rough Idle from a 300
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New User
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rough Idle from a 300
As some of you may know I just rebuilt my 1986 f150's motor and I have put about 600 miles on it since my last post. The truck drives fine lots of power and runs out well. The only problem is that It has a really rough idle. When I rebuilt this motor I converted it to the Dura-spark 2 and a Autolite 2100 2 barrel carb, so there is no computer control at all on the motor. The plugs are new the wires are new the cap button and dizzy box are all new, and the carb was rebuilt before we put it on. Any ideas or help in this matter would be great!!!! Thanks in advance!!
Post Fiend
Joined: Jul 2004
Posts: 8,786
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From: Northern California
With the vacuum leak already mentioned. Check Brake booster etc...
I'd say you need to adjust the carb. Running too lean or rich at idle perhaps?
A bad spark plug could also cause it.
Are the new plugs, wires, cap and rotor Motorcraft, or are they Chinese Quality?
Autolite hasn't been a Ford Brand since 1972, when the US Govenrment told Ford to sell off Autolite as it was a "monopoly". This is why Motorcraft came to be in 1972.
I've gotten bad Autolite plugs right out of the box.
Also something you might check... How is the compression? If the engine is unbalanced it can give it a slight "wobble" as the firing order is... 153624.
I'd say you need to adjust the carb. Running too lean or rich at idle perhaps?
A bad spark plug could also cause it.
Are the new plugs, wires, cap and rotor Motorcraft, or are they Chinese Quality?
Autolite hasn't been a Ford Brand since 1972, when the US Govenrment told Ford to sell off Autolite as it was a "monopoly". This is why Motorcraft came to be in 1972.
I've gotten bad Autolite plugs right out of the box.
Also something you might check... How is the compression? If the engine is unbalanced it can give it a slight "wobble" as the firing order is... 153624.
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
No computer... He did a DS-II conversion, with an autolite 2bbl carb.
My first instinct is it's running too lean at idle. Carb Adjustment.
My first instinct is it's running too lean at idle. Carb Adjustment.
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New User
Joined: Feb 2011
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I will look at the carb tomorrow. Might spray some carb cleaner on the intake and carb to see if there is bad gasket causing a vacuum leak. I don't think its out of balance because I have been around engines that were in the past and the shake like a jack hammer when you give them a little gas lol.
Fleet Mechanic
Joined: Jan 2011
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From: West Valley City
Also check the vacuum line tree. Sometimes the caps develop small cracks in them, and allow air to leak.
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Posting Guru
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From: Mexia, TX
Check your timing. Mine used to be off just a bit on my 302 and it had a bit of a stumble to it. You could hear it in the exhaust too. An adjustment on the timing took care of that, as well as having the air/fuel mixture correctly set.
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New User
Joined: Feb 2011
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I dont have a Vacuum gauge. I replaced all the plugs today and tuned the carb some more. it seemed to have helped just a small bit. Could my Dizzy be going bad it was a used unit that I got with all the dura spark parts from the donor truck. I'm at a loss.
Elder User
Joined: Apr 2009
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get a vacuum gauge, it will help you diagnose instead of just throwing parts / cash / time at it.
$25 @ autozone.
Actron/3 1/2 in. face display 0-30 in. vacuum and 0-10 PSI pressure tester kit (CP7803) | Vacuum Pressure Tester | AutoZone.com
or they rent them...you pay $25 and when your done with it they give you your money back.
You also use a vacuum gauge to set the carb up and set timing.
$25 @ autozone.
Actron/3 1/2 in. face display 0-30 in. vacuum and 0-10 PSI pressure tester kit (CP7803) | Vacuum Pressure Tester | AutoZone.com
or they rent them...you pay $25 and when your done with it they give you your money back.
You also use a vacuum gauge to set the carb up and set timing.
New User
Joined: May 2011
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My inline 6 does the same thing especially after removing all the emissions stuff from the engine. I was told some time back from a retired Ford mechanic that Ford went through alot of trouble getting the right distribution of vacuum to smooth out the inline 6 and the 1bbl carb (which is why they installed Carters without any mix adjustments, ie., don't mess with it), and since it is inline you will always get a slight rocking to the engine. Since you now have a two barrel, you are getting even more air into the system, so I would try to rich it out a little, and reset the timing a degree or two off to compensate, but you may never get it 100%, in my opinion.
5th Wheeling
Joined: Oct 2009
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checking with vacuum gauge
How do I check the carb adjustment/timing with that vacuum gauge. I put a new/rebuilt carb on my 84 150 with a 300 last year. Now I am having the rough idle problem also. I pull up to an intersection or light and the idle drops down after a few seconds and almost seems like it wants to shut off. The other thing is how do I check for a vacuum leak with the gauge. I will be picking one up along with a new heater core that just decided to go out on me today.
Elder User
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