383 SBC Holley 750 eye burning stinky idle

[HQM383]

Thank you guys. It’s 90% better now. You have been a big help. The stumble is even gone. Radiator is done. I did it last night. Need to do my plugs, oil, and filter. I’ll report back.

I’m going to leave my IFR’s alone due to my mixture screws being around one turn out. I figured if the IFR’s were too big, I would be at 1/2 turn or something.

You will truly know if the IFR’s are too big when you fit an AFR gauge and monitor fuel delivery through the t-slots. Right now at idle your regulating fuel through the needle point and seat that make up the idle screws. How much the exposed t-slot is pulling plus other factors such as mentioned varied manifold flow and egr have an effect on idle screw position. See pic below for where fuel travels before you regulate it with idle screw.



- BLACK = Mainwell
- BLUE = Idle/transition fuel path
- GREEN = Idle fuel regulated by idle screw
- WHITE = Upper IFR position
- YELLOW = Lower IFR position
- ORANGE = Fuel to t-slot (baseplate)
- RED = Fuel to Idle discharge hole (baseplate)
- Purple = PVCR

* upper IFR needs to lift float level fuel in order to meter and so needs to be larger and potentially less stable delivery. Lower IFR has float level head pressure to feed it and therefore can be smaller diameter to meter and more stable delivery. Lower can make for easier engine starting too as there is a column of fuel to float level after the IFR to give a little rich unmetered slug of fuel to help first fire.

[PRNDL]

Thank you guys. It’s 90% better now. You have been a big help. The stumble is even gone. Radiator is done. I did it last night. Need to do my plugs, oil, and filter. I’ll report back.

I’m going to leave my IFR’s alone due to my mixture screws being around one turn out. I figured if the IFR’s were too big, I would be at 1/2 turn or something.

You will truly know if the IFR’s are too big when you fit an AFR gauge and monitor fuel delivery through the t-slots. Right now at idle your regulating fuel through the needle point and seat that make up the idle screws. How much the exposed t-slot is pulling plus other factors such as mentioned varied manifold flow and egr have an effect on idle screw position. See pic below for where fuel travels before you regulate it with idle screw.



- BLACK = Mainwell
- BLUE = Idle/transition fuel path
- GREEN = Idle fuel regulated by idle screw
- WHITE = Upper IFR position
- YELLOW = Lower IFR position
- ORANGE = Fuel to t-slot (baseplate)
- RED = Fuel to Idle discharge hole (baseplate)
- Purple = PVCR

* upper IFR needs to lift float level fuel in order to meter and so needs to be larger and potentially less stable delivery. Lower IFR has float level head pressure to feed it and therefore can be smaller diameter to meter and more stable delivery. Lower can make for easier engine starting too as there is a column of fuel to float level after the IFR to give a little rich unmetered slug of fuel to help first fire.

Thanks for the effort man. That’s a cool image. Should I raise my float levels then? I’m at 1/3 up the sight glass window. I’m definitely getting an AFR gauge.

[HQM383]

Thank you guys. It’s 90% better now. You have been a big help. The stumble is even gone. Radiator is done. I did it last night. Need to do my plugs, oil, and filter. I’ll report back.

I’m going to leave my IFR’s alone due to my mixture screws being around one turn out. I figured if the IFR’s were too big, I would be at 1/2 turn or something.

You will truly know if the IFR’s are too big when you fit an AFR gauge and monitor fuel delivery through the t-slots. Right now at idle your regulating fuel through the needle point and seat that make up the idle screws. How much the exposed t-slot is pulling plus other factors such as mentioned varied manifold flow and egr have an effect on idle screw position. See pic below for where fuel travels before you regulate it with idle screw.



- BLACK = Mainwell
- BLUE = Idle/transition fuel path
- GREEN = Idle fuel regulated by idle screw
- WHITE = Upper IFR position
- YELLOW = Lower IFR position
- ORANGE = Fuel to t-slot (baseplate)
- RED = Fuel to Idle discharge hole (baseplate)
- Purple = PVCR

* upper IFR needs to lift float level fuel in order to meter and so needs to be larger and potentially less stable delivery. Lower IFR has float level head pressure to feed it and therefore can be smaller diameter to meter and more stable delivery. Lower can make for easier engine starting too as there is a column of fuel to float level after the IFR to give a little rich unmetered slug of fuel to help first fire.

Thanks for the effort man. That’s a cool image. Should I raise my float levels then? I’m at 1/3 up the sight glass window. I’m definitely getting an AFR gauge.

When you get the afr gauge you will need a baseline to begin with. Halfway would be a reasonable start or what puts the float level at the top e-bleed

[rp930]

Floats 1/2-3/4 up the window. Per AED.

[PRNDL]

Quick question guys. There are 4 corner idle Holleys like my 4150 dp. Then there are 2 corner idle primary only carbs live the 4160.

Are there 2 corner idle Holley with 4 corner idle discharge ports? Like the front metering block feeds all 4 idle ports and the front screws controls the front and rear sides?

Would that explain those long fuel passages on the base plates?

[rgalajda]

Quick question guys. There are 4 corner idle Holleys like my 4150 dp. Then there are 2 corner idle primary only carbs live the 4160.

Are there 2 corner idle Holley with 4 corner idle discharge ports? Like the front metering block feeds all 4 idle ports and the front screws controls the front and rear sides?

Would that explain those long fuel passages on the base plates?

The Holley carbs that used that passage were OE on some chevys in the 1960's. Such as List 3810 1967 holley 585 cfm ( later called 600 cfm ) used on 1967 Camaro and Corvette. That passage was connected to a true idle port on the secondary side .

The snake passage ( as called by some ) can be found on a lot of old aftermarket Holley carbs. I have yet to find an aftermarket Holley that has that passage fully connected. It is either blocked in the middle of the passage or never fully connects at the ends

[PRNDL]

Thanks for the info. I seen guys online convert 2 corners to 4 corner idles and they have to drill holes in the rear to make the idle ports. Then those ports connect to those snake channels. My issue is, why were those snake channels there in the first place? If not connected to anything.

[rgalajda]

As I said Original Equipment carbs such as the List 3810 used them.
Holley kept using the casting for some reason. Maybe they thought they would use it.
They can be used for conversion like the 3810. But if you are converting to 4 corner idle and secondary metering blocks with idle mixture screws , they get blocked because the rear metering block supplies the fuel.
Sometime after the 70s these passages got deleted on newer castings.
A lot of things on Holley carbs came from working with the OEM's or after market ,not Holley. Examples would be 3 circuit Holley ( Chrysler ) , Annular boosters ( Ford ) Or others , Idle Bypass AIR ( Barry Grant/Demon )
Hope this helps.

[PRNDL]

Just FYI, incase this thread helps anyone going through the same thing, this is what I did to fix my issue. Carb is running clean, smooth, and strong.

IFR’s / IAB’s: This AED 750 HP came with .035 IFR’s when I pinned them, they were .036. Tried, .033 then .031 and same issues. Rich transition. Finally tried .028 and they were spot on. A hair lean, but I switched from .075 IAB to .070 and that did the trick.

MAIN PRIMARY JET / HSAB: When my primary jet came in at 3000 rpm they came in rich then went overly lean as the RPM’s increased. Had to run a 10.5 PV just to help. Went from a .035 HSAB to .028 and the fixed the curve. This unit came with a 78 primary jet. Swapped it to 72’s and it’s running great. Dropped to 8.5 PV.

Plugged two out of 5 primary emulsion holes.
Open
Blocked
Open
Blocked
Open

Secondary jets: They were 85’s and I’m down to 83’s.

[1980RS]

Just FYI, incase this thread helps anyone going through the same thing, this is what I did to fix my issue. Carb is running clean, smooth, and strong.

IFR’s / IAB’s: This AED 750 HP came with .035 IFR’s when I pinned them, they were .036. Tried, .033 then .031 and same issues. Rich transition. Finally tried .028 and they were spot on. A hair lean, but I switched from .075 IAB to .070 and that did the trick.

MAIN PRIMARY JET / HSAB: When my primary jet came in at 3000 rpm they came in rich then went overly lean as the RPM’s increased. Had to run a 10.5 PV just to help. Went from a .035 HSAB to .028 and the fixed the curve. This unit came with a 78 primary jet. Swapped it to 72’s and it’s running great. Dropped to 8.5 PV.

Plugged two out of 5 primary emulsion holes.
Open
Blocked
Open
Blocked
Open

Secondary jets: They were 85’s and I’m down to 83’s.

Sounds to me like you gave it the old Holley 4779 750 DP tune. Good job.

[rgalajda]

A lot of off idle stumble can be to rich on the transfer slot,I always fit adjustable jets in main body.
I also don't think the rule is all 4 idle screws have to be out 1 1/2 turns or there about.
I have found on milder street deals the secondry idle screw can be leaner if required, the engine is only idling.
We set all 4 transfer slots for 20 thou or so but most have progressive secondry linkage ?
I have also seen where engines are set with lower initial mechanical timing because there is to much vacuum advance at idle,you go to accelerate away from low speed and have a stumble because timing retards .
Not saying you have this issue just thinking out loud.

Holley carbs with 4 corner idle mixture screws are not set to 1 1/2 turns out. Base setting is 3/4 turn out. Holley carbs with 2 corner idle mixture screws are set to 1 1/2 turns out, base setting.

[rgalajda]

Just FYI, incase this thread helps anyone going through the same thing, this is what I did to fix my issue. Carb is running clean, smooth, and strong.

IFR’s / IAB’s: This AED 750 HP came with .035 IFR’s when I pinned them, they were .036. Tried, .033 then .031 and same issues. Rich transition. Finally tried .028 and they were spot on. A hair lean, but I switched from .075 IAB to .070 and that did the trick.

MAIN PRIMARY JET / HSAB: When my primary jet came in at 3000 rpm they came in rich then went overly lean as the RPM’s increased. Had to run a 10.5 PV just to help. Went from a .035 HSAB to .028 and the fixed the curve. This unit came with a 78 primary jet. Swapped it to 72’s and it’s running great. Dropped to 8.5 PV.

Plugged two out of 5 primary emulsion holes.
Open
Blocked
Open
Blocked
Open

Secondary jets: They were 85’s and I’m down to 83’s.

You did a great job. Sometimes, sticking with it, can be rewarding.

[PRNDL]

I found that the bigger the IFR’s were the less turns out the idle mixtures needed, so idle was great but transition was rich.

As I reduced the size of the IFR’s to the size where the engine was the cleanest and happiest during transition, idle mixture screws fell right at 1-1/4 turns.

It seems the more overly rich these transition circuits are, the less turns are needed.

There are so many threads about rich running carbs and lean running carbs, and so much talk about jets and everything else, but what I have found is barely anyone touches on IFR’s. This carb was all around fat and nothing fixed it until I brought the IFR’s from .035 to .028.

[HQM383]

I found that the bigger the IFR’s were the less turns out the idle mixtures needed, so idle was great but transition was rich.

As I reduced the size of the IFR’s to the size where the engine was the cleanest and happiest during transition, idle mixture screws fell right at 1-1/4 turns.

It seems the more overly rich these transition circuits are, the less turns are needed.

There are so many threads about rich running carbs and lean running carbs, and so much talk about jets and everything else, but what I have found is barely anyone touches on IFR’s. This carb was all around fat and nothing fixed it until I brought the IFR’s from .035 to .028.

Well done on the journey of discovery. #1 rule when tuning a carb to run clean with good AFR on the street is size IFR first.

[Martin161]

Check your power valves. Sometimes they can rip and make a very rich idel.