74 Duster with 408 Stroker With Dieseling/Idle Issue.

[TimS]

Just bought my old 74 Duster back. Got the original 360 but it has been transplanted with a 408 Stroker motor. It also has a Holley QFT on Weind intake. I can’t get a happy medium between decent idle when cold and excessive run on at running temp. I’m thinking of adding an idle stop solenoid. Anyone have a positive experience going this route with the issue I’m having?

Thanks!!

 

[RustyRatRod]

This almost always comes from retarded timing. Since the timing is too far retarded, you have to open the throttle blades on the carburetor too much to get it to idle and that leads to engine run on. Where is your initial timing set?

 

[dano]

This almost always comes from retarded timing. Since the timing is too far retarded, you have to open the throttle blades on the carburetor too much to get it to idle and that leads to engine run on. Where is your initial timing set?

This.

 

[Bewy]

Agree with above, but the rot cause is probably idle rpm too high, caused by insufficient ign timing at idle.
- man or auto trans
- what cam specs
- what idle rpm

 

[TimS]

Car is 4 speed. It has a Comp cam with .484 lift and idles at 700. I've tried a couple of times just advancing the timing a little bit at a time but just can't seem to find a happy medium.
Why I was considering the Holley Idle stop for those carbs but that thing is salty with varying reviews.

 

[CRUZE 418]

My 418 would run on when hot, with 22 intial timing. For me, my t-slot was rectangular, so turned the idle screw out till it was square. Re adjusted the air/fuel screws and it shuts down just fine now. If it doesn't idle at start up , the choke is probably out of adjustment.

 

[TimS]

My 418 would run on when hot, with 22 intial timing. For me, my t-slot was rectangular, so turned the idle screw out till it was square. Re adjusted the air/fuel screws and it shuts down just fine now. If it doesn't idle at start up , the choke is probably out of adjustment.

I will check that out. I admit I forgot about adjusting air/fuel mixture on the carb while playing with timing and idle screw adjustments.

 

[Bewy]

The benefit of increasing idle timing is very simple: the engine makes more HP. That, in turn, allows the idle rpm to be reduced & the engine will not only idle at a lower rpm, but the idle will be smoother. A man trans car is more of a challenge with this problem, as it does not have the load of a c'ter trying to stall the engine.
Examples of more idle timing.

 

[CRUZE 418]

You did not mention the T-slot. Did you take your carb off and have a look at the T-slots? To see if they were rectangular or square? Then warm your engine and adjust the air/fuel. I always ignored this T-slot thing, it does matter.

 

[TimS]

You did not mention the T-slot. Did you take your carb off and have a look at the T-slots? To see if they were rectangular or square? Then warm your engine and adjust the air/fuel. I always ignored this T-slot thing, it does matter.

Sorry to be stupid but I’ve never heard the term t-slot. Please explain.

 

[TimS]

This almost always comes from retarded timing. Since the timing is too far retarded, you have to open the throttle blades on the carburetor too much to get it to idle and that leads to engine run on. Where is your initial timing set?

Initial timing is 24.

 

[CRUZE 418]

Remove your carb and turn it upside down. There is a vertical slot in each base or throttle plate, the throttle blades close by it. If your idle screw is turned in too far, this slot appears to be rectangular instead of square. I'm sorry, I do not have a spare Holley carb to take a picture.
Maybe someone else can take a picture, or you can Google it. Idle transfer slot, it allows fuel to enter at idle. Getting that slot close to square stopped my run on.

 

[RustyRatRod]

Remove your carb and turn it upside down. There is a vertical slot in each base or throttle plate, the throttle blades close by it. If your idle screw is turned in too far, this slot appears to be rectangular instead of square. I'm sorry, I do not have a spare Holley carb to take a picture.
Maybe someone else can take a picture, or you can Google it. Idle transfer slot, it allows fuel to enter at idle. Getting that slot close to square stopped my run on.

Along these same lines ^^^^^ Flip the carburetor upside down, locate the secondary throttle stop screw. It is only accessible from the under side. It is a small flat slot screw that gives the secondary throttle plates a stop. Sometimes, they are open too far and pull in too much air not allowing the carburetor to idle down as much as it needs. The way I adjust mine is, I turn the screw all the way out until the secondary blades start to "stick" closed and then adjust the screw until they just will not stick anymore. That's as far closed as the can go. That said, it STILL might not have enough initial timing. My Ford truck for example rumbles along with 37 degrees initial timing, BUT, I have the vacuum advance on manifold vacuum, so that's a little different. But nonetheless, keep that in the back of your mind after you double check the carburetor.

 

[TimS]

I’ll check those out on the carb. I didn’t think of the secondary plates hanging open as well.

 

[TimS]

I did find the secondary plates hanging open a bit so I corrected that issue.

I'm also considering removal of this 850 QFT and going with something a little milder and smaller.