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Jul 21st, 2025 | | | | Joined: Apr 2010 Posts: 126 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2010 Posts: 126 | i rebuilt my motor (63' 327) and i was wondering what a good starting point is for the idle adjust screws. i have a rochester 4jet (not a quadra jet) the only changes i made from stock are a little bigger cam, roller rockers, and some custom milling on a 283 intake manifold that came with the truck. nothing major- just gave the dual rise more common plenum area to help the a/f mix. other than that is stock specs.
66' Chevy C-30 Rat
| | | | | Joined: Jan 2008 Posts: 4,903 'Bolter | | 'Bolter Joined: Jan 2008 Posts: 4,903 | It is probably a Rochester 4GC. Try screwing the screws all the way in and then about a round and a half out and start from there. | | | | | Joined: Nov 2005 Posts: 6,061 'Bolter | | 'Bolter Joined: Nov 2005 Posts: 6,061 | Hy BrianToste, try 1 1/2 turns out as a baseline, you can tune it much finer while it's running. Do you have a vacuum gauge?
Last edited by 3B; 08/03/2010 5:40 AM.
| | | | | Joined: Dec 2008 Posts: 1,915 'Bolter | | 'Bolter Joined: Dec 2008 Posts: 1,915 | a little bigger cam
What duration @ .050"?
You cut down the divider wall? How much? That reduces idle vacuum somewhat, which may require richer mixture (and so may the cam).
How many turns out gives the fastest idle? | | | | | Joined: Nov 2002 Posts: 2,715 Carburetion specialist | | Carburetion specialist Joined: Nov 2002 Posts: 2,715 | a little bigger cam
What duration @ .050"?
You cut down the divider wall? How much? That reduces idle vacuum somewhat, which may require richer mixture (and so may the cam). Totally agree. Also, if you live in an area where you have to burn ethanol, you may need to increase the diameter of the idle jets (as well as the main jets). Jon.
Good carburetion is fuelish hot airThe most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes!The Carburetor Shop | | | | | Joined: Apr 2010 Posts: 126 Wrench Fetcher | | Wrench Fetcher Joined: Apr 2010 Posts: 126 | i do have a vac. gauge. the cam duration is 224 int. 234 exh. i took out most of the d1vider wall i havent tried to do any tuning on it yet- i just got it fired up over the weekend. havent timed it or anything. i was just looking for a good starting point with the idle screws
66' Chevy C-30 Rat
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