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Old 06-17-2009, 05:18 PM
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Idle RPM?

Carb is a Holley 750 DP, RPM manifold with a 1" spacer. I have the Idle screws at 1.5 turns each. Car idles when the choke is closed etc.(800rpm) When I open the choke all the way the idle jumps to like 2000rpm, can someone explain to me why? What would cause this?

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Old 06-17-2009, 10:37 PM
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Ok I did some thinking and maybe the car is running too srich and with the choke closed its acting as a restriction and keeping the RPM's down and with it open it's mixing with the fuel to make the RPM's rise?
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Old 06-17-2009, 10:52 PM
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Quote:
Originally Posted by 84TA406
Carb is a Holley 750 DP, RPM manifold with a 1" spacer. I have the Idle screws at 1.5 turns each. Car idles when the choke is closed etc.(800rpm) When I open the choke all the way the idle jumps to like 2000rpm, can someone explain to me why? What would cause this?
Have you tried simply lowering the idle speed w/the curb idle screw?

What's the carb on, cam, engine size, etc.
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Old 06-17-2009, 10:58 PM
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Choke likely has a fast idle cam that controls idle RPM when choke is activated. You want car to idle faster when choke is activated. You can fine tune this adjustment in cold weather.

But sounds like you have opposite condition. Why is choke closed in the summer anyway. Thats a good way to foul the plugs.
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Old 06-17-2009, 10:58 PM
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Doh! I cant believe I overthink things sometimes...

Once the choke is open its no longer on the fast idle, so all I would hafta do is lower the curb idle screw... Or so I think... I really need to slow down and rethink things sometimes.

Motor is a 10:1 406 sbc, 236/248* @ .050 .586/.562 on a 113 LSA
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Old 06-17-2009, 11:03 PM
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Quote:
Originally Posted by 84TA406
Once the choke is open its no longer on the fast idle, so all I would hafta do is lower the curb idle screw...
Well, the choke's fast idle cam could still be engaged, even though you've opened the choke back up. It takes a "blip" of the throttle to allow the release of the choke fast idle. The fast idle can sometimes stick, so you may need to disengage it manually.

With the choke open, fast idle OFF, adjust the curb idle screw, if needed.

And don't worry- if that's NOT it, we'll get it right, one way or another!
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Old 06-17-2009, 11:27 PM
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Good point. It is always a good idea to press gas pedal while moving a manual choke to minimize chance of binding against fast idle cam.
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Old 06-17-2009, 11:33 PM
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Thanks for the info guys! I'm usually pretty good with this stuff and other peoples cars but when it comes to my own I'm 100x more paranoid about things and dont think right. I'll let you guys know how it goes tomorrow.
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Old 06-18-2009, 05:57 PM
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Just because the choke is opening the fast idle cam may still be engaged. They are usually slotted so the choke can open without the cam having to move. Usually a blip on the gas will allow the cam to drop but is may be stuck or binding on some other piece of linkage.

If you cant figure it out, let me know and I will come over and help.
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Old 06-18-2009, 09:55 PM
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I think what was happening was I used the curb idle to try and set the fast idle(stupid mistake) Cant believe I overthough that. So when the choke was open the RPM's shot up.
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Old 06-26-2009, 12:59 PM
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Well now it doesnt want to idle with the choke all the way open...

Mixture screws at 1.5 turns out and it just doesnt want to idle. I got fed up with the manual choke so I ordered a electric choke. I can give it throttle and keep it running, and it sounds great when I give it a rev just wont idle.
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Old 06-26-2009, 01:06 PM
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You are asking for problems with the electric choke. You should tune your carb to warm idle with choke wide open. Only use manual choke briefly to start cold engine.
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Old 06-26-2009, 01:11 PM
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Yeah right now I have the choke just wide open and it just doesnt want to idle. I might pull the whole choke assembly all together and have the horn milled. I've screwed the curb idle screw in pretty far and it just doesnt seem to help anything. Could the airbleeds be affecting things?
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Old 06-26-2009, 01:17 PM
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I've had cases where I couldn't hold idle. The cause was idle circuit had orifices partially blocked with plaque.

I would make sure that the idle rich/lean screw is adjusted properly before opening up the carb.
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Old 06-26-2009, 01:21 PM
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This carb is just about brand new but I can dig into it and clean it if needed. I've had 5-6 different carbs on this car and never had a problem.
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