'74 Duster - stalls easily

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Max_118

Max_118

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Hello! I have a '74 Duster with a 318, 2 barrel carb. A very solid car, no rust, and less than 100,000 miles on it. It will start easily, and idle fine after it warms up. However, if I take it for a drive (just around the block so far) it will stall when I let off the gas. I made 100% sure the engine was at it's normal operating temp while doing this. I'm using fresh gas. (I don't know too much about cars, but I suspect this may be a carburetor issue.) I plan on giving it a tune up to see if that helps, but in the meantime if someone has some pro tips, I'd appreciate it.

Thanks!
- Dave
 
Is the idle RPM correct? You said you're new to cars, so I'm just trying to help with the basics. I'm not insulting you. Paul.
 
70dart340 said:
Is the idle RPM correct? You said you're new to cars, so I'm just trying to help with the basics. I'm not insulting you. Paul.
Click to expand...

No insult taken! I'm here to learn and get other people's thoughts, so it's all good.

I adjusted the carb idle (my ignorance is showing here - I turned the little flathead screw on the carb) until the idle was slightly higher than normal. I know by the sound about what the idle should be. This was before I tried driving it, so the issue was still there.
 
Make sure the choke blade is vertical at operating temp, & not flopping around. flopping around indicates a failure of the choke system.
 
Usually i find a dirty carb can cause this issue. Idle circuits restricted. Adjusing idle mixture screws are the telltale when there is no change when adjusting for a better idle.
 
If you never have, I would just do a through and complete tuneup

1...Do a compression test and leakdown test. I don't remember if the 74's have adjustable valves if so check them
2...Examine all ignition components condition, wires, cap, rotor, etc, and set timing. Don't set it to "stock" which is likely TDC. Set it for at least 5BTC
3...I agree that the idle problem is likely a carb "getting" (gotten) dirty. You might get lucky. Sometimes you can pull the idle screw, blow some cleaner through it, then reset. Pull it again idling, work the throttle and hold your finger over the screw hole. This will suck air through and also help clean it

Set timing BEFORE you adjust carb. Set carb fully warmed. Make CERTAIN you have not lost track of any vacuum hoses and let them open. Adjust mixture for highest vacuum or idle speed, and "finish" with screw "just leaning" towards lean (CW) side. Reajust idle speed and mixture at last
 
Orginal engine? That engine has a nylon tipped teeth on the top timing gear, they can start to fall apart at around 100,000 mi. Heat crossover is starting coke up with carbon. Flat lobes on camshaft, heads need hardened seats. This is most probably not your problem, but be aware that all of these things will start to add up. Tearing down my '72 318 right now and besides hemorrhaging oil, have found all of this, and it ran fine.
 
While engine is running take a can of starting fluid or brake cleaner and spray it around the base of your carb. Sounds to me like a bad vacuum leak. Good luck
 
Duster 72 said:
While engine is running take a can of starting fluid or brake cleaner and spray it around the base of your carb. Sounds to me like a bad vacuum leak. Good luck
Click to expand...
I used to use flammables until i lit up an engine bay. Water works too!
 
So there might be a couple different Flathead screws on that carburetor. One or two of them can be for changing the mixture and another one for changing the idle. Just to be clear the one that changes the idle you can see it slightly pull on the throttle cable as if you're actually giving it more gas. The ones probably towards the bottom are changing the air fuel mixture. But everything on a tune-up starts from timing and fresh components as in cap rotor plugs wires and likely do you have points which I always recommend to do patronics which are more expensive to start but zero maintenance in the end...
(And please let's not let that start yet another debate this guy doesn't need it)..
Oh yeah and some new spark plugs and take a look at what the old ones look like..
Post picture of a couple...
 
Max_118 said:
It will start easily, and idle fine after it warms up. However, if I take it for a drive (just around the block so far) it will stall when I let off the gas.
Click to expand...
This is a common symptom of the throttle blades being too far closed and/or the mixture screws set too lean. This is often a by-product of too much Idle-Timing.
Altho the PCV could be faulty.

EDIT
Another possibility is a wrong "wet fuel level" in the bowl; likely low, but could be high as well.or a contaminant in the fuel, but this is less likely.
In any case, your carburator circuits are all designed to function, around a specific "wet" fuel level. The spec in "the Book" is a "dry" adjustment, designed to achieve that specific "wet" level. The spec is usually pretty accurate.
Whenever working on a cantankerous carb, this is always the first go-to.
 
Last edited:
You need to do TWO things now, in this order & report back; both will tell a story......
- remove the PCV & shake it; it should rattle. If it doesn't it is gummed up or faulty.
- noting the slot position of the screw, counting the turns, back off the idle SPEED screw until the t/blades bottom out.
 
Go here and download the 72 Duster service manual. I know. It's 72 but that's the latest they have. It should get you in the ballpark. Then, follow post #7 to the Nth degree. Once you get it in proper tune, see if it's not fixed. Report back with what you find. Here's the link for the service manual. I'm sure there are differences, but it should get you close. Just scroll down and find it. It's there.

Service Manuals – MyMopar
 
Here your starting points.

67Dart273 said:
If you never have, I would just do a through and complete tuneup

1...Do a compression test and leakdown test. I don't remember if the 74's have adjustable valves if so check them
2...Examine all ignition components condition, wires, cap, rotor, etc, and set timing. Don't set it to "stock" which is likely TDC. Set it for at least 5BTC
3...I agree that the idle problem is likely a carb "getting" (gotten) dirty. You might get lucky. Sometimes you can pull the idle screw, blow some cleaner through it, then reset. Pull it again idling, work the throttle and hold your finger over the screw hole. This will suck air through and also help clean it

Set timing BEFORE you adjust carb. Set carb fully warmed. Make CERTAIN you have not lost track of any vacuum hoses and let them open. Adjust mixture for highest vacuum or idle speed, and "finish" with screw "just leaning" towards lean (CW) side. Reajust idle speed and mixture at last
Click to expand...

AJ/FormS said:
This is a common symptom of the throttle blades being too far closed and/or the mixture screws set too lean. This is often a by-product of too much Idle-Timing.
Altho the PCV could be faulty.

EDIT
Another possibility is a wrong "wet fuel level" in the bowl; likely low, but could be high as well.or a contaminant in the fuel, but this is less likely.
In any case, your carburator circuits are all designed to function, around a specific "wet" fuel level. The spec in "the Book" is a "dry" adjustment, designed to achieve that specific "wet" level. The spec is usually pretty accurate.
Whenever working on a cantankerous carb, this is always the first go-to.
Click to expand...
 
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