# Idle changes levels on its own - choke?



## 1967pontiac400 (Dec 10, 2020)

Hi all - I'll be honest, I'm stumped this time. 

So for some background, I have a 1967 GTO Convertible with a 1969 400 under the hood from a 1969 Grand Prix. It's running a 65 tri-power on top. I'm in Arizona so the temperature outside for this test was about 74 degrees.

I suspect the choke here, but let me explain the issue.

When I warm the car up I notice that I need to let it run on the fast idle for a while before I use the gas pedal since the choke opens to quickly it ends up with too little throttle and dies out. To fix this, I ended up adding more idle speed in the idle speed screw, and at this point it's most of the way in, maybe a few turns from being all the way in. With this setting, it idles around 1,100 - 1,200 RPM and eventually will settle around 800-850 once the choke cam fully opens. 

However, if I let it warm up enough, say after a 15min drive or so and the water temp gets to at least 160 or so, I notice that it's harder to hold the car at a light (it has a 3 speed), which I'm attributing to a higher idle, which is the case for sure. 

Once I return from this length or more of a drive and place the gear into park, the idle hops up to about 1,500 RPM.

So my theory here is that the choke is opening far too quickly (after 90 seconds or so it's fully open) and because its opening so fast, I've had to add a lot of turns to the idle speed screw to compensate - which as a side effect, ends up with a really high idle once the car is fully warmed up. 

I don't think I have anything a miss with the carbs at this point, but I could be wrong. My mind immediate went to another fueling issue after I noticed this on a drive today but it can't be a static part, right? Wouldn't this have to be a moving part, like the choke?

Finally, and this may or may not be related, I'm seeing about 15in on my vac gauge at idle. According to the gauge that's a little low but I suspect that with this engine that might be normal?

As always, thanks everyone for your help and wisdom =-)

-Will


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## BearGFR (Aug 25, 2008)

Since you have a vacuum gauge, I'd suggest this: Get it fully warmed up so that it's idling "right". Slow the idle back down to around 750-800. Next, using your idle mixture screws, adjust for the highest vacuum reading you can get. That might change the rpm. If it does, then set it back to 750-800. Lather, rinse, repeat until you find the point where you've got vacuum maximized and a good 750:800 idle. See what kind of result you get from that and if it changes how it acts when cold. I'm assuming you've checked all the linkages involving the choke and the fast idle cam, that it's all adjusted properly and not binding anywhere.

Bear


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## 1967pontiac400 (Dec 10, 2020)

I ran through that procedure a couple of weeks ago and feel pretty confident that it's at the highest vacuum. It could not hurt to do this again though. I set the choke according to the procedure outlined in this video (starts at 0:42) - 




Silly question, but if you cold start, about how long should it take for a vac/heat choke to fully open? Mine's opening in about 1-2min.


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## Lemans guy (Oct 14, 2014)

I think you have that throttle screw in too far. The choke opens in one or two minutes, that’s ok. But the way you went about the adjustment is the issue, maybe. When you started and you wanted to up the idle so it would not stall, you need to work the mixture screws slightly, then the throttle screw and back and forth.

if the throttle screw gets opened too far, where I think you are, the transfer slot is open way too much. So maybe open the mixture. Screws to make it a bit richer, and not so much on the throttle screw.

fast idle in park about 1200 is good ...it is not that exact, but get closer. Work those adjustments separately. Work the hot idle, by turning the mixture screw richer and closing the throttle and watching vac.

Then when totally cold, the next day, adjust the choke and fast idle cam.......see if that works...you are close..


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## 1967pontiac400 (Dec 10, 2020)

So what's tricky or at least confusing to me still is I did go through that idle setting procedure with the idle mix screws and the initial idle seems pretty good, the weird thing is that it climbs after i suppose we should call it "fully" warming u the engine, getting the water temp to 160+. At that initial setting the fast idle is at about 1,200 and once it warms up a bit I'm sitting around 800 or so. With just that all seems normal.

Just so I understand this since there are two people who are now recommending re-doing the initial idle setting procedure, that I'm perhaps not letting the car warm up enough and setting it too high?

I think I'm following in that I'm not running the right mix and the idle speed screw was improperly set to compensate for this, perhaps running too lean, and thus, we're seeing the idle rise over time?

I'm just trying to understand, it's tricky diagnosing and communicating car issues online =-).

One thought on this note, my idle mix setting right now is just about initial, which is 1.5 turns out from lightly seated. When I tuned this last that seemed to be the best spot for the best vacuum. How do I know when I'm at the right mix setting before I start playing with the idle speed screw - is there a general setting that's "normal" or "starting" that I could use as a starting place?

Thanks!


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## Old Man Taylor (May 9, 2011)

Deleted.


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## Rocketman269v (Oct 5, 2020)

The problem seems to be *how* you are increasing your "cold idle" issue. The cold idle is dependent on the position of the fast idle cam; each step of the cam increases your idle speed, until the choke opens up. Then your "curb idle" is controlled by your idle screw. In the video he referenced how to bend the tab on the air horn to adjust the fast idle cam. With your car completely warmed up, adjust your idle for highest vacuum. Let the car sit overnight to completely cool off. In the morning, see how far your choke plate is closed. You will need to open your throttle a little to allow the choke to close, because the idle screw pressure on the fast idle cam is holding the choke from closing. Adjust the choke rod to get the choke plate to the point that he showed in the video. Now see which step of the fast idle cam the idle screw is on. Adjust the fast idle cam to a higher step (or two) to increase your cold idle. Start the car with your foot off of the gas pedal and see how it idles. If it's too low, shut it off immediately and raise it one step. If too high, drop it one step. DO NOT change your idle speed screw to adjust this. Once you get it right when it's cold you should be fine. Sorry for the long dissertation, but I figured that it was necessary.


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## 1967pontiac400 (Dec 10, 2020)

This makes sense, thanks for breaking it down that's really helpful! I will give that a shot over the weekend.


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