Inconsistent Idle

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Crestline

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My Car
73 Mustang convertible
73, 351c, 2v
i cant seem to get a consistent curb idle.
I've looked through searches but didn't find one addressing my problem. Sorry if I've overlooked it..
It starts and runs good. i can adjust the idle after the engine is hot, take it for a drive of 2 or 3 miles, and it will die pulling into the driveway. it restarts easily, but then will idle down and die again.
It's new or rebuilt from fuel inlet to appetite, new fuel inlet pipe, tank, sending unit, in line fuel filter, electric fuel pump, and rebuilt the Motorcraft 2150 carb,
Carb fuel /air mix screws are adjusted out 1-1/2 turns. Pulling 16" of vacuum. i disconnected the solenoid so it doesn't try to extend and affect the idle adjustment (after several idle screw adjustments, i found out that the solenoid was messing with me). I adjust the idle using the Low Idle adjustment screw on the drivers side, towards the back of carb. if i turn up the idle in an attempt to keep it from dying, i chirp the tires when putting into gear. I've gone through the process several times and end up with a "good idle" in the shop, but die when i pull up to a stop sign or driveway. I applied a small amount of Rem oil with teflon to the Throttle Lever assembly thinking that it is hanging up a little until i drive it and vibrations or use allow the idle to drop off just enough that the engine is starved.
The carb functions normally when i set the choke on cold start and then pulls off to a lower (but fast idle) for a minute or so, so i think the choke side of the carb is working ok.
I've tried to get it into local shops, but the ones that have an older mechanic that is familiar with carb engines are really backed up. I'm in line at one of them, but wont drop my car off just to sit in the parking lot "in line".
Any thoughts or suggestions will be appreciated- I'm not a mechanic so suggestions will need to be dumbed down.
Thanks in advance
 
Joined
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Cloverdale, CA
My Car
71 429CJ
73, 351c, 2v
i cant seem to get a consistent curb idle.
I've looked through searches but didn't find one addressing my problem. Sorry if I've overlooked it..
It starts and runs good. i can adjust the idle after the engine is hot, take it for a drive of 2 or 3 miles, and it will die pulling into the driveway. it restarts easily, but then will idle down and die again.
It's new or rebuilt from fuel inlet to appetite, new fuel inlet pipe, tank, sending unit, in line fuel filter, electric fuel pump, and rebuilt the Motorcraft 2150 carb,
Carb fuel /air mix screws are adjusted out 1-1/2 turns. Pulling 16" of vacuum. i disconnected the solenoid so it doesn't try to extend and affect the idle adjustment (after several idle screw adjustments, i found out that the solenoid was messing with me). I adjust the idle using the Low Idle adjustment screw on the drivers side, towards the back of carb. if i turn up the idle in an attempt to keep it from dying, i chirp the tires when putting into gear. I've gone through the process several times and end up with a "good idle" in the shop, but die when i pull up to a stop sign or driveway. I applied a small amount of Rem oil with teflon to the Throttle Lever assembly thinking that it is hanging up a little until i drive it and vibrations or use allow the idle to drop off just enough that the engine is starved.
The carb functions normally when i set the choke on cold start and then pulls off to a lower (but fast idle) for a minute or so, so i think the choke side of the carb is working ok.
I've tried to get it into local shops, but the ones that have an older mechanic that is familiar with carb engines are really backed up. I'm in line at one of them, but wont drop my car off just to sit in the parking lot "in line".
Any thoughts or suggestions will be appreciated- I'm not a mechanic so suggestions will need to be dumbed down.
Thanks in advance
I would take a close look at the connections for your coil, making sure you have good connections. Then at next unintended engine shut down, look down the carb and listen for fuel squirting while actuating the throttle lever. Then you will know if it’s ignition or fuel and can go to next step.
 
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What is your idle RPM in both park and drive?

To clarify:
So as you pull into the driveway it dies while you are rolling or as you stop the car?
It will then instantly restart, run for a moment and then die again?

Initial idle adjustment is done with the solenoid. Make sure it is operating correctly. It should extend any time the ignition is in the ON position. But it is not strong enough to push the throttle open, it is strong enough to keep it open. It's to avoid dieseling when turning the car off. So proper operation is to turn key to ON position, press down gas (to open the throttle a bit) and then start car.

It seems like the symptoms are consistent. Let it die and then look down the carb and pump the gas once. You should see a squirt of fuel. Pump it a couple of more times and you should see it squirt again. If it does you have fuel in the fuel bowl.

Next check that the float is set correctly. It's possible the float is too high and causing it to flood at idle.

A vacuum leak is also possible. Confirm all of the connections are good. Do you have a lot of the original emissions in the vacuum system?
 

Crestline

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I would take a close look at the connections for your coil, making sure you have good connections. Then at next unintended engine shut down, look down the carb and listen for fuel squirting while actuating the throttle lever. Then you will know if it’s ignition or fuel and can go to next step.
 

Crestline

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Joined
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Messages
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My Car
73 Mustang convertible
73, 351c, 2v
i cant seem to get a consistent curb idle.
I've looked through searches but didn't find one addressing my problem. Sorry if I've overlooked it..
It starts and runs good. i can adjust the idle after the engine is hot, take it for a drive of 2 or 3 miles, and it will die pulling into the driveway. it restarts easily, but then will idle down and die again.
It's new or rebuilt from fuel inlet to appetite, new fuel inlet pipe, tank, sending unit, in line fuel filter, electric fuel pump, and rebuilt the Motorcraft 2150 carb,
Carb fuel /air mix screws are adjusted out 1-1/2 turns. Pulling 16" of vacuum. i disconnected the solenoid so it doesn't try to extend and affect the idle adjustment (after several idle screw adjustments, i found out that the solenoid was messing with me). I adjust the idle using the Low Idle adjustment screw on the drivers side, towards the back of carb. if i turn up the idle in an attempt to keep it from dying, i chirp the tires when putting into gear. I've gone through the process several times and end up with a "good idle" in the shop, but die when i pull up to a stop sign or driveway. I applied a small amount of Rem oil with teflon to the Throttle Lever assembly thinking that it is hanging up a little until i drive it and vibrations or use allow the idle to drop off just enough that the engine is starved.
The carb functions normally when i set the choke on cold start and then pulls off to a lower (but fast idle) for a minute or so, so i think the choke side of the carb is working ok.
I've tried to get it into local shops, but the ones that have an older mechanic that is familiar with carb engines are really backed up. I'm in line at one of them, but wont drop my car off just to sit in the parking lot "in line".
Any thoughts or suggestions will be appreciated- I'm not a mechanic so suggestions will need to be dumbed down.
Thanks in advance

Sorry it took so long to respond- family stuff this weekend.
I checked coil connections- coil connection is tight, and small wire connections are clean and tight.
I’ll drive it as soon as the weather settles down and check the suggestion on the carb. However, if I adjust the idle up using the idle screw, the engine doesn’t die.
What is your idle RPM in both park and drive?
i don't have a tach- any suggestions for a handheld tach?
To clarify:
So as you pull into the driveway it dies while you are rolling or as you stop the car?
It will then instantly restart, run for a moment and then die again?
It dies as I'm pulling into the driveway while rolling. It will instantly restart, and unless I feather the gas pedal, will die again. I can then readjust the low idle screw, start the car, and it will stay running.
Initial idle adjustment is done with the solenoid. Make sure it is operating correctly. It should extend any time the ignition is in the ON position. But it is not strong enough to push the throttle open, it is strong enough to keep it open. It's to avoid dieseling when turning the car off. So proper operation is to turn key to ON position, press down gas (to open the throttle a bit) and then start car.
the solenoid is functioning as you described. I electrically disconnected it to make sure that it wasn't putting any resistance to the idle mechanism when an adjustment to the low idle screw was made- trying to eliminate variables...
It seems like the symptoms are consistent. Let it die and then look down the carb and pump the gas once. You should see a squirt of fuel. Pump it a couple of more times and you should see it squirt again. If it does you have fuel in the fuel bowl.
after it died, i gave it throttle and verified that there is fuel squirting into the carb above the throttle plate.
Next check that the float is set correctly. It's possible the float is too high and causing it to flood at idle.

A vacuum leak is also possible. Confirm all of the connections are good. Do you have a lot of the original emissions in the vacuum system?
i replaced 2 vac lines that had a questionable fit- the rest of them look ok and seem to seal ok. It's pulling 16" of vacuum, so wouldn't that indicate that the vacuum is ok?
Smog stuff- I don't have a good basis for comparison, but i'm sure that it has been altered. The PCV line from the drivers valve cover connects to a port in the back of the carb. There is a line from the passengers side valve cover that connects to the air cleaner.
 

Crestline

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Update:
I drove 1/2 mile to the intersection with the highway and the engine remained running during the U-turn, but the idle was faster than what i set it at yesterday sitting in the shop. i drove it home without incident turning back into the driveway. I turned around in my drive and went for round 2; it was idling lower than the first trip (about where I had set it yesterday in the shop). It died when i pulled up to the stop sign, started right up,,
made the u turn and drove it 1/2 mile home where it died pulling into the driveway. Started it back up and nursed it into the shop by feathering the gas peddle. I let it die and verified that gas would squirt in the carb.
 
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My all time favorite vehicle is our 1969 Shelby GT500
Those 2100/2150 are simple and reliable carburetors. But, they do have some weaknesses worth looking at when the carb performance falters, such as a sudden rough idle, or higher than normal idle speed. If you are blowing black exhaust smoke at the time it is running poorly, or upon restart, you are using too much fuel. I would look at the Power Valve on the carburetor, they do tend to leak and rupture, which allows raw liquid fuel to be sucked out of the fuel bowl right into the intake manifold vacuum.

I am on the road and away from you PDF library, so you will need to dig up some YouTube videos on "2150 Power Valve" and get a feel for what you are about to do (rebuild the carb). You will need a rebuild kit, they re easy to find and still available. If you get any liquid fuel coming out of the Power Valve cover when it is removed, you found your problem. Rebuild kits have new Power Valves, and sometimes more than one Power Valve Gasket to choose from. Select one that does not block or impede the little fuel jets/openings of the inner part of the valve as it is threaded into place. YouTube is your friend when it comes to seeing the rebuild steps.
 

Crestline

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73 Mustang convertible
Those 2100/2150 are simple and reliable carburetors. But, they do have some weaknesses worth looking at when the carb performance falters, such as a sudden rough idle, or higher than normal idle speed. If you are blowing black exhaust smoke at the time it is running poorly, or upon restart, you are using too much fuel. I would look at the Power Valve on the carburetor, they do tend to leak and rupture, which allows raw liquid fuel to be sucked out of the fuel bowl right into the intake manifold vacuum.

I am on the road and away from you PDF library, so you will need to dig up some YouTube videos on "2150 Power Valve" and get a feel for what you are about to do (rebuild the carb). You will need a rebuild kit, they re easy to find and still available. If you get any liquid fuel coming out of the Power Valve cover when it is removed, you found your problem. Rebuild kits have new Power Valves, and sometimes more than one Power Valve Gasket to choose from. Select one that does not block or impede the little fuel jets/openings of the inner part of the valve as it is threaded into place. YouTube is your friend when it comes to seeing the rebuild steps.
I’ve rebuilt the carb which included new power valve, and the engine starts and runs good. No black smoke when idling or when I kick it.
 
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To get an RPM reading get a timing light that includes RPM readouts. A 'dial back' timing light is the best choice, you can adjust the flash of the timing light to the current TDC to see your current timing and then set a value of advance that you want and adjust the timing to that value. They are super handy.

16 inches is 'pretty good' vacuum but not as high as I would think you can get with a relatively stock 351C 2v car with a 2bbl carb.

Have you adjusted the idle mixture to 'best vacuum'?
 
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New power valves can be bad out of the box. All it takes is one small backfire to rupture one.
Another possibility is the accelerator pump isn't adding enough fuel on acceleration.
I suspect you have a vacuum leak. Does your car still have any of the ported vacuum valves in operation, or an EGR valve? It seems like the vacuum leak gets worse as it warms up.
 
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I had this same type of issue with my 1972 442 with a Rochester 4V carb. I know my Holley carbs in and out, but not the Rochester's. Anyways, it would do exactly what your is doing. One other issue I had was with the car being hard to start when cold. You would start the car and you had to keep it accelerated by the pedal for a long time before it would actually hold an idle even though the electric choke was working fine. This carb was rebuilt by a Rochester guru and was supposed to be a drop in and go type of deal with minimal adjustments. Anyways, I found out that these Rochester carbs have a fast idle adjustment of the passenger side of the carburetor, that if not adjusted correctly will cause all types of havoc with cold starting and idle. Seems that the Rochester guru, just left the whole adjustment loose and it was not engaging. When I finally figured this out and adjusted the fast idle screw everything magically went away. You pump the carb a couple of times, and it just starts and runs at high idle and when it warms up it will fall into a regular idle. and the idle issues with the car either being too high and snapping into gear with a chirp, or too low and just dying as soon as you put it in gear went away. I have no idea what type of fast idle adjustment the 2V Motorcraft carbs have, but I am sure that they have one. Check yours out and adjust it to factory specs.

Only other thing I can think of is that you have a problem with your needle and seats leaking. This is easy to check when the engine is running. Just take your air cleaner off and look down the carb and see if you see any fuel leaking out of the boosters, if you do then your needle and seat is not sealing correctly. When it is a small leak the engine will sometimes start and run okay as when cold you need a richer mixture, but as soon as the engine warms up the leaking will just kill the engine at idle.
 

piper62j

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Years ago, I had the same problem with a 400 C.I. Ford Mustang. After hours of chasing vacuum leaks, carburetor and ignition issues, the problem ended up being the intake manifold gaskets.. They were no longer sealing between the heads and the manifold towards the lower end where the manifold met the block. The engine was original from the factory and the gaskets had just disintegrated. Machining the manifold and installing new gaskets solved the dying out at irratic times.. The manifold was cast iron and the only conclusion we could come to was that it had warped over the years..
 

steves73

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When my points went bad it was similar to what you're experiencing. Used Pertronix ever since.
 

Superbond

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If you use Pertronics make sure you have 12 volts going to it of it will run like the issues your haven
 

Crestline

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I would take a close look at the connections for your coil, making sure you have good connections. Then at next unintended engine shut down, look down the carb and listen for fuel squirting while actuating the throttle lever. Then you will know if it’s ignition or fuel and can go to next step.
it has a pertronix flamethrower coil and pointless ignition., but I verified that the coil connections are tight and that the small wire connections are clean and tight.
I verified that there is fuel squirting onto the throttle plates after it died.
(I responded to your questions a couple of days ago on my phone, but i see that my message didnt go out- metal enclosed shop doesn't help poor signal...)
 

Crestline

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Those 2100/2150 are simple and reliable carburetors. But, they do have some weaknesses worth looking at when the carb performance falters, such as a sudden rough idle, or higher than normal idle speed. If you are blowing black exhaust smoke at the time it is running poorly, or upon restart, you are using too much fuel. I would look at the Power Valve on the carburetor, they do tend to leak and rupture, which allows raw liquid fuel to be sucked out of the fuel bowl right into the intake manifold vacuum.

I am on the road and away from you PDF library, so you will need to dig up some YouTube videos on "2150 Power Valve" and get a feel for what you are about to do (rebuild the carb). You will need a rebuild kit, they re easy to find and still available. If you get any liquid fuel coming out of the Power Valve cover when it is removed, you found your problem. Rebuild kits have new Power Valves, and sometimes more than one Power Valve Gasket to choose from. Select one that does not block or impede the little fuel jets/openings of the inner part of the valve as it is threaded into place. YouTube is your friend when it comes to seeing the rebuild steps.
As mentioned, i have rebuilt the carb. there isn't any black smoke coming out at start up, idle or while driving.
 

Hotstang

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As Superbond said check for 12 volts going to the petronix. This might require bypassing the internal resistor wire that connects the ignition switch to the + side of the coil. The resistor wire cut the voltage down to prolong point life but will play havoc on a petronix.
 

Crestline

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I had this same type of issue with my 1972 442 with a Rochester 4V carb. I know my Holley carbs in and out, but not the Rochester's. Anyways, it would do exactly what your is doing. One other issue I had was with the car being hard to start when cold. You would start the car and you had to keep it accelerated by the pedal for a long time before it would actually hold an idle even though the electric choke was working fine. This carb was rebuilt by a Rochester guru and was supposed to be a drop in and go type of deal with minimal adjustments. Anyways, I found out that these Rochester carbs have a fast idle adjustment of the passenger side of the carburetor, that if not adjusted correctly will cause all types of havoc with cold starting and idle. Seems that the Rochester guru, just left the whole adjustment loose and it was not engaging. When I finally figured this out and adjusted the fast idle screw everything magically went away. You pump the carb a couple of times, and it just starts and runs at high idle and when it warms up it will fall into a regular idle. and the idle issues with the car either being too high and snapping into gear with a chirp, or too low and just dying as soon as you put it in gear went away. I have no idea what type of fast idle adjustment the 2V Motorcraft carbs have, but I am sure that they have one. Check yours out and adjust it to factory specs.

Only other thing I can think of is that you have a problem with your needle and seats leaking. This is easy to check when the engine is running. Just take your air cleaner off and look down the carb and see if you see any fuel leaking out of the boosters, if you do then your needle and seat is not sealing correctly. When it is a small leak the engine will sometimes start and run okay as when cold you need a richer mixture, but as soon as the engine warms up the leaking will just kill the engine at idle.
I've watched the choke/ fast idle pull off move as the engine warms.
If you use Pertronics make sure you have 12 volts going to it of it will run like the issues your haven
The voltage at the terminals on the Pertronix is running at 10.6v (+/- a few tenths due to readout of the digital VOM bouncing a little). Isn't there a dropping resistor on some systems?
 

Crestline

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73 Mustang convertible
As Superbond said check for 12 volts going to the petronix. This might require bypassing the internal resistor wire that connects the ignition switch to the + side of the coil. The resistor wire cut the voltage down to prolong point life but will play havoc on a petronix.
I'm measuring 10.3v at both small terminals on the Pertronix. How do I bypass the internal resistor?
 
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