to full manifold vacuum. That's how an EFI timing table is programmed at idle.
                            May God's grace bless you in the Lord Jesus Christ.
                            '92 Ford Mustang GT: 385"/6.3L SBF, Dart SHP 8.2 block, forged steel crankshaft & H-beam rods, forged pistons, Trick Flow Twisted Wedge 11R 205 CNC Comp Ported heads, 12:1 compression ratio, 232-244 duration/.623" lift/114 LSA camshaft, TFS R-Series FTI Comp Ported intake, BBK 80mm throttle body, Holley Dominator MPFI & DIS, Holley 36-1 crank trigger, MSD 1x cam sync, PA PMGR starter, PA 200A 3G alternator, Optima 34/78 Red battery, 100HP progressive dry direct-port NOS, R134a A/C, Spal Dual 12" HP 3168 CFM fans, Frostbite 3-core aluminum radiator, Pypes dual 2.5" exhaust, off-road X-pipe, shorty headers, Earl's -6AN fuel system plumbing, Walbro 255 LPH in-tank pump & Pro-M -6AN hanger, S&W subframe connectors, BMR upper & lower torque box reinforcements, LenTech Strip Terminator wide-ratio AOD, 3000 RPM C6 converter, B&M Hammer shifter, Stifflers transmission crossmember & driveshaft safety loop, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

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