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Thread: Idle Mixture Screws All The Way In

  1. #1

    Default Idle Mixture Screws All The Way In

    I've got a 4160 (0-80551) marine carburetor on a Ford 351W engine (PCM marine conversion). The carburetor is new (not rebuilt). The engine is a 1994. Ignition timing is set per OEM specs at 10 degrees BTDC at idle. The carburetor and engine seem to working well. Power is strong and transitions are good. I am at 4,000 feet of altitude, so I replaced the main jets going from the stock #66 jets down one size to #65 jets. Additionally, I had to deal with a hesitating / stumbling problem when going from idle to wide-open-throttle. I changed out the accelerator pump nozzle from a number 31 to a number 35. Hesitation / stumble is now gone. Everything seems to be OK. Here is my question:

    When setting up the carburetor initially, I set the idle mixture screws using the following procedure:

    Initial idle mixture screw setting started at 1 1/2 turns out (counter-clockwise) from lightly seated.
    750 RPMs.
    No vacuum leaks (that I could determine).
    Transmission in gear, idling forward (on boat trailer, in the water).
    Vacuum gauge hooked up to PCV hose connected to carb/manifold spacer (should be true manifold vacuum).


    Going with 1/4 turn increments, I could not get the vacuum gauge to read much higher than 12 inHg.

    I went up to 5 turns out and the vacuum only decreased by 2 inHg down to 10. The best vacuum reading I got was at the starting point of 1 1/2 turns out.

    Then I tried something...I turned the idle mixture screws all that way closed (clockwise) to "lightly seated". The engine did not quit -- and didn't appear to be surging or sputtering, which is what I was expecting.

    Is this normal? Does this indicate a problem? Is this due to the fact that even though the carb is in the idle circuit, is already starting to transition to the main circuit?

    Thanks for your input.

    JQ

  2. #2

    Default

    Take it out of gear when adjusting the mixtures.
    You have about 4" less vacuum, and the idle is too high when in gear.

  3. #3

    Default

    Too much transfer slot exposed. This is why your idle mixture screws are having little affect. You should be able to kill the motor at idle by seating the idle mixture screw.

    Float bowl adjustment too high maybe. You would see fuel dripping from the main boosters if so.

    Mains won't start flowing till about 2000 RPM, give or take a little. You need a good amount of airflow to cause a negative pressure change, to get the mains to start flowing. At idle, mains will not flow, unless something seriously wrong with carb. Again, you would see fuel dripping from the main boosters.
    Last edited by 69cam; 04-20-2015 at 10:51 AM.

  4. #4

    Default Very similar issue.

    Quote Originally Posted by 69cam View Post
    Too much transfer slot exposed. This is why your idle mixture screws are having little affect. You should be able to kill the motor at idle by seating the idle mixture screw.

    Float bowl adjustment too high maybe. You would see fuel dripping from the main boosters if so.

    Mains won't start flowing till about 2000 RPM, give or take a little. You need a good amount of airflow to cause a negative pressure change, to get the mains to start flowing. At idle, mains will not flow, unless something seriously wrong with carb. Again, you would see fuel dripping from the main boosters.
    I'm having basically the exact same problem with a new Holley 4160, I just purchased on a 351 marine engine. The engine will not idle with the mixture screws out 1.5 turns, without rich black smoke with all possible adjustments to the idle speed screw. The only way I could get it to idle, was with both mixture screws fully closed (seated). Now the idle speed screw adjusts engine idle fine with no black smoke, but still a tad rich. 65 jets, 2.5 power valve, 16 psi vacuum at idle out of gear. Initially, I was getting some fuel drip from the primary booster venturis after shutting the engine off, so I lowered the float level in the primary bowl a bit, which eliminated this issue (motor is pitched in this boat since it is a direct drive). Timing at idle 10 deg. I'm baffled. Any ideas? Is it possible to be getting too much fuel pressure from a mechanical fuel pump? Planning on checking fuel pressure. Talked to a Holley rep today while on the water, and they confirmed my settings and list of carb matched the application. Any insight would be much appreciated!

  5. #5

    Default

    It has to be pulling fuel from somewhere.

    Try a new power valve on the primaries, or at the very least vacuum test the valve that's in there.
    Same as above, your idle mixture screws should be able to shut down your engine. If you screw them all the way in and it's still running, your pulling fuel from somewhere.

    Double check your vent tubes. Make sure they are free flowing. A clogged vent tube would push unwanted fuel out during idle.

    Is it possible to get too much pressure from a mechanical pump...sure...probable? Not likely, but I wouldn't overlook it.
    Too much fuel pressure would over pressure the needle & seat. If this were the case, you would see fuel dripping out of the booster.
    Last edited by 69cam; 09-17-2015 at 10:36 AM.

  6. #6

    Default

    Yes, you may need a fuel pressure regulator, 4.5-6 psi.

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