A question for the Holley/Demon carb experts
08-16-2017, 04:35 PM
#1
Melting Slicks
Thread Starter
A question for the Holley/Demon carb experts
Is it possible to have too rich of an idle yet be lean just off idle.
I have a 406sbc with a BG 650 and a Comp Cam XE280HR cam that has an off idle stumble but also seems to run rich at idle. I'm talking about a steady throttle, 1500-2000 rpm stumble, not a hesitation coming off idle. Under even the slightest of accelleration, there's no stumble, only when trying to maintain a constant rpm.
And if I back off from any low speed (<35 or so) while in 4th or 5th, it really jerks (trailer hitches) badly.
I've set the transfer slot exposure for both primary and secondary ends and have varied them in both directions with little change. I've also plugged all vacuum ports with no change.
All those things point to a lean condition but at idle I have the mixture screws at barely 3/4 turns out for optimum idle vacuum (17") and it smells very rich and will burn your eyes if idling in the garage for more than a minute.
Based on all I've read, and it's been a bit over several days, the rich idle would be addressed by enlarging the Idle Air Restrictors while the lean off idle would be addressed by enlarging the Idle Fuel Restrictor. Those are generally opposite tuning actions so I'm puzzled.
I have a 406sbc with a BG 650 and a Comp Cam XE280HR cam that has an off idle stumble but also seems to run rich at idle. I'm talking about a steady throttle, 1500-2000 rpm stumble, not a hesitation coming off idle. Under even the slightest of accelleration, there's no stumble, only when trying to maintain a constant rpm.
And if I back off from any low speed (<35 or so) while in 4th or 5th, it really jerks (trailer hitches) badly.
I've set the transfer slot exposure for both primary and secondary ends and have varied them in both directions with little change. I've also plugged all vacuum ports with no change.
All those things point to a lean condition but at idle I have the mixture screws at barely 3/4 turns out for optimum idle vacuum (17") and it smells very rich and will burn your eyes if idling in the garage for more than a minute.
Based on all I've read, and it's been a bit over several days, the rich idle would be addressed by enlarging the Idle Air Restrictors while the lean off idle would be addressed by enlarging the Idle Fuel Restrictor. Those are generally opposite tuning actions so I'm puzzled.
08-16-2017, 05:32 PM
#2
Race Director
It might also help if you post the timing specs you are using - initial timing, centrifugal timing, if vacuum advance is being used, etc. I sort of wonder if some more timing might not clear it up - but maybe you already have as much in it as is safe.
In the opposite direction, if you have too much advance in it with a lot of vacuum advance at cruise speed the resulting kickback/missing you get at around 2000 rpm could be what you are describing and also could feel like trailer hitching.
In the opposite direction, if you have too much advance in it with a lot of vacuum advance at cruise speed the resulting kickback/missing you get at around 2000 rpm could be what you are describing and also could feel like trailer hitching.
Last edited by DansYellow66; 08-16-2017 at 05:35 PM.
08-16-2017, 07:00 PM
#3
If it's four corner idle 3/4 on the emulsion screws sounds OK. Have you tried to close the secondary to where the botton of the transfer slot is at the top of the blade and give the engine air from small holes in the primary blades while maintaining the. 020 primary transfer slot.
Do you know the ifr and iab specs for your carburetor.
Do you know the ifr and iab specs for your carburetor.
08-16-2017, 09:00 PM
#4
Melting Slicks
Thread Starter
It might also help if you post the timing specs you are using - initial timing, centrifugal timing, if vacuum advance is being used, etc. I sort of wonder if some more timing might not clear it up - but maybe you already have as much in it as is safe.
In the opposite direction, if you have too much advance in it with a lot of vacuum advance at cruise speed the resulting kickback/missing you get at around 2000 rpm could be what you are describing and also could feel like trailer hitching.
In the opposite direction, if you have too much advance in it with a lot of vacuum advance at cruise speed the resulting kickback/missing you get at around 2000 rpm could be what you are describing and also could feel like trailer hitching.
I should add that by 2000 rpm under steady throttle that the stumbling is about gone.
08-16-2017, 09:03 PM
#5
Melting Slicks
Thread Starter
If it's four corner idle 3/4 on the emulsion screws sounds OK. Have you tried to close the secondary to where the botton of the transfer slot is at the top of the blade and give the engine air from small holes in the primary blades while maintaining the. 020 primary transfer slot.
Do you know the ifr and iab specs for your carburetor.
Do you know the ifr and iab specs for your carburetor.
And no, I have no specs for the carb. It's an early Speed Demon carb without the Idle-eze base plate.
08-17-2017, 08:07 AM
#6
Race Director
If it was too much total timing at cruise speed I don't think the missing would start to fade away at 2000 rpm and above. But you might try a quick experiment of plugging your vacuum advance and taking a quick spin in it. But I acknowledge that with those timing figures none of this would explain the rich/lean carb conditions.
08-17-2017, 08:49 AM
#7
Melting Slicks
Thread Starter
If it was too much total timing at cruise speed I don't think the missing would start to fade away at 2000 rpm and above. But you might try a quick experiment of plugging your vacuum advance and taking a quick spin in it. But I acknowledge that with those timing figures none of this would explain the rich/lean carb conditions.
Short of welding in a bung and install an O2 sensor, I'm only guessing at rich/lean conditions and am now questioning my sense that it's rich at idle. As 'tbarb' pointed out, 3/4 turn on a 4 corner carb isn't unusual and I have to acknowledge that it won't idle on a cold startup for several minutes since there's no choke. I have to keep the revs up to 1500 or so until it's sufficiently warm so the smell and burning eyes may be nothing more than operating in a closed space.
At this point, the simplest course of action is to open up the IFR incrementally and retest. If I go too far, I can drill and tap and insert homemade jets to bring the diameter back down.
Last edited by 65air_coupe; 08-17-2017 at 09:54 PM.
08-17-2017, 05:02 PM
#8
Burning Brakes
I had disconnected all vacuum lines at one point with no effect.
Short of welding in a bung and install an O2 sensor, I'm only guessing at rich/lean conditions and am no questioning my sense that it's rich at idle. As 'tbarb' pointed out, 3/4 turn on a 4 corner carb isn't unusual and I have to acknowledge that it won't idle on a cold startup for several minutes since there's no choke. I have to keep the revs up to 1500 or so until it's sufficiently warm so the smell and burning eyes may be nothing more than operating in a closed space.
At this point, the simplest course of action is to open up the IFR incrementally and retest. If I go too far, I can drill and tap and insert homemade jets to bring the diameter back down.
Short of welding in a bung and install an O2 sensor, I'm only guessing at rich/lean conditions and am no questioning my sense that it's rich at idle. As 'tbarb' pointed out, 3/4 turn on a 4 corner carb isn't unusual and I have to acknowledge that it won't idle on a cold startup for several minutes since there's no choke. I have to keep the revs up to 1500 or so until it's sufficiently warm so the smell and burning eyes may be nothing more than operating in a closed space.
At this point, the simplest course of action is to open up the IFR incrementally and retest. If I go too far, I can drill and tap and insert homemade jets to bring the diameter back down.
08-17-2017, 06:58 PM
#9
I had disconnected all vacuum lines at one point with no effect.
Short of welding in a bung and install an O2 sensor, I'm only guessing at rich/lean conditions and am no questioning my sense that it's rich at idle. As 'tbarb' pointed out, 3/4 turn on a 4 corner carb isn't unusual and I have to acknowledge that it won't idle on a cold startup for several minutes since there's no choke. I have to keep the revs up to 1500 or so until it's sufficiently warm so the smell and burning eyes may be nothing more than operating in a closed space.
At this point, the simplest course of action is to open up the IFR incrementally and retest. If I go too far, I can drill and tap and insert homemade jets to bring the diameter back down.
Short of welding in a bung and install an O2 sensor, I'm only guessing at rich/lean conditions and am no questioning my sense that it's rich at idle. As 'tbarb' pointed out, 3/4 turn on a 4 corner carb isn't unusual and I have to acknowledge that it won't idle on a cold startup for several minutes since there's no choke. I have to keep the revs up to 1500 or so until it's sufficiently warm so the smell and burning eyes may be nothing more than operating in a closed space.
At this point, the simplest course of action is to open up the IFR incrementally and retest. If I go too far, I can drill and tap and insert homemade jets to bring the diameter back down.
08-17-2017, 10:01 PM
#10
Melting Slicks
Thread Starter
I've opened the IFR by .002 and got a smoother idle that requires less warm up time but can't discern any difference in the stumbling/bucking at low steady rpm. Dave is probably right that it's going to take some instrumentation to get where I want to be but I'll try your suggestion next. As I understand it, changing the air bleed moves the mixture curve around so I'll have to re-read some earlier data on the subject.
Last edited by 65air_coupe; 08-17-2017 at 11:03 PM.
08-18-2017, 10:54 AM
#11
No argument there, just not ready to take it to that level...yet.
I've opened the IFR by .002 and got a smoother idle that requires less warm up time but can't discern any difference in the stumbling/bucking at low steady rpm. Dave is probably right that it's going to take some instrumentation to get where I want to be but I'll try your suggestion next. As I understand it, changing the air bleed moves the mixture curve around so I'll have to re-read some earlier data on the subject.
I've opened the IFR by .002 and got a smoother idle that requires less warm up time but can't discern any difference in the stumbling/bucking at low steady rpm. Dave is probably right that it's going to take some instrumentation to get where I want to be but I'll try your suggestion next. As I understand it, changing the air bleed moves the mixture curve around so I'll have to re-read some earlier data on the subject.
If your warm up problems go away there is a good chance it's rich after the plenum warms up. A .002 IFR change is large enough to make a difference but the problem is still there so all you did was richen the mixture.
The smaller IAB will extend the idle circuit to a higher rpm and allow the fuel curve to be a little richer in that higher rpm where the air bleed has it's most influence. If you take the carburetor apart it's a good idea to verify the hole sizes so you have a tuning baseline. Is the idle feed restriction up high or low in the block.
08-18-2017, 12:00 PM
#12
Melting Slicks
Thread Starter
You can drill tap the IAB for 8-32 brass set screws and drill tap the IFR for 6-32 brass set screws. That way you can always go back exactly where you were if the change does not take out the stumble.
If your warm up problems go away there is a good chance it's rich after the plenum warms up. A .002 IFR change is large enough to make a difference but the problem is still there so all you did was richen the mixture.
The smaller IAB will extend the idle circuit to a higher rpm and allow the fuel curve to be a little richer in that higher rpm where the air bleed has it's most influence. If you take the carburetor apart it's a good idea to verify the hole sizes so you have a tuning baseline. Is the idle feed restriction up high or low in the block.
If your warm up problems go away there is a good chance it's rich after the plenum warms up. A .002 IFR change is large enough to make a difference but the problem is still there so all you did was richen the mixture.
The smaller IAB will extend the idle circuit to a higher rpm and allow the fuel curve to be a little richer in that higher rpm where the air bleed has it's most influence. If you take the carburetor apart it's a good idea to verify the hole sizes so you have a tuning baseline. Is the idle feed restriction up high or low in the block.
It's interesting but probably not surprising how the different engine requires these kind of changes. This wasn't a problem when it was on my L79 but that's a lot of difference in air flow and volume to a 406 with a roller cam.
Last edited by 65air_coupe; 08-18-2017 at 09:26 PM.
10-31-2017, 06:04 PM
#13
Melting Slicks
Thread Starter
O2 sensor installed - no voltage!
I finally relented, pulled off the header pipe and welded in a bung for an O2 sensor. I bought the recommended SG12 single wire, narrow band sensor, installed it and went for a ride of several miles at speeds of 50-60mph.
At no time did I see any voltage reading on the meter with the scale set to 0.00 volts (hundredths). I see two possible reasons for this - first, the sensor is not getting hot enough, second I'm so lean that I'm out of range of the sensor.
The sensor is located approximately 6 inches back from the collector flange for the headers so I'd think that would be hot enough. I pulled some plugs the other day to confirm that I'm running lean and sure enough, they were borderline white.
I've opened the idle feed jet by 0.002" to 0.037" and dropped the size of the air bleed from 0.064" to 0.055" in several steps with no appreciable change in the uneven behavior at steady throttle, low speed driving. I even opened the air bleed to 0.070" on the off chance I was making an incorrect assumption but that indeed made it worse.
What's next - 3 wire sensor so I can heat it up or even more expensive wide band sensor? Or drop the air bleed to 0,050"?
This is a 650 CFM carb on a 406 SBC that in my thinking should have a tendency to run rich, not lean. Thoughts?
At no time did I see any voltage reading on the meter with the scale set to 0.00 volts (hundredths). I see two possible reasons for this - first, the sensor is not getting hot enough, second I'm so lean that I'm out of range of the sensor.
The sensor is located approximately 6 inches back from the collector flange for the headers so I'd think that would be hot enough. I pulled some plugs the other day to confirm that I'm running lean and sure enough, they were borderline white.
I've opened the idle feed jet by 0.002" to 0.037" and dropped the size of the air bleed from 0.064" to 0.055" in several steps with no appreciable change in the uneven behavior at steady throttle, low speed driving. I even opened the air bleed to 0.070" on the off chance I was making an incorrect assumption but that indeed made it worse.
What's next - 3 wire sensor so I can heat it up or even more expensive wide band sensor? Or drop the air bleed to 0,050"?
This is a 650 CFM carb on a 406 SBC that in my thinking should have a tendency to run rich, not lean. Thoughts?
11-01-2017, 01:36 AM
#14
Have you matched the power valve to the engines vacuum characteristics? Is the power valve leaking?
Take a new clean spark plug and install it in #1. Let the car idle for an hour, pull the plug and see what it looks like.
Take a new clean spark plug and install it in #1. Let the car idle for an hour, pull the plug and see what it looks like.
11-01-2017, 05:21 AM
#15
I finally relented, pulled off the header pipe and welded in a bung for an O2 sensor. I bought the recommended SG12 single wire, narrow band sensor, installed it and went for a ride of several miles at speeds of 50-60mph.
At no time did I see any voltage reading on the meter with the scale set to 0.00 volts (hundredths). I see two possible reasons for this - first, the sensor is not getting hot enough, second I'm so lean that I'm out of range of the sensor.
The sensor is located approximately 6 inches back from the collector flange for the headers so I'd think that would be hot enough. I pulled some plugs the other day to confirm that I'm running lean and sure enough, they were borderline white.
I've opened the idle feed jet by 0.002" to 0.037" and dropped the size of the air bleed from 0.064" to 0.055" in several steps with no appreciable change in the uneven behavior at steady throttle, low speed driving. I even opened the air bleed to 0.070" on the off chance I was making an incorrect assumption but that indeed made it worse.
What's next - 3 wire sensor so I can heat it up or even more expensive wide band sensor? Or drop the air bleed to 0,050"?
This is a 650 CFM carb on a 406 SBC that in my thinking should have a tendency to run rich, not lean. Thoughts?
At no time did I see any voltage reading on the meter with the scale set to 0.00 volts (hundredths). I see two possible reasons for this - first, the sensor is not getting hot enough, second I'm so lean that I'm out of range of the sensor.
The sensor is located approximately 6 inches back from the collector flange for the headers so I'd think that would be hot enough. I pulled some plugs the other day to confirm that I'm running lean and sure enough, they were borderline white.
I've opened the idle feed jet by 0.002" to 0.037" and dropped the size of the air bleed from 0.064" to 0.055" in several steps with no appreciable change in the uneven behavior at steady throttle, low speed driving. I even opened the air bleed to 0.070" on the off chance I was making an incorrect assumption but that indeed made it worse.
What's next - 3 wire sensor so I can heat it up or even more expensive wide band sensor? Or drop the air bleed to 0,050"?
This is a 650 CFM carb on a 406 SBC that in my thinking should have a tendency to run rich, not lean. Thoughts?
Is this a vacuum or double pump carburetor?
Closing the secondary blades completely gives no improvement with the .037/.055
What is the IFR and IAB in the secondary metering block
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65air_coupe (11-01-2017)
11-01-2017, 06:37 AM
#16
Drifting
If the exhaust is making your eyes burn as you say, then you aren't too rich, it's the opposite; you are too lean. A lean burn makes your eyes burn.
Last edited by Tonio; 11-01-2017 at 06:38 AM.
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65air_coupe (11-01-2017)
11-01-2017, 08:12 AM
#20
Melting Slicks
Thread Starter
Take a new clean spark plug and install it in #1. Let the car idle for an hour, pull the plug and see what it looks like.