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950 cam, .038 IFR, .082 IAB, idles very good. Engine is 400cid SBC, alum block and heads, Edlebrock Victor E intake, Comp cams 258 intake, 264 exhaust, 108 lobe separation, .630 lift.
Car is 2100#, Turbo 350 w/4500 stall ATI converter. Using wide band have lean idle which suddenly transitions pig rich at 2200 rpm light load. Under heavy load, 12.5-13:1 aft so mains are close.

My problem is how to delay mains to eliminate rich transition. This occurs during cruise from 40-70 mph.
 

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Not sure if you are running a power valve but you may be able to go down on the mains and up on the PVCR.
 

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Mains are coming in early, how do I delay? Well above power valve vacuum when problem occurs and mains are slightly small when on the power mid-range. MSD 7A box so no lack of spark even when rich enough to smoke however at times it is rich enough to smoke and misfire at around 2500.
 

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You missed the point, you can lean up the mains by smaller jet, put it back later in the curve by PVCR.

You can delay the mains by larger HSAB, but that comes with other effects.
 

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All the above is good advice. SOmetimes the T Slot comes into,play around that rpm too. To keep mine clean I put a restriction in the main body where the t slot feeds from. Sorting this out, a wide band will be your friend
 

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You missed the point, you can lean up the mains by smaller jet, put it back later in the curve by PVCR.

You can delay the mains by larger HSAB, but that comes with other effects.
No, you delay the main with a smaller HSAB.

Main air bleed air pushes the fuel through the main well and nozzle, like the aspirator in a paint spray gun.

At the beginning of the main flow MAB and emulsion air encourages flow, but in most - not all - cases it leans the WOT.

Larger MAB = richer start of main and small throttle opening and leaner WOT.

Smaller MAB = delayed and leaner start of main nozzle and richer WOT.

Just last week this was an issue with a QFT 950 1.450 V x 1.75 TB, on a 496 BBC, Brodix heads, 246 x 254 @ .050, Erson mechanical roller, Victor intake. After selection of main jet that gave best performance in the mid-pedal, appropriately lean for reasonable fuel consumption but rich enough to be snappy, the idle circuit was best with .033" IJ, .081" IAB with .052" T-slot restriction. Also, moved the IJ from top of idle well to bottom of idle well.

This particular QFT billet base plate has T-slots .028" wide and .320" long, which is probably why it wanted such a small T-slot jet and large IAB. It's not unusual to use .052" in Dominators which have 5/16" T slots, but 4150 type don't usually like that small. Whatever runs best is right.

With that (to me seems crazy) lean idle circuit calibration it still had a rich place where the T-slot and start of main overlapped. Changing the MAB from .032" to .025" made it sweet so the progression from the 12.8/1 idle A/F leaner to the ~ 15/1 cruise and mid pedal light acceleration was a smooth change, no more rich dip in the slightly above just off-idle range.

Metering blocks were set up with two .028" E-bleeds in old-school Holly configuration. Previously (as delivered) had 4 x .028 with the .032" MAB which made it stupid rich at part-throttle. Again, MAB and E-bleed air encourages flow by reducing viscosity and surface tension of the fuel. Fuel flow at very low velocity at the beginning of flow in the main circuit is more influenced by viscosity and surface tension than jet size. A jet change within a few numbers range will not affect the A/F at the start of flow as much as it affects A/F at higher load in mid pedal. A several jet size change will have an effect in the nozzle start range, but will change the A/F about 1/2 ratio per jet # in the mid load, 50% power range.

Of course, with carbs every carb/engine combination is unique, but the above is generally true with the average Holley or clone in use on street driven hot rod engines.


The action of the Main Air Bleed and Emulsion Bleeds in Holley and Holley clones is grossly misunderstood by most folks, thanks to years -- decades -- of BS in magazines and people online repeating the same misinformation over and over.
 

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Too much emulsion. Take off metering block, you probably have 4 or 5 of the ebleeds unblocked
 

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Discussion Starter #10
No, you delay the main with a smaller HSAB.

Main air bleed air pushes the fuel through the main well and nozzle, like the aspirator in a paint spray gun.

At the beginning of the main flow MAB and emulsion air encourages flow, but in most - not all - cases it leans the WOT.

Larger MAB = richer start of main and small throttle opening and leaner WOT.

Smaller MAB = delayed and leaner start of main nozzle and richer WOT.

Just last week this was an issue with a QFT 950 1.450 V x 1.75 TB, on a 496 BBC, Brodix heads, 246 x 254 @ .050, Erson mechanical roller, Victor intake. After selection of main jet that gave best performance in the mid-pedal, appropriately lean for reasonable fuel consumption but rich enough to be snappy, the idle circuit was best with .033" IJ, .081" IAB with .052" T-slot restriction. Also, moved the IJ from top of idle well to bottom of idle well.

This particular QFT billet base plate has T-slots .028" wide and .320" long, which is probably why it wanted such a small T-slot jet and large IAB. It's not unusual to use .052" in Dominators which have 5/16" T slots, but 4150 type don't usually like that small. Whatever runs best is right.

With that (to me seems crazy) lean idle circuit calibration it still had a rich place where the T-slot and start of main overlapped. Changing the MAB from .032" to .025" made it sweet so the progression from the 12.8/1 idle A/F leaner to the ~ 15/1 cruise and mid pedal light acceleration was a smooth change, no more rich dip in the slightly above just off-idle range.

Metering blocks were set up with two .028" E-bleeds in old-school Holly configuration. Previously (as delivered) had 4 x .028 with the .032" MAB which made it stupid rich at part-throttle. Again, MAB and E-bleed air encourages flow by reducing viscosity and surface tension of the fuel. Fuel flow at very low velocity at the beginning of flow in the main circuit is more influenced by viscosity and surface tension than jet size. A jet change within a few numbers range will not affect the A/F at the start of flow as much as it affects A/F at higher load in mid pedal. A several jet size change will have an effect in the nozzle start range, but will change the A/F about 1/2 ratio per jet # in the mid load, 50% power range.

Of course, with carbs every carb/engine combination is unique, but the above is generally true with the average Holley or clone in use on street driven hot rod engines.


The action of the Main Air Bleed and Emulsion Bleeds in Holley and Holley clones is grossly misunderstood by most folks, thanks to years -- decades -- of BS in magazines and people online repeating the same misinformation over and over.
Currently, #32 MAB's .041" Will change to .035". E bleeds are top #29, next #29, Blank, #29, Blank on bottom. How should I proceed to change them?
 

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Currently, #32 MAB's .041" Will change to .035". E bleeds are top #29, next #29, Blank, #29, Blank on bottom. How should I proceed to change them?
MAB is too big. E-bleed locations are close, and could be reduced in size some, down to a 26-28. I would drop the IFR to a 0.035-0.038. Going from a .041 to a 0.035 should be a very noticeable change.
 

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Currently, #32 MAB's .041" Will change to .035". E bleeds are top #29, next #29, Blank, #29, Blank on bottom. How should I proceed to change them?
Try e bleeds: 026 blank 026 blank blank
^^ This ^^ Do you mean the MAB is .032" and the idle jet is .041" ? Move the idle jet to the bottom of the idle well and use .033" or smaller. Use 6-32 x 3/16" brass hex-key set screws to make idle jets and thread the metering block passages at the bottom of the idle wells. You may also need to put a jet in the passage to the T-slot in the metering block gasket face of the body. This depends on how long the T-slot is, therefor how far into the throttle opening it continues to increase fuel delivery. The carb I described in the post above has a longer T-slot than most 4150 type Holleys, which is probably why it wanted such a small T-slot jet, usually the 4150 carb slots are less than .250" long & only .026" wide and they use near .070"~+.
 

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^^ This ^^ Do you mean the MAB is .032" and the idle jet is .041" ? Move the idle jet to the bottom of the idle well and use .033" or smaller. Use 6-32 x 3/16" brass hex-key set screws to make idle jets and thread the metering block passages at the bottom of the idle wells. You may also need to put a jet in the passage to the T-slot in the metering block gasket face of the body. This depends on how long the T-slot is, therefor how far into the throttle opening it continues to increase fuel delivery. The carb I described in the post above has a longer T-slot than most 4150 type Holleys, which is probably why it wanted such a small T-slot jet, usually the 4150 carb slots are less than .250" long & only .026" wide and they use near .070"~+.
Careful with an Ultra XP carb, a 0.033" IFR maybe be too small, even in the bottom. On my Ultra XP it liked .035 IFR... smaller and I would get a hesitation on acceleration that I couldnt get rid of with squirters. I would try a 0.033", but if you get a hesitation go up to a .035".
 
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