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Old 08-21-2020, 02:37 AM   #10
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Default Re: '40 Ford carb fine tuning help needed

Originally Posted by GB SISSON View Post
I'm sorry, but do the twin idle screws enrichen the mixture when turned clockwise ? I have twisted these two screws for 25 years to get that sweet spot, but still don't get if they control fuel or air.

The idle screws are needle valves. They open counter clockwise on the idle fuel circuit, directly behind the venturi, which sucks the fuel from that circuit. The further they are opened, the faster the idle within the fuel/air optimal ratio, during and after which point the idle speed adjusting screw is adjusted outward, lowering the idle back down by restricting the air intake. This procedure is done individually on the two idle screw valves, repeating until the highest steady vacuum is achieved, all of which is done on a fully warmed up engine.

All that said, I've had experts improve on my adjustments just by ear, not bothering with a vacuum gauge at all.
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