Please help my 5.9 Magnum Duster keeps stalling at idle and light throttle

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carmart1975

carmart1975

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DRIVEABILITY DIAGNOSTIC – 1975 PLYMOUTH DUSTER


VEHICLE / ENGINE


1975 Plymouth Duster
5.9 Magnum V8 swap (LA timing cover)
904 Torqueflite transmission
Chrysler 8.25 rear, 3.21 gears, open differential

SYMPTOMS

  • Severe drivability issue at idle and light throttle
  • Engine instantly transitions from normal idle to stall (“on/off switch” behavior)
  • Random cut-out / stall at idle and part throttle
  • Restart behavior inconsistent:
    • Starts then immediately stalls
    • Will start and continue running if slight throttle is applied during crank
  • Light throttle cruise has consistent miss/stumble
  • Runs strong and clean at wide open throttle
  • Issue worsens when engine is warm
  • No issues at WOT or heavy load

DIAGNOSTIC NOTES
  • Condition is throttle-position / light-load related, not RPM related
  • Initial timing tested at 8°, 10°, 12°, 15° with no change
  • Total timing ~40°+
  • Vacuum advance connected
  • Mechanical advance verified working
IGNITION SYSTEM

Stock Mopar-style ignition box
Multiple ignition boxes tested
Multiple distributors tested
New coil
New ballast resistor
All ignition wiring and connectors replaced
Engine and chassis grounds cleaned and verified
Distributor pickup air gap checked and reset on two distributors (~0.008”)

FUEL SYSTEM

Holley Blue electric fuel pump (110 GPH)
Fuel pressure regulated at 6.5 PSI
Holley 4150 Double Pumper 750 CFM
Power valve: 6.5

ENGINE

5.9 Magnum V8
Clay Smith camshaft (cam card attached) it says on the cam card measured at 1.6 ratio, but it was really measured at 1.5 but stock Magnum rockers are 1.6. I wanna say it was like 533 and 509 at 1.6 not too sure.
It’s on stock replacement jegs Magnum heads
Has Hughes spring kit


Please help me — I’ve been fighting this for weeks now and it’s extremely frustrating. I don’t even know where to go from here.

IMG_5578.jpeg
 
I would say try a different carburetor on it. Takes a lot of guess work out of it for diagnostics.

Edelbrock 1406 600 cfm are good running straight up simple carburetors, that are bolt on and go, without a bunch of adjusting required.

If you were closer by I would let you try one of my known good working spares.

I don't like having to goof around with carburetors to get things to run right, that's why I go to the 1406 on the small blocks. Frees me up to do the more enjoyable things on the cars.

Screenshot_20250410-212641_Gallery.jpg




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OK diagnostic guy.....
How long has his combination been in this car and did it ever runs properly. ?
The 5.9 was a computer controlled Throttle body injected truck engine.
Since you are now using an LA timing cover, you are also using a camshaft designed to be used with an eccentric pump cam. and Standard distributor. Is this supposed to be a performance upgrade ?

No computer, TPS or any other sensors designed for this engine.
Again has this engine ever run properly in this application ? Any reason why a 904 trans, or the 8.25 rear ?
Without any information other than taking your word for ir that it runs fine at WOT, that just says it needs lots of fuel for the amount of air that you are putting through it. Could be Bad in take/ vacuum leak of sometype. Internal or ??
you have already been trowing all the possible darts at it.
Just some advice for you.
Always start at the beginning and never assume that anything is working properly.
When doing diagnostic work, never make more than one change at a time.
Always start with a fully charged battery that will pass a load test easily.
Everything runs off of the battery. Ignition, Fuel pumps, charging system etc..
Always go back to the beginning. 90+ percent of the time, running problems are just very basic problems.
Already replaced something twice ? probably not there.

Spark, fuel, timing
A picture is worth a thousand words. Typed out notes don't show a thing.
Really want good help ? Post a lot of clear, close up pictures. Remember that you are the only one there.
We can't see anything. Try to diagnose a problem with your eyes closed...

Click here -----> Magnum Swap: Chrysler LA parts - what works and what doesn't? |
 
Last edited:
About 15 days ago, I did the full swap on my first drive. The car ran really decent for a first drive. None of these things that I listed were a problem. The next day I go to drive it again. It starts to run pretty bad. It’s having all the same symptoms that I have listed and also it would not accelerate or except wide open throttle. That’s when I checked the pick up coil air gap and it was way too big. I redid it to 8000 and it slowly walked itself back out made a warranty claim on the distributor. They sent me a new one and And the only problem that went away was the stumble and backfire, and what not at wide open throttle, but I still have a stumble and miss at partial throttle and randomly stalls at idle.
 
what coil? have you swapped that to see if the problem persists.

Ω of coil & ballast are correct for box?
 
OK again, Spark, fuel, timing.
Ran great but not next day.
No change in weather, put new fuel in empty tank before you ran it the first time ?
Timing s different since you changed the distributor.
Correct firing order ? 18436572 clock wise.


 

Things I would check

Power valve bad?

Also are you running a standard fuel pressure regulator or a bypass regulator? Should be a bypass with a return to the tank when running an electric pump.

Since it gets worse when hot I would say it fuel related

I would get this style fuel line at the carb and a bypass regulator. The regulator then goes on the return line to the tank.

Pardon Our Interruption

Steve 107.JPG
 
As others have suggested swapping out the Holley for a known good (not a used one that has been sitting for a long time) carburetor would be an easy first step. If you don't want to do that troubleshooting step I would start with the basics of setting up the carb, timing, and fuel pressure. What you are describing could be something as simple as a stuck carb needle valve, improper fuel float adjustment, maybe combined with an accelerator pump problem. The fire up and then engine dying from initial idle without a stall trying to increase engine RPM sounds like too little or too much fuel entering the intake manifold.

Edit to note: Any carb you use on an engine, especially a modified one, should be setup correctly first and then tuned, after basic setup steps, along with timing and spark (plug heat range)
 
George Jets said:
I would say try a different carburetor on it. Takes a lot of guess work out of it for diagnostics.

Edelbrock 1406 600 cfm are good running straight up simple carburetors, that are bolt on and go, without a bunch of adjusting required.

If you were closer by I would let you try one of my known good working spares.

I don't like having to goof around with carburetors to get things to run right, that's why I go to the 1406 on the small blocks. Frees me up to do the more enjoyable things on the cars.

View attachment 1716546839



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Click to expand...
My 4779 Holley 750 DP worked great right out of the box. Weird ain't it? Maybe Holleys aren't as finicky as you believe. I ran an air cleaner like yours about 40 years ago. Every time I took the element off, I could see dust on top of the air cleaner base and carb.
 
Do you have a way to hook up a fuel pressure gauge that you can see while driving? I'm thinking weird stuff like crap in the fuel tank intermittently blocking the pickup. Under hard accel, the g-forces could move the garbage away from the pickup and run fine?
 
George Jets said:
View attachment 1716547417


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Click to expand...
Lmao. Rich idle caused by a blown power valve. More AI crap. Anyone who knows how any carb functions knows that the power enrichment (power valve) adds fuel to the mains, IE the booster. The booster does not flow ANY FUEL at idle, and the power valve adds no fuel to the idle circuit. Therefore you can take the power valve out of the metering block and it will not cause a rich idle. I feel like AI is making people dumber.
 
George adds a lot of good information to this site. Simple is good George! Keep posting
 
Did you use Edelbrock intake gaskets by chance? I had a set delaminate and suck into the ports. It ran great and idled good but would miss bad in high vacuum conditions
 
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