Rather Erratic Idle

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Jun 8, 2004 | 11:35 AM

78RangerXLT

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Senior User

Joined: Jul 2002

Posts: 145

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Rather Erratic Idle

Hey Guys, been a while since i posted. Well I've gotten a new truck. Its a 1979 F100 Custom SWB, its got a 302 with a c-6 behind it. freshly rebuilt carb new cap and rotor, plugs and wires as well as air filter. I was wondering about the idle. When the motor is cold it idles smooth as glass in gear but after driving the idle is choppy enough to shake the truck. once in a blue moon after its hot she'll idle smoothly. I took it to a shop to have the carb and timing professionally adjusted and still the same thing. Guys I'm stumped. Perhaps a coil that's heating up or a engine computer going out? (the irony of saying engine computer, the little box mounted on the drivers side firewall). All help is appreciated thanks guys.
btw.. the motor has 55,000 miles on it

Jun 8, 2004 | 11:50 AM

78fordman

Cargo Master

Joined: Oct 2003

Posts: 2,833

Likes: 4

From: Jefferson, Iowa

the motor could just have a big cam in it. I assum they adjusted teh choke and everything, otherwise i would say the choke is sticking. I just wiring mine open, pump 2 or 3 times give it a minute to warm up and you are just fine.

Jun 8, 2004 | 12:01 PM

dfisher1

Posting Guru

Joined: Jul 2001

Posts: 1,613

Likes: 2

From: Gainesville, FL

XLT,
I need more detail:
1. Is it backfiring? No. It's idle fuel circuit is too lean. Yes. Ignition problem.
2. How does it start hot? Does the starter lag or turn over slowly at all? If so, timing is too far advanced. retard timing. This would make the idle even worse.
3. I think its a fuel problem at this point. Adjust the idle fuel mixture.
4. Does the idle go up and down over and over? If so, its a vacuum leak.
KingFisher

Jun 8, 2004 | 02:14 PM

beartracks

Lead Driver

Joined: Nov 2000

Posts: 6,635

Likes: 349

From: Albuquerque

It could be vapor lock/running a bit hot. Also check out you EGR system. It's not supposed to function at idle. It might leak.

Jun 8, 2004 | 07:32 PM

78RangerXLT

Thread Starter

Senior User

Joined: Jul 2002

Posts: 145

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Lets see, I dont have emissions so egr isnt an issue. No backfiring. I have no trouble starting it hot or cold. The air/fuel mixture, idle speed, choke and fast idle cam were adjusted professionially by an accreditied shop near me. They set the air/fuel mixture with a vacuum gauge. I also put it on a tach and the idle is just choppy, no drastic rpm change maybe +-50 rpm. I have a see through fuel filter by the carb and it always remains full of gas. Im stumped still.

Jun 8, 2004 | 07:54 PM

mhg

Fleet Mechanic

Joined: Sep 2003

Posts: 1,279

Likes: 1

From: savannah,ga

sounds like a small vacuum leak
check intake bolts, carb bolts, all screws on the carb
and all vacuum lines
I had a similar problem with my pvc hose
just a small crack and slightly brittle at the manifold connection