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Holley 4150 tuning questions. I rebuilt the carburetor last week. During disassembly I found that the secondary power valve has a plug installed. My primary jets are 71's and the secondaries are 80's (mechanical secondaries). After rebuilding and tuning I found that the idle mixture screws have an uneven effect on the vacuum reading (max vacuum is approximately 15 inches). Is this because the jets are so rich? The engine runs smooth at idle and is fairly smooth in response throughout the range, though there is a slight hesitation when opening up the secondaries - which I am hoping to fine tune and eliminate. Your thoughts? The engine is a fresh 355 with a mild cam, 9 to 1 compression.
 

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My thoughts on the subject are 750cfm might be a lot for the 355 with a mild cam. No? Wouldn't a 650 be better suited for a near stock c3 motor? When we oversize or undersize the carb for the engine we are using we find ourselves with these idle issues because the airflow requirements and the placement of passages on either side of the throttle plates for the idle circuit is in a fixed location and can not be changed with re jetting. I think 750 would be a good carb for a 396 or a 427. A big block. You cant treat the carb like it is fuel injection. There is no idle air control motor or valve here. That is decided by the flow rate stamped on the carb.

If you want performance you should consider a Speed Demon Carb. They are owned by Holley. Just a higher tier of carb than your Holley. But you will still do better with the 650 for all around drivability. Alternatively you could consider an aftermarket fuel system like the FAST easy efi 2.0 that comes with a throttle body that flows 1150cfm but it can still hold a great idle with even a 4 cylinder engine because of the precise control from the GM stepper motor regulating airflow for the idle circuit(IAC).
 
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