427 idle issues
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
427 idle issues
Hey guys I have a 427/4sd car with a larger than stock cam and 850 cfm Holley. This car is new to me so I will tell you what I can.
I hook up a vacuum guage and get about 20in so I tried turning to mixture screw but no change, so then I disconnected vacuum port under the front fuel bowl and the car seemed to idle higher and stumble less. I then turned one of the front mixture screws in and the car seemed to idle better, could it be the carb is jetted to high?.The po did say his buddy rebuilt the carb fir performance and the car does seems run rich meaning if you are near the car your eyes will start to burn, this is a side pipe car.
I cannot verify timing yet because of an aftermarket harmonic balancer but I do suspect the carb is the culprit....any thoughts would be appreciated.
I hook up a vacuum guage and get about 20in so I tried turning to mixture screw but no change, so then I disconnected vacuum port under the front fuel bowl and the car seemed to idle higher and stumble less. I then turned one of the front mixture screws in and the car seemed to idle better, could it be the carb is jetted to high?.The po did say his buddy rebuilt the carb fir performance and the car does seems run rich meaning if you are near the car your eyes will start to burn, this is a side pipe car.
I cannot verify timing yet because of an aftermarket harmonic balancer but I do suspect the carb is the culprit....any thoughts would be appreciated.
Race Director
Joined: May 2006
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Before diving into the carburetor did you check the ignition timing and if still running points did you first check the dwell?
Next, make sure the distributor vacuum advance is hooked to a constant vacuum source, quite possibly the one you found in the front on the base plate.
Then adjust the idle speed to what you want, possibly 900 rpm if it is a "large" cam.
Then start with the mixture screw on one side turning it clockwise a quarter turn and watch the idle speed.
If it drops turn it back out a quarter turn.
If it doesn't drop turn the opposite side screw in a quarter turn.
Continue this side to side adjustment till you get a drop and then back out the quarter turn.
If they fully seat without causing a drop in rpm you have other issues, such as a blown power valve or incorrectly adjusted secondary throttle stop.
Next, make sure the distributor vacuum advance is hooked to a constant vacuum source, quite possibly the one you found in the front on the base plate.
Then adjust the idle speed to what you want, possibly 900 rpm if it is a "large" cam.
Then start with the mixture screw on one side turning it clockwise a quarter turn and watch the idle speed.
If it drops turn it back out a quarter turn.
If it doesn't drop turn the opposite side screw in a quarter turn.
Continue this side to side adjustment till you get a drop and then back out the quarter turn.
If they fully seat without causing a drop in rpm you have other issues, such as a blown power valve or incorrectly adjusted secondary throttle stop.
Last edited by MelWff; May 22, 2021 at 01:45 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Jetting has no effect on idle mixture or idle quality. You need to balance out your primary/secondary idle speeds (and verify no more than .020" transition slot exposure on the primary side) and then correctly balance and adjust your idle mixture screws. Most of those carbs will have an idle mixture screw setting of about 5/8 - 7/8 turn when correctly set up. But before you fiddle with the carb, fix your timing, making sure you have 36 total (most big blocks will actually take 38 degrees if you're running factory iron heads), initial in the mid-teens, and 10-12 degrees of additional vacuum advance hooked up to manifold vacuum (with the vacuum advance control unit meeting the "2-inch rule"). You cannot adjust the carb without first correctly setting up the timing.
Lars
Lars
Last edited by lars; May 22, 2021 at 03:15 PM.
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
Currently the car has an hei distributor and I don't think the advance it working, while it does hold vacuum it may be disconnected because the po ran the car without the advance hooked. I will inspect the distributor tomorrow and check the timing again.
As far as the carb is concerned I turned in each front mixture screw one at a time about 1.5 turns and the car made no change in idle or running so I turned the screws back out 1.5. I remember 1.5 being a baseline setting.
I think the timing may be off a little also because thr car starts fine but when it's hot it cranks over rral slow like I have a heat soaked starter.
As far as the carb is concerned I turned in each front mixture screw one at a time about 1.5 turns and the car made no change in idle or running so I turned the screws back out 1.5. I remember 1.5 being a baseline setting.
I think the timing may be off a little also because thr car starts fine but when it's hot it cranks over rral slow like I have a heat soaked starter.
Race Director
Joined: May 2006
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Have you determined the mechanical advance isnt working by removing the distributor cap and rotor and examining the weights and springs?
I understand the vacuum advance is disconnected but should be reconnected before adjusting the carburetor.
Did you stop at 1.5 turns because the mixture screw bottomed out?
Did you fully understand what Lars stated in the prior post?
I understand the vacuum advance is disconnected but should be reconnected before adjusting the carburetor.
Did you stop at 1.5 turns because the mixture screw bottomed out?
Did you fully understand what Lars stated in the prior post?
Race Director
Joined: Mar 2018
Posts: 10,713
Likes: 2,168
From: Golden Colorado
Currently the car has an hei distributor and I don't think the advance it working, while it does hold vacuum it may be disconnected because the po ran the car without the advance hooked. I will inspect the distributor tomorrow and check the timing again.
As far as the carb is concerned I turned in each front mixture screw one at a time about 1.5 turns and the car made no change in idle or running so I turned the screws back out 1.5. I remember 1.5 being a baseline setting.
I think the timing may be off a little also because thr car starts fine but when it's hot it cranks over rral slow like I have a heat soaked starter.
As far as the carb is concerned I turned in each front mixture screw one at a time about 1.5 turns and the car made no change in idle or running so I turned the screws back out 1.5. I remember 1.5 being a baseline setting.
I think the timing may be off a little also because thr car starts fine but when it's hot it cranks over rral slow like I have a heat soaked starter.
Last edited by Vette5311; May 22, 2021 at 07:18 PM.
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
Have you determined the mechanical advance isnt working by removing the distributor cap and rotor and examining the weights and springs?
I understand the vacuum advance is disconnected but should be reconnected before adjusting the carburetor.
Did you stop at 1.5 turns because the mixture screw bottomed out?
Did you fully understand what Lars stated in the prior post?
I understand the vacuum advance is disconnected but should be reconnected before adjusting the carburetor.
Did you stop at 1.5 turns because the mixture screw bottomed out?
Did you fully understand what Lars stated in the prior post?
The carburetor mixture screws were only turned in 1.5 turns and then backed out the same amount.
I will verify the timing tomorrow also.
Race Director
Joined: May 2006
Posts: 18,734
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If the screws were 6 turns out from fully seated, the 1.5 turns you went in did not indicate anything . After confirming timing and attaching vacuum line to distributor set idle speed and then start turning each screw in until you get a change in speed or they fully seat, do not stop at 1.5 unless they fully seat.
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Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
If you can turn the idle mixture screws all the way in until seated, and the engine keeps running, you are running the carb on the main metering system, and have an inoperable idle system. This is either due to the throttle opening on the primary side being excessive, or you have a plugged idle circuit in the carb.
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
If you can turn the idle mixture screws all the way in until seated, and the engine keeps running, you are running the carb on the main metering system, and have an inoperable idle system. This is either due to the throttle opening on the primary side being excessive, or you have a plugged idle circuit in the carb.
1- with the car idling if I apply vacuum to the advance nothing happens.
2- with the #1 cylinder at compression stroke and TDC confirmed the rotor seems to point to the front of the engine and not # 1 cylinder.
3- aftermarket balancer does not read 0° but closer to 50°before TDC.
Herr are some pics of the #1 and 2 plugs and the rotor direction on compression stroke.
Also when I had the car idling and applied a vacuum tester to the advance I did not notice a change in rpm's but the advance does work..
Race Director
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"here are some of my findings so far..
1- with the car idling if I apply vacuum to the advance nothing happens.
2- with the #1 cylinder at compression stroke and TDC confirmed the rotor seems to point to the front of the engine and not # 1 cylinder.
3- aftermarket balancer does not read 0° but closer to 50°before TDC.
Herr are some pics of the #1 and 2 plugs and the rotor direction on compression stroke.
Also when I had the car idling and applied a vacuum tester to the advance I did not notice a change in rpm's but the advance does work.."
Your last sentence states the advance does work but at the same time you say applying vacuum to the vacuum advance diaphragm does nothing, how is that possible?
You dont see the timing mark move when you apply vacuum to the diaphragm while the engine is running?
If so, how can it be working?
You state the rotor at TDC points to the front of the engine. If it points to the drivers side front cylinder in the cap, that is supposed to be #1 cylinder on an HEI distributor. What plug does that post connect to?
How are you determining the engine is on TDC #1 cylinder on compression stroke?
1- with the car idling if I apply vacuum to the advance nothing happens.
2- with the #1 cylinder at compression stroke and TDC confirmed the rotor seems to point to the front of the engine and not # 1 cylinder.
3- aftermarket balancer does not read 0° but closer to 50°before TDC.
Herr are some pics of the #1 and 2 plugs and the rotor direction on compression stroke.
Also when I had the car idling and applied a vacuum tester to the advance I did not notice a change in rpm's but the advance does work.."
Your last sentence states the advance does work but at the same time you say applying vacuum to the vacuum advance diaphragm does nothing, how is that possible?
You dont see the timing mark move when you apply vacuum to the diaphragm while the engine is running?
If so, how can it be working?
You state the rotor at TDC points to the front of the engine. If it points to the drivers side front cylinder in the cap, that is supposed to be #1 cylinder on an HEI distributor. What plug does that post connect to?
How are you determining the engine is on TDC #1 cylinder on compression stroke?
Last edited by MelWff; May 23, 2021 at 12:21 PM.
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
Ok so if I apply a hand pump to the vacuum advance cannister it does pull in and move the distributor advance, what I was trying to say is that if I do the same test while the engine running I don't hear a change in the engine.
The rotor seems to be pointing toward the water pump and not the #1 cylinder. Could the distributor be off a tooth ?.
My cell is 203-952-1069.
Thanks ~Jim
The rotor seems to be pointing toward the water pump and not the #1 cylinder. Could the distributor be off a tooth ?.
My cell is 203-952-1069.
Thanks ~Jim
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
That carb is a badly hacked unit.... It's been commercially rebuilt, sandblasted and painted. It needs a complete teardown and a thorough inspection to see what they've done to it - I'm betting it has a very bad setup, and probably has plugged fuel and air bleeds from the sandblasting that was done.
The timing can be set with the distributor in any position. Do a TDC verification to make sure the timing pointer is correct, and then just set the timing. Being "one tooth off" has no effect on timing at all - it only affects the cosmetic clocking of the distributor once the timing has been set.
Lars
The timing can be set with the distributor in any position. Do a TDC verification to make sure the timing pointer is correct, and then just set the timing. Being "one tooth off" has no effect on timing at all - it only affects the cosmetic clocking of the distributor once the timing has been set.
Lars
Last edited by lars; May 23, 2021 at 02:38 PM.
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
That carb is a badly hacked unit.... It's been commercially rebuilt, sandblasted and painted. It needs a complete teardown and a thorough inspection to see what they've done to it - I'm betting it has a very bad setup, and probably has plugged fuel and air bleeds from the sandblasting that was done.
The timing can be set with the distributor in any position. Do a TDC verification to make sure the timing pointer is correct, and then just set the timing. Being "one tooth off" has no effect on timing at all - it only affects the cosmetic clocking of the distributor once the timing has been set.
Lars
The timing can be set with the distributor in any position. Do a TDC verification to make sure the timing pointer is correct, and then just set the timing. Being "one tooth off" has no effect on timing at all - it only affects the cosmetic clocking of the distributor once the timing has been set.
Lars
Lars that car was rebuilt according to the po but I was curious why you say it was sand blasted?, obviously it was painted but are there tell tale signs you use to determine it being blasted?
Could I take it apart and check all the passeges to see if it is indeed clogged up?.
I was really hoping to enjoy this car this summer and do some shake down cruises to see what it needs before really diving into the car.
My suspension needs a rebuild for sure.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Race Director
Joined: Mar 2017
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From: Virginia
OP, do you have a timing light, or are you doing this by feel?
If you can find TDC, and put marks on your harmonic balancer, or the correct size MSD tape, you may save yourself a lot of grief and speculation.
If you can find TDC, and put marks on your harmonic balancer, or the correct size MSD tape, you may save yourself a lot of grief and speculation.
Last edited by Bikespace; May 23, 2021 at 07:31 PM.
Thread Starter
Racer
Joined: May 2016
Posts: 492
Likes: 35
From: Norwalk, Ct
Lars, I did see where the carb was repainted but I never thought about it being blasted.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Judging from the photos.....you have way too much advance......this needs to be sorted before you can do anything. Timing HAS to be done first.....verify it and lock it in with the 36 degrees at or around 3000 RPM. This is important. Then record the number at "idle".........this is with the vacuum can unplugged. Try plugging the vacuum can in again and verify change or no change.......
That carb is sketchy......Lars is correct in saying that you are not on the idle circuit at all. I would borrow or purchase another Holley......anything but the Street Avenger. A 750 HP would work nicely here.....
Jebby
That carb is sketchy......Lars is correct in saying that you are not on the idle circuit at all. I would borrow or purchase another Holley......anything but the Street Avenger. A 750 HP would work nicely here.....
Jebby