Calling on Holley carb experts!!!!!!!!

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duster360

duster360

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I have a Holley carb with 4 corner idle. I have researched and cannot find a straight forward answer to what I am about to ask.
I have been read and been told by a professional carb building company that the 4 corner idle carb should be turned out about 1 1/2 turns. I have tried 2 different Holley based carbs and on my engine neither did very well at idle with all 4 corners turned out more than 1/2 turns.
I have an afr gauge to help with tuning. If adjusted to 1 full turn on each corner it will bellow black smoke out the exhaust. My question is, are there any drawbacks to only being able to gon1/2 turns vs the 1 1/2 turns they suggest?
 
Im no expert.
But did you start with setting your transfer ports?
Carb off,upside down.
Adjust timing to get idle up and then work on your 4 corners.
Thats all i learnt so far.
 
Find yourself some new experts. You're already showing your using your noodle more than they have. The reason for having adjustable idle mixture screws is to adjust them. The number of turns out is not the way to determine what the engine needs. :rolleyes: As you observed, its clearly too rich for your engine at just one turn out.

As long as the engine responds to the adjments, its not terribly important if they are 1/4 turn out, 2 turns out or something in between. Keeping track of the number of turns out is useful but it's not a target.

With a four corner idle, the manifold vacuum is acting on a second set of idle ports and supply. This is why 4 corner carbs generally need less turns out on the mixture screws. Compared to a similar carb with 2 corner idle, it doubles the idle port fueling. So if the engine produces reasonable decent vacuum at idle, having them end up at less the 1 turn out is not surprising.

PS. Sadly its not not the first time I've heard of a professional give this type of advice.
PSS. Judge best power at idle by using rpm and vacuum in gear if an automatic. Your AFR gage may or may not be terribly accurate at idle depending on the reversion and quality of the burn.
 
There was a thread all about this in the last week.
A little long but informative. Will try to link it.
 
Tooljunkie said:
Im no expert.
But did you start with setting your transfer ports?
Carb off,upside down.
Adjust timing to get idle up and then work on your 4 corners.
Thats all i learnt so far.
Click to expand...
Yes, I have done all of that.
 
Mattax said:
Find yourself some new experts. You're already showing your using your noodle more than they have. The reason for having adjustable idle mixture screws is to adjust them. The number of turns out is not the way to determine what the engine needs. :rolleyes: As you observed, its clearly too rich for your engine at just one turn out.

As long as the engine responds to the adjments, its not terribly important if they are 1/4 turn out, 2 turns out or something in between. Keeping track of the number of turns out is useful but it's not a target.

With a four corner idle, the manifold vacuum is acting on a second set of idle ports and supply. This is why 4 corner carbs generally need less turns out on the mixture screws. Compared to a similar carb with 2 corner idle, it doubles the idle port fueling. So if the engine produces reasonable decent vacuum at idle, having them end up at less the 1 turn out is not surprising.

PS. Sadly its not not the first time I've heard of a professional give this type of advice.
PSS. Judge best power at idle by using rpm and vacuum in gear if an automatic. Your AFR gage may or may not be terribly accurate at idle depending on the reversion and quality of the burn.
Click to expand...
Thanks for clarifying what I was thinking.
 
The more you turn out the screws the more fuel, open the throttle plates with the screw in the right rear and you get more air, If it idles to high close the front plates a little. If you close the front plates all the way then turn the screw in 1 1/2 turns the t slots will be in the right place, start there and adjust the mixture screws.
 
The Carb Tuning Forum.
racingfuelsystems
Some pictures were lost in the tapatalk takeover of myfunforums but lots of good stuff there and no BS.
For basic downdraft carb concepts, the Chrysler Master tech stuff is as good as most, and Fisher and Urich's Holley Carburators and Manifolds is my favorite for starters.
 
The idle mixture screws are trims to the real low speed circuit which is the transfer ports.
If the transfer ports are flowing too much, then the trims will be small, and vice versa. It's just a balancing act between them. If the engine will not idle with the T-ports in the ballpark, then you add idle air from a bypass( usually incorporated in the PCV line, or add ignition timing until it does.
Both of these are also balancing acts.
Too much bypass air leads to tip-in hesitations, and really really stinky burns-your-eyeballs stinky, exhaust; that cannot be trimmed with the mixture screws.
And too much idle-timing CAN lead to low-speed/low-rpm detonation; usually with a manual trans or near stock TC.
Also when advancing thechit out of it, the idle speed may rise so high that you want to back out the curb-idle speed screw........ and thus mess up the T-port sync...... and introduce hesitations,and an unstable idle crappy idle.

So the primary part of the solution is to get the T-port sync established and then LEAVE THE SPEED SCREW ALONE.Then ensure your streeter has a properly plumbed and working PCV.That your streeter has a Vcan hooked to the spark-port and is dead at idle. And for all typical street cams even up to about 276* intake duration cam,that the secondaries are closed up tight but not sticking- both valves. Then set the front mixture screws in the middle of their sweetspot, about 3/4 to 1 turn for Holleys, and shut off the rear screws.For a SBM, set the timing to about 14*.......
This is your starting point.

Ok so what is the starting point T-port exposure? IDK; but I have an opinion. The exposure under the primary valves should be square to slightly taller than wide, and the smaller the cam or the higher the Scr or the closer you are to spot-on, the shorter the slot can be. Your engine will tell you by the settings of the trims.
Now, if this is a 360 with a 276 or less cam, and matching compression ratio,and you have yours already set-up this way, then it should already be idling nearly perfect. How can I say that? Cuz that's what I am driving.

But if not, then NOW we can try to figure out the why of it.
And here are the principle players;
1) low and/or unstable wet fuel level. this can include air blowing over the bowl-vents
2) unstable ignition timing. This can include the Vcan dicking around.
3) Vacuum leaks, including into the valley, and the P/B booster, or anything plumbed to the intake
4) unstable engine temperature, including an electric fan cutting in and out
5) uneven cylinder pressure; this can be from an entire subset of issues including;
..............2a)leakage from rings or valves
..............2b) bad lifter preload or valve lash
..............2c) bad cam
............. 2d) bad valve gear
............. 2e) Bad valve timing.
..............2f) bad headgasket.
..............2g) fuel-washed cylinders I guess.
6) hot and/or overheated sparkplugs
7) Not likely but; an overfilled crankcase
8) lemmee think, I think I got it all.
 
Last edited:
11" seems low with that cam at 900 rpm. I'd square up the transfer slots and use the IM screws to give you max vacuum while adjusting the idle speed screws after each IM adjustment. If a IM adjustment raises rpm, the engine wants it.
 
mopowers said:
11" seems low with that cam at 900 rpm. I'd square up the transfer slots and use the IM screws to give you max vacuum while adjusting the idle speed screws after each IM adjustment. If a IM adjustment raises rpm, the engine wants it.
Click to expand...
When I get the trans back in I will be checking vacuum at idle and cruise and go from there as suggested. Woulda had it done this weekend but UPS delayed shipping again.
 
Last edited:
T-slots are squares in front and i adjust idle in the rear, which doesn’t require much opening.
 
New carb. Dynoed 340 with 400HP. Engine has approximately 1000 miles on it.
 
AJ/FormS said:
The idle mixture screws are trims to the real low speed circuit which is the transfer ports.
If the transfer ports are flowing too much, then the trims will be small, and vice versa. It's just a balancing act between them. If the engine will not idle with the T-ports in the ballpark, then you add idle air from a bypass( usually incorporated in the PCV line, or add ignition timing until it does.
Both of these are also balancing acts.
Too much bypass air leads to tip-in hesitations, and really really stinky burns-your-eyeballs stinky, exhaust; that cannot be trimmed with the mixture screws.
And too much idle-timing CAN lead to low-speed/low-rpm detonation; usually with a manual trans or near stock TC.
Also when advancing thechit out of it, the idle speed may rise so high that you want to back out the curb-idle speed screw........ and thus mess up the T-port sync...... and introduce hesitations,and an unstable idle crappy idle.

So the primary part of the solution is to get the T-port sync established and then LEAVE THE SPEED SCREW ALONE.Then ensure your streeter has a properly plumbed and working PCV.That your streeter has a Vcan hooked to the spark-port and is dead at idle. And for all typical street cams even up to about 276* intake duration cam,that the secondaries are closed up tight but not sticking- both valves. Then set the front mixture screws in the middle of their sweetspot, about 3/4 to 1 turn for Holleys, and shut off the rear screws.For a SBM, set the timing to about 14*.......
This is your starting point.

Ok so what is the starting point T-port exposure? IDK; but I have an opinion. The exposure under the primary valves should be square to slightly taller than wide, and the smaller the cam or the higher the Scr or the closer you are to spot-on, the shorter the slot can be. Your engine will tell you by the settings of the trims.
Now, if this is a 360 with a 276 or less cam, and matching compression ratio,and you have yours already set-up this way, then it should already be idling nearly perfect. How can I say that? Cuz that's what I am driving.

But if not, then NOW we can try to figure out the why of it.
And here are the principle players;
1) low and/or unstable wet fuel level. this can include air blowing over the bowl-vents
2) unstable ignition timing. This can include the Vcan dicking around.
3) Vacuum leaks, including into the valley, and the P/B booster, or anything plumbed to the intake
4) unstable engine temperature, including an electric fan cutting in and out
5) uneven cylinder pressure; this can be from an entire subset of issues including;
..............2a)leakage from rings or valves
..............2b) bad lifter preload or valve lash
..............2c) bad cam
............. 2d) bad valve gear
............. 2e) Bad valve timing.
..............2f) bad headgasket.
..............2g) fuel-washed cylinders I guess.
6) hot and/or overheated sparkplugs
7) Not likely but; an overfilled crankcase
8) lemmee think, I think I got it all.
Click to expand...

T-slot is squared on primaries.
Distributor is a tuned and curved unit for my engine and has full manifold vacuum all the time with 20* initial and 34* total.
I use the secondaries to set idle.
 
1/2 turn is fine if the idle circuit AFR is good until the primary jet comes in, usually starts around 2k rpm. You will see it transition on the AFR gauge. The idle mixture screws add air and fuel mix provided by the IAB and IFR. If you start to go lean on idle cruise you'll need to tweek the IAB and IFR.
 
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