[chiphead]

i guess Tom V and Shaker are on break LOL

I've been doing a TON of reading on holley tuning and emulsion bleeds in particular. There is precious little information about it online. There are about 5 good threads on it, and they all refer to each other. I'm gonna do a brain dump so future readers get an idea of WTF is going on with their 4150 style DP. Feel free to correct anything if I'm wrong somewhere in this.

The emulsion holes simply help control when main circuit flow starts, as well as how aggressive the fuel curve is, in response to booster signal.

The HSAB controls how much air is allowed to enter the emulsion system. Larger HSAB will allow the main system to start at lower suctions, but will tend to lean the main system out as suction increases.

The Kill bleed, or anti-siphon bleed, is located in the emulsion well, above fuel level. It bleeds booster vacuum signal to ambient air through the HSAB. It delays the onset of main flow, as well as provides a siphon break when the mains shut off in response to throttle plate closure.

The uppermost emulsion bleed should be at or just above the fuel level. If it is above fuel level, it acts as another kill bleed before pullover. After pullover, it adds more emulsion to the fuel flow in the main well.

Emulsion bleeds at fuel level help start pullover. The addition of air bubbles reduces the viscosity of the fuel in the main well. The lighter, bubbly fuel is easily lifted by the booster vacuum and starts pullover (main fuel flow). Smaller holes start flow at higher vacuums but lean the system out less as flow increases. Larger holes start flow at lower booster signal (vacuum) but tends to lean out faster as the fuel in the main and emulsion wells drops and exposes the hole to air from the HSAB.

Emulsion holes below fuel level in the bowl help increase flow at higher main flow rates. As the main system begins to deliver fuel at higher flows, the level of the liquid fuel in the emulsion well and main wells drops, uncovering more emulsion holes to HSAB air. This air also produces bubbles in the fuel, allowing the emulsified fuel to be drawn up the main well and out of the boosters easily.

Similar behavior happens with the lower emulsion holes. The larger holes promote fuel flow as you go to WOT. But once WOT flow is established, the larger holes allow more air to enter, reducing the amount of liquid fuel pulled out of the main jet and up the main well. There is a point where the emulsion hole size starts to lean out the AFR as carb flow increases.

I believe this was the thinking with all the extra emulsion holes. If 2 holes control the start and the WOT AFR, then more emulsion holes should help the progression of the flow, right? This works, to a point. From what I've read, more than 3 emulsion holes per side will produce a rich spike as flow starts, then start to lean out more and more as RPM increases. There is too much total air being added to the main well. This reduces vacuum in the well and also displaces liquid fuel that should be traveling up the well, lifted by the appropriate amount of emulsion air. The boosters start to spit and fuel delivery gets inconsistent. It's harder to increase jet size and get an appropriate AFR at all openings. This makes tuning one of these carbs on the street a real challenge.

The other thing that happens is that when the main system first starts to flow, the emulsion well is full of fuel. That fuel is pulled through all the bleeds, into the main well and out the booster. So what happens is you can get a rich condition right after pullover. Then the system starts going leaner as the emulsion well is emptied and air from all the emulsion bleeds starts flowing into the main well.

So the way to re-baseline one of these new-school holleys is to go back to the classic metering block setup, which had 2 emulsions. The upper should be open right at fuel level, and the second hole should be 7/16-1/2" below that one. On a 4-hole emulsion block, leave 1 and 3 open and block 2 and 4. This also may require reducing the HSAB size. Typically you want the HSAB to be the size of the kill bleed, to maybe + .003 of the kill bleed. That's a good place to start. With less emulsion and HSAB, sometimes it necessary to reduce main jet size to maintain desired AFRs. Just depends on test drives and watching the AFR gauge and listening to what the engine wants.

The other thing is the IFR. The upper location meters fuel and air, and is less consistent than the old, lower location. Moving the IFR to the low location really helps maintain a consistent AFR at idle and cruise before the mains start. The upper location is a hot mess, as (at least on my engine) it produces rich curb idle but leans out way too much on the T-slot cruise. Tuning the transition to the mains is much harder with the IFR in the upper location, IMHO. This is what Tom V and others were trying to tell me, but I didn't fully get it, until now.