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Very Rough Idle When Cold

16K views 48 replies 6 participants last post by  Tony Lochmandy 
#1 ·
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.
 
#3 ·
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?
 
#4 ·
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.

Motorcraft 2150 2V Carburetor
 
#5 ·
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.
 
#6 ·
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.
 
#7 ·
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.
 
#9 ·
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.
 
#10 ·
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 .
 
#12 ·
It still idled very rough this morning, with it being somewhere in the 40's. There's also a vacuum controlled valve on the air cleaner assembly that when closed pulls in air from along the exhaust manifold. It was wide open, so not sure if that's related or not.
 
#13 ·
I read that forum post, and it looks like I have the choke set correctly according to the sticker on the valve cover: 3 rich. Guess I'll have to wait until the weekend to really try anything further, though. At present this car is my daily driver since one car won't pass emissions and the plates expired, and I'm still fighting with the brakes on yet another.
 
#15 ·
I gave it a couple firm pumps of the pedal this morning, and as usual it started up without any trouble. It got into high idle, and after a few seconds of smooth idle it got rough. But, unlike previously, nowhere nearly as rough. Before it was so rough that the entire car would shake like it was going over a rather bumpy road, but this was far less pronounced. The roughness resolved a bit more quickly, as well.
 
#23 ·
It's been warmer outside the last couple mornings when first starting the motor. Idling smoothly on start up. Guess I'll have to wait for the weather to cool down again before I know if anything I do helps.
 
#24 ·
Ok, so it looks like when the ambient temperature drops below about 50 degrees, and the engine has cooled enough to match, that's when it get it running rough until it's run for a few minutes to heat up. When the ambient temperature is warmer, I do not encounter the issue. Adjusting the choke to full rich has improved the situation, but not resolved it. I also found that if I hold the gas pedal down at about half throttle for something like 5-10 seconds it will smooth out a lot quicker while remaining at high idle when I release the pedal. Goose it and it drops the idle speed as expected.

I want to do vacuum testing, as well, but I don't know which vacuum line is being referenced. I've already had to do some vacuum line repairs due to some broken plastic junctions. Not sure if there are more leaks. Concerning the test mentioned in the previous post, which line do I test? Also, based on the description, may I correctly assume this test is to be done without the engine running? I do have a handheld vacuum pump with a gauge on it, which would seem to be the appropriate tool.
 
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