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Thread: Cold start tuning questions.

  1. #1

    Default Cold start tuning questions.

    Hey guys, new to the forum of course. But I'm having a head scratcher of a time figuring out the cold tuning of this New carb. I picked up a Holley Street Avenger 870cfm for my truck. Current truck is an '89 Chevy 2500 LD ext long box. I have a fresh built 496 under the hood currently. 10.76:1 compression ratio. Comp cams XE 268 grind 268°/280° Duration 224°/230° Duration @ .050. I have 781 heads with 2.19" intake valves and 1.720" exhaust. .060" over pistons and 6.385" rods. Intake is a GM cast iron 948. Timing is set at 10°F advanced with a total at 3000 RPM of 20°. I removed the QuadraJet 850 I had on it do to it needing bushings installed in the lower rods. It has recently dropped below freezing around the 0°F. I'll go out pump the gas twice with the ignition on to set the choke. Fires right up in about half a turn, RPM shoots up to the high idle numbers and drop off right away to hot idle. The truck idles for around one minute at 600 RPM and will stall out. My question is, what am I missing on why it won't stay on the high idle? On a warm day, it will drop RPM and slowly climb in RPM until it's at the high idle range until I blip the throttle. Now since it's cold, it does not climb back up. Just looking for some input. Thanks in advance.

  2. #2

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    You need to adjust the fast idle linkage for the choke. Then you need to adjust how long the choke runs at high idle. Should all be based on engine temperature. Go YouTube and search for Holley choke adjustment: https://www.youtube.com/watch?v=OoW9pWZmtBI
    Last edited by 69cam; 12-12-2017 at 01:04 PM.

  3. #3

    Default

    I appreciate the response. This is where I’m confused. I have everything set on the truck to do just that. Choke pulls off in time without flooding, and once the few minutes go by, it finally gets up to high idle speed. What I'm confused by, is why does it have this gap in there where it doesn’t go to high idle right off the bat? Changed plugs again from a 7 heat range to a 5 heat range, and I’ll adjust the choke to see if this helps anything now.

  4. #4

    Default

    Quote Originally Posted by 90guy View Post
    I appreciate the response. This is where I’m confused. I have everything set on the truck to do just that. Choke pulls off in time without flooding, and once the few minutes go by, it finally gets up to high idle speed. What I'm confused by, is why does it have this gap in there where it doesn't go to high idle right off the bat? Changed plugs again from a 7 heat range to a 5 heat range, and I’ll adjust the choke to see if this helps anything now.
    I think you might be confused on how the choke functions. The choke does not change fuel flow only air flow. The choke butterfly closes to where it only allows so much air in, until the engine warms up & releases that choke butterfly.

    Choke pulls off in time without flooding...
    Choke does not FLOOD an engine. Engines typically want more fuel when cold. The colder they are, the more fuel they want to get over the initial cold start. The idea of the choke is to high idle the engine until it gets warm enough to idle on its own without requiring you to feather the throttle until it gets warm enough. Since choke does not change fuel, it changes the amount of air allowed in. So that now the air fuel mixture/ratio is rich and will allow the engine to warm up.

    On a cold engine you should be able to turn on the ignition, pop the hood, touch the throttle and see the choke engaged. When the engine starts from this setting it should instantly go to a HIGH IDLE of 1500-1600 RPM. If it doesn't either the fast idle has to be adjusted, there is something binding, the fast idle adjustment screw as shown in video, the fast idle cam is worn. Something else is wrong if it won't perform as stated above.

    ...and once the few minutes go by, it finally gets up to high idle speed.
    You have this backwards. It should start at fast idle and then idle down to wherever you have your curb idle set. Your idle screw and choke fast idle screws are two completely different screws. Plug Heat of that small of a heat range change won't affect idle that much if any.

    What I'm confused by, is why does it have this gap in there where it doesn’t go to high idle right off the bat.
    Something has to be hanging up for it to take so long to get the fast idle up to speed. Something is sticking or binding.
    Last edited by 69cam; 12-12-2017 at 02:24 PM.

  5. #5

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    Awesome writeup & explanation. It’s a brand new carb from Holley. When I get more time this weekend, I’ll have to play with the choke. As for the plugs, noticeably a difference on cold starts with this engine, compared to the other set when the teams where the same. But the old plugs are the short tip to the long tip UR5s that are in it now. I really appreciate all the info and will have to update it when I figure something out. Just seemed odd to me the way it has been starting like that.

  6. #6

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    That was a good video. Short and to the point. Thanks for posting it.

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