Quick Fuel SS750AN Tuning

[Tuner]

With 33 HSABs all around, what would be a number to start with on the primary HSAB? And if im understanding correctly, change just the primary ones, correct?

I was thinking about the cruise situation today. At 60 mph, I assume the throttle blade is past the transition slot. What is responsible for enrichment in this "window" where the throttle blade angle is increasing (i.e. beginning to accelerate up a hill), but prior to the PV's opening? Strictly the booster sensitivity? And lowering the primary side HSAB should help increase this sensitivity?

The T-slot continues to feed long after the main starts, so IJ, IAB, MJ and MAB all conspire to affect the A/F.

At some point in the large throttle opening range the lowest pressure in the carb moves up to the booster nozzle, in which condition the pressure is higher at the T-slot than the booster so the idle circuit reverses and the idle jet becomes an air bleed to the main well. In the part-throttle range up to significant load the T-slot continues feeding long after the main nozzle has begun to discharge so the A/F is a combination of idle/T-slot circuit and main nozzle. Keep in mind the in the mid-range both T-slot and main nozzle are metered by the restriction of the main jet because the restriction of the main jet affects the head pressure on the idle jet.

The problem with the high idle jet is because it is above float level air from the idle air bleed can easily flow back upstream of the jet orifice when intake manifold pulsations cause oscillating flow. This condition causes the fuel flow through and downstream of the high idle jet to be erratic and also causes the pressure in the main well to oscillate, resulting in erratic discharge from the main nozzle.

All this carb metering behavior of air and fuel was figured out over a hundred years ago and is reported in detail in NACA TR-49, and in fact this forum has a few descriptions of this in posts over the last decade or so.

[eric8]

I would test with 31 or a 28 bleed and see what it does,when you play with carbs you soon accumulate parts,part of the fun

Oh yeah the "Holley drawer" in my tool box is getting bigger and bigger

[eric8]

With 33 HSABs all around, what would be a number to start with on the primary HSAB? And if im understanding correctly, change just the primary ones, correct?

I was thinking about the cruise situation today. At 60 mph, I assume the throttle blade is past the transition slot. What is responsible for enrichment in this "window" where the throttle blade angle is increasing (i.e. beginning to accelerate up a hill), but prior to the PV's opening? Strictly the booster sensitivity? And lowering the primary side HSAB should help increase this sensitivity?

The T-slot continues to feed long after the main starts, so IJ, IAB, MJ and MAB all conspire to affect the A/F.

At some point in the large throttle opening range the lowest pressure in the carb moves up to the booster nozzle, in which condition the pressure is higher at the T-slot than the booster so the idle circuit reverses and the idle jet becomes an air bleed to the main well. In the part-throttle range up to significant load the T-slot continues feeding long after the main nozzle has begun to discharge so the A/F is a combination of idle/T-slot circuit and main nozzle. Keep in mind the in the mid-range both T-slot and main nozzle are metered by the restriction of the main jet because the restriction of the main jet affects the head pressure on the idle jet.

The problem with the high idle jet is because it is above float level air from the idle air bleed can easily flow back upstream of the jet orifice when intake manifold pulsations cause oscillating flow. This condition causes the fuel flow through and downstream of the high idle jet to be erratic and also causes the pressure in the main well to oscillate, resulting in erratic discharge from the main nozzle.

All this carb metering behavior of air and fuel was figured out over a hundred years ago and is reported in detail in NACA TR-49, and in fact this forum has a few descriptions of this in posts over the last decade or so.

Just when I start to think I know a little, I realize I know nothing. Awesome info. Guess I'll add "NACA TR49" to my reading list

[Tuner]

With 33 HSABs all around, what would be a number to start with on the primary HSAB? And if im understanding correctly, change just the primary ones, correct?

I was thinking about the cruise situation today. At 60 mph, I assume the throttle blade is past the transition slot. What is responsible for enrichment in this "window" where the throttle blade angle is increasing (i.e. beginning to accelerate up a hill), but prior to the PV's opening? Strictly the booster sensitivity? And lowering the primary side HSAB should help increase this sensitivity?

The T-slot continues to feed long after the main starts, so IJ, IAB, MJ and MAB all conspire to affect the A/F.

At some point in the large throttle opening range the lowest pressure in the carb moves up to the booster nozzle, in which condition the pressure is higher at the T-slot than the booster so the idle circuit reverses and the idle jet becomes an air bleed to the main well. In the part-throttle range up to significant load the T-slot continues feeding long after the main nozzle has begun to discharge so the A/F is a combination of idle/T-slot circuit and main nozzle. Keep in mind the in the mid-range both T-slot and main nozzle are metered by the restriction of the main jet because the restriction of the main jet affects the head pressure on the idle jet.

The problem with the high idle jet is because it is above float level air from the idle air bleed can easily flow back upstream of the jet orifice when intake manifold pulsations cause oscillating flow. This condition causes the fuel flow through and downstream of the high idle jet to be erratic and also causes the pressure in the main well to oscillate, resulting in erratic discharge from the main nozzle.

All this carb metering behavior of air and fuel was figured out over a hundred years ago and is reported in detail in NACA TR-49, and in fact this forum has a few descriptions of this in posts over the last decade or so.

Just when I start to think I know a little, I realize I know nothing. Awesome info. Guess I'll add "NACA TR49" to my reading list

You've had a week to study, you must be about due to turn in some homework, how's your carb tuning project coming along?

[eric8]

Funny you should ask..... No fault of Summit Racing (but all credit to our US Postal Service), my package that was supposed to be here Monday just showed up today that has the 68 secondary jets and the 10.5 PV. It should be on and tested by tomorrow so stay tuned!

[Tuner]

Don't forget to wear your woohan flu mask when you install the parts.

[eric8]

Hahaa will do. The size 31 HSABs arrived too, so we'll see what that does. Had 33's in it so I don't know if that'll be enough to make a difference or not.

[eric8]

Well I got the new jets installed (didn't mess with the PMAB's yet). It finally allowed me to see mid 12 AFRs under full throttle. But here's the thing....it has to "build" up to those AFRs. Seems like under a higher load, it wants to pull on the boosters too much and stay in the low 11's.
The truck is a 3 speed manual. Say I'm cruising along at 45 mph in 3rd. Not lugging, but still under a good load. Mat the gas and it will go high 10's and low 11's AFR, but will (slowly) climb into the 12's from 4,000 to 6,000 rpm. Now do that same pull at 45 mph in 2nd gear, under less load, and it jumps into the mid 12's on AFR much quicker.
Another scenario: wind out 2nd gear to redline (nice mid 12's afr), shift to 3rd, rpms drop, back on the gas full throttle, and it's back to 11:1 until the revs build back up.
Ideas?

[Tuner]

Change the MAB.

[eric8]

Change the MAB.

So go up on all 4 MAB's? It has 33's right now.

[Tuner]

You have .031" ? put them in. It probably needs .028" or .026".

[eric8]

You have .031" ? put them in. It probably needs .028" or .026".

Are we referring to smaller MAB's all around to fix the WOT condition I described above? This seems counterintuitive to me?

[treyrags]

I think what Tuner is getting at is the smaller MAB is "usually" later/stronger pulling on the mains.

[Tuner]

You have .031" ? put them in. It probably needs .028" or .026".

Are we referring to smaller MAB's all around to fix the WOT condition I described above? This seems counterintuitive to me?

Yes, smaller all around. It is getting leaner as the RPM climbs because the bleed is larger than ideal.

The years, decades, of exactly backwards misinformation about air bleed behavior in Holley type carbs has resulted in a totally opposite than reality false understanding of how the main and emulsion bleed system affects A/F over the total carb air flow range.

In Holley type carbs the main and emulsion air increases fuel flow at low air flow and can increase or decrease fuel flow at high air flow. The emulsion configuration, number and size of holes vs. how far below float level, has predominant effect at low flow, and the main air bleed has predominant effect at high air flow. Obviously the MAB and E-bleeds are co-dependent and together affect the entire range of whatever the particular engine's total air flow may be, the more HP = the wider range of air flow.

Your situation is rich low end and progressing leaner as air flow increases. You have smaller MAB and that is an easy change in your carb, simple screwdriver and no mess (if you don't drop one). Change it and see what happens.

Don't be surprised if the result is counter-intuitive, this is just a simple carburetor you are playing with and they do that. If you do this enough times you will come to expect it.

[eric8]

You have .031" ? put them in. It probably needs .028" or .026".

Are we referring to smaller MAB's all around to fix the WOT condition I described above? This seems counterintuitive to me?

Yes, smaller all around. It is getting leaner as the RPM climbs because the bleed is larger than ideal.

The years, decades, of exactly backwards misinformation about air bleed behavior in Holley type carbs has resulted in a totally opposite than reality false understanding of how the main and emulsion bleed system affects A/F over the total carb air flow range.

In Holley type carbs the main and emulsion air increases fuel flow at low air flow and can increase or decrease fuel flow at high air flow. The emulsion configuration, number and size of holes vs. how far below float level, has predominant effect at low flow, and the main air bleed has predominant effect at high air flow. Obviously the MAB and E-bleeds are co-dependent and together affect the entire range of whatever the particular engine's total air flow may be, the more HP = the wider range of air flow.

Your situation is rich low end and progressing leaner as air flow increases. You have smaller MAB and that is an easy change in your carb, simple screwdriver and no mess (if you don't drop one). Change it and see what happens.

Don't be surprised if the result is counter-intuitive, this is just a simple carburetor you are playing with and they do that. If you do this enough times you will come to expect it.

Tuner,
Thanks for taking the time to explain all this to me. I'll add four .028 bleeds to my next order. So that takes care of my wot situation, but listen to what I have going on at part throttle.

Today, I swapped in a 10.5 PV when I did the jet change. I thought this for sure would fix the the lean "hole" that I'm seeing, but it's still there. Let me explain.

Take for instance a 30-45 mph cruise in 3rd gear. Nice 14.9ish afr. On the transition slots I assume. Mash on the pedal slightly (1/4 way down) to accelerate, and it goes to 17:1 and isn't happy at all. Mash the pedal further to around halfway and it richens up. I had the vacuum gauge hooked up and I was well past the opening point of the PV when it goes lean. 1/4 throttle in 3rd brings it down to 8 inches vacuum and it's like the PV isn't letting the fuel flow.

So basically, any low speed steady state cruise my afr is happy. Tip in some throttle and it goes lean. Tip in more throttle, and it clears up.