Super high Idle speed?

[August07]

I just got my 72 Grande, it’s a 351C with automatic transmission. It seems to hate starting if it’s not warm and whenever I start it it idles at about 3000 rpm until i drive it for about 20 minutes and it heats up. Only then does it have a normal idle speed. Any suggestions on a fix?

 

[Sheriff41]

What carburetor do you have? It will have a fast idle adjustment, but they vary on different carbs.

 

[Stanglover]

Hello and welcome from SW Ontario,, Canada.
As it seems like you just got your Mustang, you may not be too familiar with them.
As Sheriff says, can you first tell us what carb is on it, take a pic and post if not sure.
If it seems to run better after it's warmed up fully, it could also be the choke mechanism.
It may also need a proper cleaning or even a rebuild, but first do the simple things, check choke or sticking linkages.
We have an "Introductions" section and we'd love to learn more about you and your car WITH pics, we love pics!!
All the best getting it running better.

 

[Big Red Mach 1]

What's it drop to when put in gear? I'd be curious to know your base timing also since it's a new to you car, but you need to get that rpm down to about 800 first.

 

[basstrix]

Those sympoms indicate that either the choke isn't getting proper heat (either from an exhaust driven Choke Stove, or electric) or the choke is too tight.

 

[vintageman]

I agree with basstrix above, sounds like choke related if it drops down after getting to operating temp. If it stayed at the higher RPMs afterwards, I would look to the fast idle needing adjustment. What is your idle after it gets to operating temps when it drops down?

Good luck
Tom

 

[giantpune]

My guess is on fast idle screw or the idle speed solenoid, depending on what carb it is.

 

[Stanglover]

I agree with basstrix above, sounds like choke related if it drops down after getting to operating temp. If it stayed at the higher RPMs afterwards, I would look to the fast idle needing adjustment. What is your idle after it gets to operating temps when it drops down?

Good luck
Tom

My thoughts too.
Also, what do the plugs look like after run at operating temp? But do one thing at a time.

 

[August07]

What's it drop to when put in gear? I'd be curious to know your base timing also since it's a new to you car, but you need to get that rpm down to about 800 first.

It drops down to about 950-1000rpm

 

[August07]

Hello and welcome from SW Ontario,, Canada.
As it seems like you just got your Mustang, you may not be too familiar with them.
As Sheriff says, can you first tell us what carb is on it, take a pic and post if not sure.
If it seems to run better after it's warmed up fully, it could also be the choke mechanism.
It may also need a proper cleaning or even a rebuild, but first do the simple things, check choke or sticking linkages.
We have an "Introductions" section and we'd love to learn more about you and your car WITH pics, we love pics!!
All the best getting it running

What carburetor do you have? It will have a fast idle adjustment, but they vary on different carbs.

 

[August07]

What carburetor do you have? It will have a fast idle adjustment, but they vary on different carbs.

It has a double barrel carb

 

[Sheriff41]

 

[Sheriff41]

 

[Stanglover]

So, it looks like the original carb. My unqualified opinion is it's mostly choke related and may just need a good thorough cleaning. I would start simple by buying a can of SeaFoam Carb cleaner with the spray tube (I love Seafoam products) or any good carb cleaner and go to it. Check that the choke is free moving and not sticking. In the diagrams, it shows how to adjust the choke, set it in the middle for now. If necessary, Google for more information.
With the engine hot and running, spray SeaFoam gently through the bores as well as adding a can to a full tank of gas, or proportionally.
One thing I did notice in your pic is a "gear clamp" on the fuel inlet to the filter. You may want to change that filter as well and the please use a proper fuel line clamp on ALL fuel line hoses. Gear clamps are known to break, causing leaks and fires. You can buy proper clamps at any auto parts store.

 

[basstrix]

Some quick things to check.

1. The connection at the choke housing (see picture). The metal tube going into the intake must be connected and will get HOT when fully warmed up (so be carful when you check its temperature).

After the 20 min run time, the choke plate should be straight up and snap open if you close it with your finger. If it doesn't, either the chokes not getting heat or it's way too tight. If the connection is hot, then the choke is too tight. If it's not hot, you'll need to determine if it's pulling air through the Choke Stove. Pull the tube off the Fresh Air port and feel for air pulling into the stove with engine running. If it's not pulling then you need to dig deeper and find out why.

2. Choke Pull Off diaphragm (also in picture). This pulls the choke plate open to some minimum amount so the engine will run when very cold. I don't think this is the root cause of your issue, but these don't usually need replacement on a first or 2nd rebuild, but by now, they are well past their prime and most likely leaking.

3. Vacuum hoses. Make sure the fast idle after the 20 min warmup isn't due to a vacuum leak. This applies if the choke snaps open when you manually try to close it.

 

[rcadd1ct]

when running try and close the secondaries by hand and see if that helps. If so you might be able to bend the linkage to force it closed.

Also you might look for a vacuum leak. I have had a worn throttle shaft allowing more air in and messing with the idle.

 

[mrgmhale]

So, it looks like the original carb. My unqualified opinion is it's mostly choke related and may just need a good thorough cleaning. I would start simple by buying a can of SeaFoam Carb cleaner with the spray tube (I love Seafoam products) or any good carb cleaner and go to it. Check that the choke is free moving and not sticking. In the diagrams, it shows how to adjust the choke, set it in the middle for now. If necessary, Google for more information.
With the engine hot and running, spray SeaFoam gently through the bores as well as adding a can to a full tank of gas, or proportionally.
One thing I did notice in your pic is a "gear clamp" on the fuel inlet to the filter. You may want to change that filter as well and the please use a proper fuel line clamp on ALL fuel line hoses. Gear clamps are known to break, causing leaks and fires. You can buy proper clamps at any auto parts store.

I am thinking it is also choke related, as have some of our other esteemed contributors to this thread. The eectric heated choke cap on an oem carb did not appear on Mustangs until 1973. If you have the oem carb it gets its heat from a stove pipe tube that is notorious for not being inserted properly onto its inlet on the passenger side exhaust manifold. Thus not enough really hot air is drawn into the inner choke cap area to cause the choke to come off timely. Other issues can interfere with the air being drawn from the air horn down to the exhaust manifold how air stove, and back up to the carb. The carb itself may just need a good cleaning. When the choke comes off the high speed idle cam will drop away and the engine should be running as its normal idle speed. It is very rare that I see the high speed idle cam needing to be adjusted, as it is really hard to get to for many folks. I do have a video on the 2100 carb that does cover adjusting the high speed idle cam in the following YouTube link.

https://youtu.be/mwLwwUtI1cE

But wait! There's more! If the carburetor is a replacement unit that has an electric choke, you should be aware that unless it is another 2100 carb, the electric choke will require 12 volts of current. I say this because I have seen filks use voltage from the "I" terminal on the starter relay for aftermarket carb electric chokes, not realizing they are gertting the same voltage the positive primary terminal on the ignition coil is getting, usually around 8 volts or so. Or, in the case of 1973 Mustangs, the oen electric choke circuitry is powered via Circuit #4, a White wire with a Black stripe that is fed by the alternator Stator terminal. The stator voltage is unusual, both cause it is AC cirrent (not DC), and it is 1/2 of battery voltage. In either case the coltage is inadequate to warm the electric choke up as quickly as needed for the high speed idle cam to drop away in a timely manner.

In the off chance your carb does have an electric choke, there are a two places to get 12 volts of switched power under the hood. The first is Circuit #640, which feeds the carburetor Throttle Position Solenoid (Red wire with Yellow hashing). The other is Circuit $63, a Red wire, used to power the Windshield Wiper motor. Splicing into either circuit will get you 12 volts without having to install a power relay (although that is a viable option it is more complicated than needed). I have attached a schematic showing those two splice points. I also have another YouTube video covering the wiring options for 12 volts of switched power under the hood/ That viddeo is at the following link:

https://www.youtube.com/watch?v=WDcskIPfLcE

i hope that helps you get past this situation