Very Rough Idle When Cold - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars
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post #1 of 48 (permalink) Old 11-04-16, 10:03 AM Thread Starter
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Very Rough Idle When Cold

Now that we've gotten to these colder mornings, I'm getting a very rough idle for a few minutes. My best guess is that the vacuum advance isn't working correctly, or there's a problem with the vacuum connected to it. Does that sound right? What's the best way to go about testing it? This is for a 351M with a 2bbl. This is actually my first car with a carb. Even in the 90's I had a 10yo car from the 80's with EFI, so this is a learning experience for me.


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post #2 of 48 (permalink) Old 11-04-16, 05:59 PM
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Re: Very Rough Idle When Cold

More likely the automatic choke on the carburettor has failed or is incorrectly adjusted. See

and

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post #3 of 48 (permalink) Old 11-05-16, 05:38 PM Thread Starter
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Re: Very Rough Idle When Cold

Thanks for these posts. The engine was cold, and when I checked the carb, this is how it looked:



There seems to be some sort of sticking linkage, and after fiddling with it, I got closed even further:



The linkage is too stiff for it to completely close all the way on its own. Should it be able to close even further than this, though?

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post #4 of 48 (permalink) Old 11-05-16, 07:29 PM
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Re: Very Rough Idle When Cold

Could be the Fast idle adjustment, could also be a bad choke. I know Fords have had a couple of different designs for different applications. Often they all look the same until you pop the cap on the choke and actually see the spring. Many don't realize that there's different choke mechanisms , but the outsides all look the same. This sounds more like the fast idle.

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post #5 of 48 (permalink) Old 11-05-16, 10:58 PM
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Re: Very Rough Idle When Cold

If those pics are on cold start after goosing (flipping) the throttle peddle/accelerator with the engine off, the choke plate is not closing fully which suggests the automatic choke needs adjustment; in any event you'll then probably need to adjust the fast idle too and it may be out of adjustment. Also see
and
shows how the choke and fast idle works and how to adjust them.

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post #6 of 48 (permalink) Old 11-06-16, 09:01 AM Thread Starter
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Re: Very Rough Idle When Cold

Well, checking on it this morning, I saw the choke open like in my first photo, and then decided to tap the gas pedal, and this was the result:



So, not sure if adjusting the choke helped or not, but at least it looks appropriate for now. I did look around the carb, though, and couldn't find the identifier tag. I figure this thing could use a rebuild after sitting for so long before I bought it, but I'm not sure how to identify it so I can get the correct rebuild kit. The only marking I can find is the Motorcraft logo on the top.

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post #7 of 48 (permalink) Old 11-06-16, 02:55 PM
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Re: Very Rough Idle When Cold

Ok choke is now working so if it still plays up increase the fast idle. BTW you are meant to goose the throttle on cold morning starts before turning the key to activate the automatic choke. If you have the owners handbook there should be instructions in it about cold starting. Sometime it's a matter of gently opening and closing the throttle twice before turning the key or rather than goosing it.

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post #8 of 48 (permalink) Old 11-06-16, 04:05 PM Thread Starter
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Re: Very Rough Idle When Cold

I typically give it 2-3 pumps of the pedal before starting it up in the morning, warm or cold outside.

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post #9 of 48 (permalink) Old 11-07-16, 05:14 AM Thread Starter
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Re: Very Rough Idle When Cold

I found a venturi diameter of 1.21 stamped into the side. So at least that's a start. Supposedly there's the carb ID stamped elsewhere on the carb itself, but I'm not certain where to look.

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post #10 of 48 (permalink) Old 11-07-16, 03:05 PM
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Re: Very Rough Idle When Cold

Hope you don't mind me cutting in . A long time since I worked on a carb but I seem to remember when setting the choke on Webbers for instance , setting the choke to fully close was too much and tended to flood the carb.The second picture with a gap looks more like the way we used to set the clearance .Also with them it was pedal fully down once prior to starting .But might be wrong , memory plays tricks with old age at times .

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