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Thread: Idle Problems With Performance Camshaft

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    Default Idle Problems With Performance Camshaft

    Yes, this is another thread about idle problems together with camshaft with big overlap. I have read almost every article and threads on the Internet about this problem, but haven’t found a working solution for my setup.

    My engine is a modified Chevy 350 with ported double hump heads, roller lifters and lots of more performance stuff. The engine also have a very aggressive camshaft (I don’t have any specs of it). Finally the engine is equipped with a brand new Holley 750cfm vac carb (3310-SA) with manual choke.

    I have tried to set the idle mixture screws but they are not responding at all and it feels like it running too rich at idle which for me indicates that the main circuit is in use instead of the idle circuit.

    Here are what I have done so far:
    • Manifold vacuum when idling at 900 RPM in neutral is about 9 Hg.
    • Manifold vacuum when idling in drive is low and jumping up and down (around 5, I guess).
    • I have tried to bump up the initial timing to 16° BTDC.
    • The vacuum advance is connected to the ported vacuum.
    • The carb is new and not modified.
    • I have tried different power valves (6.5, 2.5 and 3.5).
    • I have drilled the primary throttle blades.
    • The transfer slot is not exposed to much on any of the throttle blades (I have checked by having the carb upside down).
    • If I use the choke the engines dies.
    • I have searched for vacuum leaks but not found any (I replaced all hoses and used carb cleaner spray around the intake)
    • I have done all tests with the brake booster disconnected and the vacuum port plugged (if it was a vacuum leak there).
    • I use a PCV valve in the drivers side valve cover and the passenger side valve cover is connected to the air cleaner. I did also the test with breathers instead of the PCV system but with no difference.


    After all the above tests, I finally found some discussions about Holleys running to rich at idle when using cams with big overlap and the suggestions was to restrict the fuel into the idle circuit by putting a thin wire into the idle feed restrictor. So yesterday I also tested this but it didn’t help at all.

    I think my problem is the carb isn’t using the idle circuit or the idle circuit isn’t setup according the low vacuum my engine produces. Should I perhaps try to increase the fuel to the idle circuit instead of restrict it with the wire in the idle feed restrictor?

    Does anyone have any ideas or tips how I should tune the carb to be used at the street with my monster camshaft?
    Last edited by chevycsb; 08-20-2015 at 04:05 PM.

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