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Discussion Starter · #1 ·
I have researched this to death, but can't find a good explanation for this adjustment.

There are a lot of people who insist the primary butterfly must be adjusted so that a small portion of the transfer slot is below the throttle blade at idle (about 0.020" so it appears as a "square"). In fact, many go to the extreme of leaving it there and only adjusting the secondary to get their idle rpm.

Now why would that matter? I can see if the throttle is too high up and too much of the slot is exposed, you run out of transition metering during light cruising. But I can't really grasp why you need "some" slot showing at idle. There is the idle discharge hole to feed the enigne, and it seems to me that if the idle is adjsuted and the throttle blade windes up between the idle discharge hole and the transfer slot (so no slot showing), it should work OK.

And the reason I am asking is that an awful lot of info states how important this idle/transition relationship is to prevent hesitation/boggin etc. They must mean light accelration, but they don't seem to be talking about that.....they seem to mean "stepping on it".

In fact, as most people have a heavy foot when it comes ot accelrating, the throttle goes way up past the top of the transfer slot and accelration is based on main metering (jets and PV) with the accelerator pump covering things until the main system starts flowing. So how does showing the transfer slot "square" affect "stepping on it"?

None of my Holley books mention having the slot showing as a requirement, but do mention it as a bad thing if too much slot is showing.

I'm confused. Can anyone offer an explanation for this "square" slot requirement?

ps: I'm thinking of it with the only Holley I am familiar with, the 4160 vacuum secondaries.
 

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the carb has a tough timing controlling small amounts of fuel delivery (idle) when past of the transfer slots. Like trying to fill a shot glass with a garden hose.

more timing when typically cure that problem by increasing the rpms. Then you can turn down the idle screw.

do a web search " How to tune a Holley the scientific way" and print out a nice article of this subject.
 

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Discussion Starter · #4 ·
Thank-you. I do understand the info in the lin as well as what you're saying....or at least I think I do......LOL.

I'm just in the process of rebuilding my carbs and I notice my previoius/current primary throttle plate position doesn'tshow any of the transfer slot at all below the plate. I htought this was a good thing, and still do.

However, I keep seeing this "square slot" amount that's supposed to be showing. Just wondering why that is, or maybe it's incorrect.

It's easy enough to do. I can just open the primary up a tiny bit and close the secondary about the smae amount (if there's enough room left on the secondary). But it's a pain as the secondary is almost impossible to adjust once on the enigne....if the idle has to be further adjusted.
 

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The reason you want part of the transfer slot exposed at idle is to allow a
small amount of fuel to flow at that point. Then when the throttle opens there
is no delay in fuel flow helping to eliminate a lean spot.

If the carb is set where no transfer slot is exposed at idle, it takes time to
initiate flow wen the throttle is opened.

If too much slow is exposed, idle will be rich and you will lose control of the
mixture at the idle adjustment screws.

The recommendation of the "square slot" is just a reference point, give the
engine what it likes. Ideally you should have complete control of the idle
mixture with the idle adjustment screws and instant throttle response when
the throttle is opened. If you have that you are OK
 

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Discussion Starter · #6 ·
That makes sense. Thank-you. Right now the bottom edge of the throttle blade lines up with the bottom edge of the slot, not a hair difference, so it probably was close. However, I can do the "square" thing. No big deal to reverse it back if I get trouble, other than I may have to remove the carb to re-adjust the secondary idle if necessary.
 

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Discussion Starter · #8 ·
The bottom of the secondary slot is very high up above the secondary throttle plate. If I have both primary and secondary throttle blades showing a bit of transfer slot, the engine may idle at several thousand rpm. This carb has vacuum secondaries, so that may be why the secondary slot is so high. Perhaps a mech secondary carb with 4 corner idle has lower slots, but I really don't know.
 

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I was referring to a double pumper, with these the secondary can open very
fast requiring a good signal to the secondary transfer slot.

With a vacuum secondary, the back blades open slowly so there is no need to
have a active transfer slot at idle, it's just a wast of fuel.

So I would close the secondary side enough to have about 0.020" of slot
exposed on the primary side. This will assure you have the slot active at idle
and should give good throttle response.
 
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