Quick fuel Q750 tune ?

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Junior340

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On my 340 with a Q750 the left rear idle mixture screw wants to be pretty much bottomed out to get best idle/vacuum, all the others are to me are normal around one turn out. If I turn that one out much the engine will die.
Is this anything to worry about.
KIMG0964.jpeg


Thank you
 
Take it apart for a clean and inspection. I've seen this happen when a feed hole or bleed is plugged.

I've also seen the butterflies come loose and move around, causing less air flow through one Venturi and change screw adjustment.
 
OK thanks yellow I will look into that. The carb. is brand new but still could get some dirt in there.
 
I agree. Doesn't matter if its new. Could be a shaving from the drilling.

If you have a setup the really wants stagger jetting, could be reflected in the idle mixes.

Also if your setup doesn't really need the extra fuel from a four corner idle, then 1 turn out is too much. There is no normal for a carb. There's a range in which it will respond to what the engine wants. If its down around 1/4 to 1/2 turn out on each mix screw, or 2 turns out on each mix screw - then somethings up. Otherwise its just a fuel trim adjustment for what the engine wants.
 
Due to variations in manufacturing tolerances, the needles may not be made exactly the same, nor the seats. That means they will not be the same amount of turns. This does not matter at all. Adjust them to how it runs best and don't worry about it.
 
Blow that circuit port out , with air compression, and an air gun. My thoughts.
 
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Use a pencil eraser to block each idle bleed one at a time. It should go rich while doing this. If you get very little change that air bleed circuit is restricted. I run a dual plane and stagger jet. All 4 main jets are different sizes and idle mixture screws are set to best idle and off idle transition. Plug reading at cruise and WOT gets a tune dialed in, it's time consuming but worth the effort.
 
Thanks for the input people, I need to get out there and try some of these out
 
Thanks for the input people, I need to get out there and try some of these out
Running the same carb on my 68 340 cuda, had to jet down to 66 with a 4.5 powervalve in the primary & 72"s in the back with no pv. Idle screws are about 1 1/4 turns on all 4 corners. Running mopar purple cam, 260/260 @ .50. Hope this helps.
 
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