Barry Grant even offers carbs with holes already in the baseplate, eliminating the need to drill. Start at 1/16 inch, then drill progressively larger holes until the desired idle speed is reached. To compensate, the secondaries can be cracked open slightly or two small holes can be drilled on the primary throttle blades. McFarland advises that no more than 0.025 to 0.030 inch of the transfer slot should be exposed past the throttle plate at idle. This exposes more of the transfer slot, resulting in a rich condition. Design a fuel system with minimum pressure and maximum volume." Things get a bit trickier with a big cam, as low vacuum requires opening up the throttle blades to maintain a steady idle. The most pressure you want to run is 7 psi, and 10 psi is excessive. "Carbs love volume, but hate excessive pressure. "The biggest problem we encounter in our shop is cars with excessive fuel pressure," explains Murphy. Solution: When a motor runs rich at idle even with the right size power valve and the adjustment screws turned all the way in, it may be time to bust out the fuel pressure gauge. Problem: My Idle Circuit Must Be Messed Up, Because The Idle Is Way Too Rich.
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