Holley 390 carb

[kiwitony]

Hi guys, I've just installed a new out of the box 390 holley on my 59a in my 37 pickup.It runs great,but I can't get the idle speed below 800 rpm.I've checked that the choke ,fast idle screw are not touching,nothing interfering with the linkage etc,I can back the idle speed screw right out.The butterflys look completly shut.If I clamp the pcv hose the revs drop by about 100 but still not enough.I have good steady vacumn of around 20 whatsits,I can't belive it's getting enough air to rev so high.If I lean out the mixture screws the speed drops,but it then runs like a sack of shite.I don't believe I have any vacumn leaks but am open to any and all suggestions.Thanks in advance Kiwi Tony.

[JWL]

I have played with a number of Holley 4 barrel carbs and had 2 instances where the secondary throttle stop screw did not allow that set of throttle plates to completely close. I do not remember whether these were 390, 600, 800 or whatever size carbs, but it is worth checking.

[Flathead]

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Yes check the secondaries as JWL has suggested. Also (if that hasn't fixed it) be sure the base gasket isn't leaking. With the wide Holley bolt pattern and the need for an adaptor on the flathead intake, you never know. If there is still a problem, back off the idle speed screw and hold the throttle linkage closed while you tap gently on one end of the throttle shaft followed by tapping the other end, a side to side motion. This will center the throttle plates in the carb. and reduce the amount of air that can get past them. This will work on the secondaries too. Hope this helps.