Missing at idle
Thread Starter
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Trailering
Joined: Mar 2025
Posts: 22
Likes: 3
Missing at idle
Have a 1981 f100 with a 300 6 cyl. At idle it misses. Not a constant miss just every few seconds.
New plugs ,wires, distributor cap and rotor button. When you rev it higher it sounds like it stops missing.
Ideas?
New plugs ,wires, distributor cap and rotor button. When you rev it higher it sounds like it stops missing.
Ideas?
Moderator
Joined: Jan 2001
Posts: 56,949
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From: Virginia
Mess with the idle mixture adjustment screw a little bit and see if you can tune it out.
Lead Driver
Joined: Apr 2002
Posts: 5,240
Likes: 1,222
I agree with Franklin, try closing the idle mixture screw all the way, counting the turns as you go. Gently seat the screw. Gently. I said gently twice so you don't damage the seat. Then turn it out the same number of turns it took to close it. Should be around 1.5 to 2 turns. Clearing the jet by turning in the screw will often heal a weak idle.
Laughing Gas
Joined: Aug 2010
Posts: 982
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From: Northern Kentucky
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Lead Driver
Joined: Nov 2013
Posts: 7,154
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Is this a Duraspark system? If so, check for wear marks on the face of the distributor pickup coil. My experience here, complete with pictures and a heart-warming resolution:
https://www.ford-trucks.com/forums/1...tator-gap.html
Also check the flexible wires to the pickup coil. They can rub against the distributor body and wear through the insulation.
https://www.ford-trucks.com/forums/1...tator-gap.html
Also check the flexible wires to the pickup coil. They can rub against the distributor body and wear through the insulation.
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FTE Legend
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From: Angier, NC
Tell us you did a compression test on the motor when the plug were out.
If you have a low hole that can cause a miss.
As for doing the test hot or cold and the throttle at wide open who works on a hot motor?
I have never dont a WOT compression test and as I cant see it making that much of a difference.
You are looking for even compression across all holes high or low but if too low can cause uses I would say no lower than 100 PSI.
Dave ----
If you have a low hole that can cause a miss.
As for doing the test hot or cold and the throttle at wide open who works on a hot motor?
I have never dont a WOT compression test and as I cant see it making that much of a difference.
You are looking for even compression across all holes high or low but if too low can cause uses I would say no lower than 100 PSI.
Dave ----
Thread Starter
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Trailering
Joined: Mar 2025
Posts: 22
Likes: 3
I agree with Franklin, try closing the idle mixture screw all the way, counting the turns as you go. Gently seat the screw. Gently. I said gently twice so you don't damage the seat. Then turn it out the same number of turns it took to close it. Should be around 1.5 to 2 turns. Clearing the jet by turning in the screw will often heal a weak idle.
Thread Starter
|
Trailering
Joined: Mar 2025
Posts: 22
Likes: 3
Tell us you did a compression test on the motor when the plug were out.
If you have a low hole that can cause a miss.
As for doing the test hot or cold and the throttle at wide open who works on a hot motor?
I have never dont a WOT compression test and as I cant see it making that much of a difference.
You are looking for even compression across all holes high or low but if too low can cause uses I would say no lower than 100 PSI.
Dave ----
If you have a low hole that can cause a miss.
As for doing the test hot or cold and the throttle at wide open who works on a hot motor?
I have never dont a WOT compression test and as I cant see it making that much of a difference.
You are looking for even compression across all holes high or low but if too low can cause uses I would say no lower than 100 PSI.
Dave ----
Lead Driver
Joined: Apr 2002
Posts: 5,240
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https://www.garysgaragemahal.com/carter-yfa-1bbl.html
Click on the Walker tab. Part number 48 is the idle mixture needle valve.
FTE Legend
Joined: Nov 2015
Posts: 30,828
Likes: 4,094
From: Angier, NC
If you dont have a compression gauge also pick up a vacuum gauge as that can be used to adjust the carb.
After you lightly turn the idle mix screw in counting turns and back out to where it was get the motor up to temp.
Hook the vacuum gauge up to manifold vacuum and turn the idle mix screw in and out to get the highest vacuum reading on the gauge and if you want turn in 1/8 to 1/4 turn and see how it idles.
Also what is the timing set to? I think factory is around 6* BTDC you can bump this up to 10* BTDC or a little more if you dont get pinging or hot restart kick back.
IIRC mine is around 12* BTDC but I do get a little pinging under load and low RPM of around 1000 RPM in overdrive @ 35 - 40 MPH
When I get pinging I just back out of the throttle a little or pull it out of overdrive.
Dave ----
After you lightly turn the idle mix screw in counting turns and back out to where it was get the motor up to temp.
Hook the vacuum gauge up to manifold vacuum and turn the idle mix screw in and out to get the highest vacuum reading on the gauge and if you want turn in 1/8 to 1/4 turn and see how it idles.
Also what is the timing set to? I think factory is around 6* BTDC you can bump this up to 10* BTDC or a little more if you dont get pinging or hot restart kick back.
IIRC mine is around 12* BTDC but I do get a little pinging under load and low RPM of around 1000 RPM in overdrive @ 35 - 40 MPH
When I get pinging I just back out of the throttle a little or pull it out of overdrive.
Dave ----
Tuned
Joined: Sep 2014
Posts: 454
Likes: 92
Definitely pick up a vacuum gauge, it will tell you a lot.
Try some different gasoline even some premium fuel.
see if it makes a difference. you posted you have some pinging.
You don't have to run it for ever.
I have found the quality of gas varies .
These distributors will wear out, check for slop in the shaft and marks on the pick up unit.
Try some different gasoline even some premium fuel.
see if it makes a difference. you posted you have some pinging.
You don't have to run it for ever.
I have found the quality of gas varies .
These distributors will wear out, check for slop in the shaft and marks on the pick up unit.