400 with poor idle
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Junior User
Joined: May 2008
Posts: 69
Likes: 0
From: Friendswood, TX
400 with poor idle
I've got a 77 F150 with the 400. It runs great and smooth at anything above idle. The idle is rough. It will usually continue to idle after being warmed up, but it's rough. Basically sounds like it has a lumpy cam, but I'm pretty sure it does not. Just barely touch the throttle, and it becomes completely smooth.
Very smooth going down the highway and power, even from a stop, is good, and about what you'd expect out of a stock engine that's 34 years old.
Any ideas on what would cause that particular symptom? I'm gonna start checking the vacuum hoses first, and probably just go ahead and replace them due to their age.
Thanks in advance!
Very smooth going down the highway and power, even from a stop, is good, and about what you'd expect out of a stock engine that's 34 years old.
Any ideas on what would cause that particular symptom? I'm gonna start checking the vacuum hoses first, and probably just go ahead and replace them due to their age.
Thanks in advance!
Cargo Master
Joined: Sep 2003
Posts: 2,735
Likes: 0
From: TX
What kind of carb do you have?
If you have a vacuum guage, hook it up, and adjust the screws for until you get the highest vacuum. If not, do it by ear really. I know on my edlebrock (and I think this is true for most 4 bbl carbs?) I start off at 2 1/2 turns out. You could turn in one at a time until it bottoms out and turn it out 2 full turns, then do the other.
Or adjust each for the highest idle.
If you have a vacuum guage, hook it up, and adjust the screws for until you get the highest vacuum. If not, do it by ear really. I know on my edlebrock (and I think this is true for most 4 bbl carbs?) I start off at 2 1/2 turns out. You could turn in one at a time until it bottoms out and turn it out 2 full turns, then do the other.
Or adjust each for the highest idle.
Lead Driver
Joined: Nov 2000
Posts: 6,625
Likes: 342
From: Albuquerque
It could be the EGR plate under the carb if you still have it. They are notorious for burning out/warping and basically creating a bad vacuum leak, especially the aluminum ones. If it's bad you can't just block it off because it will leak internally.
Posting Guru
Joined: Mar 2010
Posts: 1,196
Likes: 2
Could be vacuum leaks, could be timing, could be your carb. For the record, mixture screws don't make you run more rich/lean...jets do that. I've never seen a rebuild manual or owners manual that states to start at 2.5 turns out...I usually see 1-1.25 turns out. I've had some stockers that are good at one turn out and then I've had some 400hp 400's that settle out at 1.75-2 turns out. That being said, I think it's more of a vacuum issue since you have the problem when you have the least amount of vacuum in the system...at idle.
Senior User
Joined: Feb 2009
Posts: 308
Likes: 0
From: Southern NM
When you install your vacuum test gauge,I think you will discover that you have the most vacuum AT IDLE,and when the engine decelerates from high idle to low.Install your test gauge and let us know what you discover for readings.
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Senior User
Joined: May 2010
Posts: 244
Likes: 0
From: Oregon
I have the same problem. Specially when she is cold, and been sitting for awhile. Once warmed up, she runs better, but is still lumpy at idle. I have a 2 barrel carb on it now.
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