Q. I have a Chevy 350 engine with a Holley 650 double pumper and a COMP Cams Xtreme Energy camshaft in my street rod. I’m unable to get the idle fuel mixture lean enough. The idle mixture screws are turned all the way in, and the engine still idles a little rich. Can you give me some pointers on idle tuning?
A. Improper carburetor float level setting or excessive fuel pressure can cause a rich idle condition. A carburetor power valve opening prematurely because of an incorrect vacuum opening point (indicated by the number printed on the valve) or insufficient idle vacuum can make the situation worse. Additionally, fuel can be drawn in through a ruptured power valve diaphragm; it should be inspected and ruled out.
Engines equipped with camshafts that have increased overlap need a higher curb rpm to idle correctly. As the idle screw is turned to raise the curb idle speed, the transfer slots located just above the blades may be exposed (photo A), allowing unmetered fuel to be drawn into the engine at idle.
One technique to reduce the primary throttle opening at idle (while maintaining the correct curb idle speed) is to increase the secondary throttle blade opening slightly (photo B) to balance idle airflow through all four throttle blades.
Another trick is to drill a small hole in the throttle blades (photo C); this allows airflow through the blades while they remain closed. When doing this, you want to start small because it’s much easier to increase hole size than go back and replace blades. We recommend starting with an .080″ hole in each one. Ignition timing can also play a role in idle speed and quality. Increased valve overlap reduces cylinder pressure, increasing the amount of time it takes to burn the mixture.
Advancing the base timing will allow the ignition more time to complete the combustion process, effectively increasing idle speed and improving throttle response. Pay attention to the overall timing curve, so you don’t over-advance total timing. Limiting the mechanical advance in the distributor may be necessary.