IDLE AFR QUESTION

[rgalajda]

[kgb]

My experience with MAB changes was early on in my use of a Mighty Demon 750. My experimentation with IFR changes is a recent thing and not related to MAB or main circuit. The carb has been working fine, no real need to change from the overly rich cruising A/F ratios, but curious how it would go. Next step might be to move the IFR's to the bottom of the circuit from their current top position, but the thing seems to work fine as-is.

Re: MAB's, when I experienced the stumble from the MD750, as-installed, I read up on people having the same problem and solving it via smaller HSAB (MAB) jets. Made sense then and makes sense now, the bleed is a hole in a straw between the outlet side and supply side, the outlet side has to suck harder to overcome the hole in the straw, the sucking comes from venturi effect which increases with engine demand that pulls air through the venturi to make it all happen. Smaller hole, less suck required (lower engine draw/rpm), it activates the system sooner. That all makes sense, provided the issue is the point of transfer from idle circuit to main circuit, which seems to be the case. Problem disappeared; I have a measure of faith I'm seeing what I'm seeing but won't put money on anything having to do with carburetors.

What also makes sense is what I'm reading from RHD here, emulsion system bubbles help the fuel get sucked through the pipe easier. Something I'm not sure of, is the use of terms like "help" the fuel get moving through the main circuit. The part that rings true is, aerated fuel should lift easier. But just how much easier (and we're talking fine points of when the system initially starts to flow) do the introduced bubbles make that process? Since the emulsion holes are so much smaller, and in my mind would activate in order from the top to the bottom since the upper ones should see less resistance from the weight of fuel on the other side (and assuming the lower ones that are below bowl fuel level are submerged until fuel starts to move through the main well), any commonly used MAB is more than big enough to supply all the emulsion air required. Its effect as a direct bleed to the venturi draw through the main circuit discharge tube is what should determine initiation of the main circuit, in my mind, and making it smaller should bring that system into play earlier. I believe that's what happened with my MD750, prejudiced because I was told that's what was going on so easier for me to believe. I can't prove it, of course.

Or is it supposed to be a matter that the bleed through the main discharge, above the fuel since it's not moving yet to cut it off, is fully utilizing a smaller MAB and there is not enough to be drawn through the emulsion holes, requiring a larger MAB to serve both demands so that emulsion may begin?

I can see both sides of the theory, how do we prove it in actuality? Plug every emulsion bleed, accept a crappy, inefficient maybe even impossible to burn a/f mixture, but depict when it actually starts flowing? Backing up a bit, completely plug the main circuit first, to establish the exact point when the idle system cuts out, for a baseline. Then uncover just one emulsion at a time and find those resulting points of activation, in pairs, whatever combinations are available.  

[GTO Geoff]

Regarding MABs.
I believe what is stated in the original Holley book, written by engineer & later VP, Mike Urich. Somebody I know, also an engineer, worked at Holley during the halcyon days, knew MU & said he was a very smart engineer.

Page 23 says this.......& it is unambiguous: " The strong signal from the discharge nozzle is bled off or reduced by the MAB so there is is less effective pressure difference to cause fuel flow. The mixture will become leaner as the size of the bleed is increased."


While it doesn't say it in so many words, it should be obvious from that statement that a smaller bleed starts the system earlier...& vice versa.

[rgalajda]

There is know argument that as you increase the size of the MAB that the AFR will lean at high flow .

There are many different booster designs straight / straight notched/ downleg/ stepped downleg /Annular. Add to that booster leg sizes.
Now add to that the size of the venturi. We know that in a large venturi such as 1.56” that some boosters are less effective at producing a good signal. Also we have different venturi shapes.
In the metering block ( Holley ) we can have different emulsion stacks and the presence of an emulsion tube plus different sized main wells .
What was the original size of the MAB around .028” or .036”.
Is it possible that with all these variations that the size of the MAB could have a different effect at low flow in some situations?

Here is one persons experience. I am not saying this is what you will experience nor does it address the question of start up of the main system. Take note here that he was using a .036” HSAB where as an old school set up would have had about a .028” HSAB
“My carb A/F ratio at cruise was between 13.9 to 14.5 then when I swapped HSAB from .036 to .028 my light load and low speed A/F ratio leaned out a bunch and cruise ratio is now 14.5 to low 15's, Sure wasn't the effect I was expecting 
I was trying to log a change to the WOT curve with just a swap of HSAB, my WOF A/F ratio didn't change any great amount and still is between 12.3-12.6 “
Tuner
I’ve been trying to explain the air bleed function in Holley type carbs to folks for some time now and your experience is a good example of the air bleed system behavior and its effect on low flow. This could help explain why they don’t always do what you would expect from reading the magazines. "


These new carbs with .036” HSAB ,such as the 750 Mighty Demon down leg booster carb, questions how did they come up with such a large HSAB.

I don’t believe kgb mentioned what size his was?

[Right hand drive]

[kgb]

.039" were in the MD750, they chart their 650 as having .041", and bigger carbs having smaller.

https://static.summitracing.com/global/ ... 282010.pdf


From the suggestions I read back when I noticed the stumble (over 10 years ago), I wound up putting in .025's at all 4 HSAB. Didn't have any way of logging A/F then, was just happy it stopped the stumble as the only thing I was aware at the time of maybe causing it, was the accelerator pump shot.