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Idle Enrichment Screws and Flow


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I am starting to look deeper into the claim that later DCOE's Idle Enrichment Screws are typically turned-out further.

The details in the screw interface to the carb body distill to:

1. For the same flow: A narrow tapered needle needs to be tuned out further than a wider tapered needle if the orifice size is the same. It is proportionate to 1/tan

2. For the same fuel flow, a needle with the same taper needs to be turned out more from a smaller orifice than for a larger orifice.

So to compare an early carb's Idle Enrichment Screw turn-out to a later, one must know the taper of the needle and the orifice size.

In another post I measured the threads on early and later DCOE Enrichment screws and it was 0.8 for both.

The part number for 40DCOE151 Enrichment screws is 64750.003


The DCOE151's I am tuning have this needle: 64750.004


64750.001 is stated to be "38 degrees" and for "early DCOE carbs"


64750.002 is the air bypass screw


From the above, the 003 and 004 do seem narrower than the early 001.



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With respect to the orifice diameter, if the early needle's tip just protrudes through, then a thinner later needle will protrude further. I am guessing that later DCOE's have smaller orifices to match the narrower needles to prevent the tip from poking out into the throat or the casting is thicker and the orifice starts higher above the roof of the throat...... there are a few more variables here to be explored.

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This is good to finally know Blue. It makes perfect sense what you are saying. And it is a good indication that the rules of thumb of DCOE tuning very greatly on what carb model you have and how old it is. It also just adds to the realization that you really need a wide band O2 to get them spot on, as there are way to many variables for a one tune fits all approach.

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