Incredibly Fast Rev After Routine Maintenance

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J

Jim McLain

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Jacksonville, FL
I bought this 75 Dart (225 slant 6, Holley 1945 single barrel carb) about 4 weeks ago and have been fiddling with something or other just about every day since. The car has started up fine and idled okay every time I fired it up. Only problem is it coughs and sputters when you give it some gas. So, last weekend, I changed out the plugs and wires and the fuel filter and pump. Upon completion, I fired it up and it revved like it was trying to take flight. When I went to look at the fast idle adjustment screw, it noticed it and the spring are missing, and the lever was stuck in the wide open throttle position. I touched it and it released back to the normal idle position.

Question - Is the fast idle lever supposed to set back in the wide open throttle position like this or do I have a kink in my linkage somewhere?

I know the engine revs higher when cold and then releases to normal when warm, but full throttle? That seems a bit much.

Also, I don't want to be going down the road and hit it and have the carb stick in the full open position.

I appreciate your thoughts on this.

Jim
Jacksonville, FL
 
The fast idle cam only swings around to function with the closing of the choke. When the engine initially starts cold, it is opened slightly by a vacuum diaphragm which sets the fast idle cam to about mid-ramp. Then as the choke opens it swings out of the way to curb idle. Sounds like diaphragm is not working (not allowing the cam to reset) or you bumped the linkage and jammed it temporally. You need to replace that screw and spring to restore fast idle function, not doing so opens the possibility of this happening again.
 
Roger that. Thanks Murray. I'll have to check to see if the diaphragm is even connected. Since it's a 43-year-old Dart, no telling what else I'm going to discover.
 
Usually the choke on a carb is thermostatically set with a spring that pushes/pulls when hot. When cold it pulls (eg) on a choke cam that wants to set the choke plate but is blocked by the throttle linkage. When you give the throttle 1 push prior to a cold start, the lever that blocks the choke actuator releases and the spring sets the choke
When it's set the fast idle cam swings up and the cam sets a new baseline for the throttle to stop at via that fast idle screw. There is no spring that I'm aware of on the carb itself besides the integrated throttle return spring. The vacuum unloader pot pulls the choke open a tad at full throttle If i remember right. When the bimetallic spring heats up, it loses tension on the choke and gradually releases the choke plate/cam to lower and lower fast idle stops until it completely drops it down to normal idle. It does this automatically but I believe the tension of the fast idle screw on the cam is enough to hang it until you tap the throttle again and then it falls down to it's new state. Sometimes in stages, sometimes in one step depending on how long you warm it up. When it works, it's literally a pump once and turn the key with no other throttle interaction. It'll fire right up due to the very strong vacuum signal the choke plate creates and the slight crack of the throttle that the fast idle hangs up on. Those vids of guys pumping the throttle like its CPR on the motor obviously have a choke issue.
 
Jim McLain said:
When I went to look at the fast idle adjustment screw, it noticed it and the spring are missing, and the lever was stuck in the wide open throttle position. I touched it and it released back to the normal idle position.
Click to expand...
What was stuck in the "wide open"position?
Was the choke blade wide-open? or
was the main throttle valve wide open? or
what?

With no fast idle-screw and no link-rod, the fast-idle cam, by your description, is inoperative. and nothing was actually "wide-open". It sounds more like your throttle return spring is missing, and the main butterfly simply stuck a lil open
But IDK
Maybe somebody before me guessed right,my head hurts
 
AJ/FormS said:
What was stuck in the "wide open"position?
Was the choke blade wide-open? or
was the main throttle valve wide open? or
what?

With no fast idle-screw and no link-rod, the fast-idle cam, by your description, is inoperative. and nothing was actually "wide-open". It sounds more like your throttle return spring is missing, and the main butterfly simply stuck a lil open
But IDK
Maybe somebody before me guessed right,my head hurts
Click to expand...
Sorry AJ. I wish I knew more about carburetors to explain it more clearly. Unfortunately, this is my first experience with carbs.
 
I'm obviously confusing some with my lack of knowledge.

Here's a couple of pics to hopefully explain what I'm trying to say (hopefully I uploaded these images correctly). Where I say the "full open" position in the first image, I mean a high idle position. I understand that "full open" would be set much further back.

Am I missing any linkages that anyone can tell?

Thanks for looking.

Jim

carb1.jpg


carb2.jpg
 
Here's what my 1945 looks like. Please disregard the incorrect screw used on the throttle cable. It has since been replaced with the correct parts.
IMG_20180210_110734_183.jpg
 
Jim McLain said:
Looks like mine is missing a spring on the fast idle lever.
Click to expand...

Yeah, throttle return spring missing? When you let up on the gas pedal, the spring returns the throttle plate to the idle position. Is there any resistance when you step on the gas? That would explain the high engine speed when you started it up again.

And you ARE missing the fast idle adjustment screw as you thought.

If nothing else, you can go to your local hardware store and rummage through their spring selection to get you back on the road. Measure the length you need to go from point A to B and pick one a little shorter. Or adjust the length with wire cutters and needle nose pliers.

Since you are in Florida you can wire the choke open temporarily to keep it from getting in the way of normal operation until you fix it.

If you have not moved the car recently, the parts may be on the ground under the car.
But the best thing to do is get the correct parts. Can anyone on here help him out?
 
Thanks Kosmic -

I don't feel resistance when I step on the gas, but when I do, the throttle sticks open and the engine continues to rev high (because of the missing spring, I'm assuming). I've checked all around the garage floor and found nothing, so I'm gonna go the auto parts store route.

Thanks again and keep that advice coming. Maybe one day I'll actually understand what I'm doing.

Jim
 
Keystone -

Thanks for the image. It and Kosmic's discussion explains a lot. One more question though - I see where your fast idle return spring connects to the fast idle lever, but where does the other end attach? On mine, I see the accelerator cable and it looks like there's a connection point, but I'm just not sure.

Thanks, Jim
 
Here's another picture I took when looking for the pin for the throttle cable.
2.PNG


And here's an illustration from the FSM.
3.PNG
 
A quick update: Got all of the springs and screw I needed from the auto parts store, hardware store, and my bench spares bins and now she idles good. No longer sticking in hi rev. Now she just needs a real tune up.

Thanks to all. You guys rock!

Jim
 
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