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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 10-16-2011, 12:56 AM   #1
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Cold engine idle speed with carburetor

I have a 1987 Z28 305 with Edelbroch carburetor (and other external engine mods).

As with most other carbureted engines, RPM will be higher on a cold start up, mine being around 1800 or so. Is there a way to lower engine speed on cold start up to around say 1000 RPM? Would this be bad thing to do? I wouldn't drive the car for several minutes and would still let it warm up. I'm mainly looking to not wake up the world every time I start up the car whether it sits for a couple weeks or a couple hours...and it's starting to be a pain to me lol. (Please don't say convert it to TPI lol). I've had the car in my possession for almost 15 years and prior to that it was in the family.

I tried to research this but couldn't find direct answers. Sorry for my ignorance with this topic.
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Old 10-16-2011, 02:41 AM   #2
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Re: Cold engine idle speed with carburetor

The choke is closed whenthe air is cold, keeping the heat in the engine and mixing the fuel very rich in the carb, this allows the carbed engine to run when it's cold. I have the same problem with a modded 350 and custom exhaust. 1800 is a very tame high idle number. However, you should have two spots. The high idle and the "second" idle is what I call it. once you start the car and it jumps to high idle, pump the gas pedal once and the rpm should drop anywhere from 100-300rpm down to the "second" idle. Then after the 3 min or so when the car is warm enough, you can kick it down to normal idle. The main difference is I have a Qjet and you have an Edelbrock, so I don't know if your carb has the "Second" idle or not.

If, you drop the high idle to 1000rpm, I belive you'll find the car impossible to drive in cold temps because I don't think it'll stay running.

Last edited by Ozz1967; 10-16-2011 at 02:50 AM.
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Old 10-17-2011, 06:44 PM   #3
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Re: Cold engine idle speed with carburetor

Quote:
Originally Posted by Ozz1967 View Post
The choke is closed whenthe air is cold, keeping the heat in the engine and mixing the fuel very rich in the carb, this allows the carbed engine to run when it's cold. I have the same problem with a modded 350 and custom exhaust. 1800 is a very tame high idle number. However, you should have two spots. The high idle and the "second" idle is what I call it. once you start the car and it jumps to high idle, pump the gas pedal once and the rpm should drop anywhere from 100-300rpm down to the "second" idle. Then after the 3 min or so when the car is warm enough, you can kick it down to normal idle. The main difference is I have a Qjet and you have an Edelbrock, so I don't know if your carb has the "Second" idle or not.

If, you drop the high idle to 1000rpm, I belive you'll find the car impossible to drive in cold temps because I don't think it'll stay running.
Hey Ozz, thanks for your reply. I know what you mean about the high idle and second idle. If I were to not touch the gas, the high idle would probably go up to 2200rpm and when I blip the gas it will go down to 1800rpm. It seems to function perfectly in that regard.

I think you also responded to my other thread, but here is the link to the video on that post http://www.youtube.com/watch?v=LybkYgRk7wk This was a cold start. The car actually idles very low for a bit in this video until finally getting up to the high idle. I don't want to mix or confuse two separate threads so I won't touch the other questions on here.

So you think that if the cold start idle was somehow bypassed and placed at 1000-1200rpm it could crap out once in gear even if it warmed up for say 5 minutes? I know that I have had to move the car when it was started in this condition (like the video, cold start) and it was idling at 800rpm and did not stall out. I don't know too too much about the tech of this stuff.

Last edited by Jon87Z28; 10-17-2011 at 06:49 PM.
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Old 10-17-2011, 11:34 PM   #4
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Re: Cold engine idle speed with carburetor

turn your idle down in general. its kinda cool sounding after its warm and it idles low, as long as it keeps running good. then your cold idal will be slightly lower
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Old 10-18-2011, 01:47 AM   #5
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Re: Cold engine idle speed with carburetor

Once the car warms up, the point is moot. The issue is that the car won't stay running at normal idle when it's cold. You need the higher RPM and rich fuel mixture with the carb. Now, once the motor is warm enough to kick down and run on normal idle, as I said, the point is moot. Since you are having such an issue getting the car revved up initially, could e that the choke needs to be rebuilt/replaced. Now, there is a way that you can turn down the high idle. Mine is set to 1800, second idle is 1500. ON my Qjet, the adjustment is a screw that's under the choke pull off and faces the front on the passenger side.

Quote:
Originally Posted by Jon87Z28 View Post
Hey Ozz, thanks for your reply. I know what you mean about the high idle and second idle. If I were to not touch the gas, the high idle would probably go up to 2200rpm and when I blip the gas it will go down to 1800rpm. It seems to function perfectly in that regard.

I think you also responded to my other thread, but here is the link to the video on that post http://www.youtube.com/watch?v=LybkYgRk7wk This was a cold start. The car actually idles very low for a bit in this video until finally getting up to the high idle. I don't want to mix or confuse two separate threads so I won't touch the other questions on here.

So you think that if the cold start idle was somehow bypassed and placed at 1000-1200rpm it could crap out once in gear even if it warmed up for say 5 minutes? I know that I have had to move the car when it was started in this condition (like the video, cold start) and it was idling at 800rpm and did not stall out. I don't know too too much about the tech of this stuff.
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Old 10-18-2011, 01:50 PM   #6
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Quote:
Originally Posted by Ozz1967 View Post
The choke is closed when the air is cold ...
A nit, perhaps, but more accurately the choke is closed when the engine is cold. It can be 100 degrees outside, but if the engine hasn't been run, it needs a choke to start and run properly.

To review the process: Before starting the car when cold, tap the throttle. This does two things: 1) sets the choke; 2) gives a little shot of gas into the intake manifold. After the engine starts and vacuum builds up, the choke has a "pull-off" built into it to open the choke butterfly slightly. This also will allow the idle to drop slightly, but you have to tap the throttle again in order to release the pressure on the high idle cam. As the engine continues to run, the choke thermostat heats up (via an electric current), gradually opening the choke butterfly and allowing the high idle cam to drop down farther (if the throttle is opened at least slightly, as would be the case in driving). When the choke thermostat has heated sufficiently after a few minutes, the choke butterfly will be fully open and the high idle cam will have dropped so that it is no longer controlling idle.

If your initial cold idle is 1800 RPMs, try giving the throttle another tap after the engine has been running to see if the idle RPMs drop. If it doesn't, then the vacuum pull-off mechanism may not be operating properly.

If all seems to be operating properly, but you still think the high idle is too high, then adjust it down until you are comfortable with the idle quality.
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Old 10-18-2011, 09:40 PM   #7
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Re: Cold engine idle speed with carburetor

Thanks everyone for the explanations and ideas. I'm going to tinker with the idle adjustment screw and see what the results are.

Would I be correct to think that turning the screw to the right will lower the idle?
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Old 10-31-2011, 09:12 PM   #8
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Re: Cold engine idle speed with carburetor

Quote:
Originally Posted by Jon87Z28 View Post
Thanks everyone for the explanations and ideas. I'm going to tinker with the idle adjustment screw and see what the results are.

Would I be correct to think that turning the screw to the right will lower the idle?
Turning to the right will raise the idle.
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Old 11-01-2011, 09:24 AM   #9
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Re: Cold engine idle speed with carburetor

I have and edelbrock carb also.
And I use to kind of hate it.

Now I love my edelbrock carb and it works great and is so simple.

Here is how I made the change:
#1 Down load the edelbrock performer carb PDF.
http://www.edelbrock.com/automotive_...ers_manual.pdf
Read it find your carb model if you dont know it. Understand it.
#2 get a tachometer if you dont have one.
#3 get a wide band air fuel ratio meter (use the AEM gauge and it was the best $200 I have spent so far on my car).
#4 get an edelbrock 600 serries rebuild kit and carb tuning rod and jet kit, maybe the accelerator nozzle kit later on if you need them and additonal rods and jets if you need them too.
#5 rebuild and tune your carb and you will be very happy with it.
#6 Kick your self for not doing this years ago.
#7 you can do nearly all your tuning and seasonal adjustments with very quick rod changes only.

A carb tuned with an A/F meter, tachometer and vacuum gauge is as good as fuel injection for most users.
^^^ I use to think that wasn't possible with out 20+ years of carb tuning experance ^^^
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Old 12-10-2011, 03:38 PM   #10
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Re: Cold engine idle speed with carburetor

Just wanted to post this video as a follow up to the link I posted earlier in the thread. This new video is the same problem but just viewed from the outside of the car instead. Done with a phone camera, but the quality is good until you reach near the end of the video with the higher rpms.

http://www.youtube.com/watch?v=cBkr4...ature=youtu.be

Notice that around 1:40 when it figures itself out, the smoke changes to white (cold outside) and it runs perfectly smooth. This is always the case during a cold start (not used for over a day or longer). Car runs great except for the cold start. Like mentioned before, maybe it needs a carb rebuild.....unless it's completely a different issue?
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Old 12-10-2011, 03:38 PM
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