Need Holley 850 Idle Help
Thread Starter
Pro
Joined: Nov 2008
Posts: 680
Likes: 5
From: Denham Springs LA
Need Holley 850 Idle Help
Chasing the idle on 454 (not stock) engine.
I can't get a steady idle.
I adjust the idle mixture screws to get maximum vacuum (5 in Hg). Then when I rev it up it release throttle it idles about 1100 rpm and won't come back down. I back off the throttle adjustment screw and lower the idle then it settles back down.
Here's engine details (close to old LS6 specs)
468 cu in
Holley 850 mechanical secondaries (recently rebuilt)
990 rect port heads
Ls6 dual plane aluminum intake
10.25 CR
MSD Dist vacuum advance disconnected
Initial timing 14-deg, total timing 35-deg
10 in Hg at 1100 rpm
5 in Hg at 700 rpm
Running 2.5 in power valves
Thanks
I can't get a steady idle.
I adjust the idle mixture screws to get maximum vacuum (5 in Hg). Then when I rev it up it release throttle it idles about 1100 rpm and won't come back down. I back off the throttle adjustment screw and lower the idle then it settles back down.
Here's engine details (close to old LS6 specs)
468 cu in
Holley 850 mechanical secondaries (recently rebuilt)
990 rect port heads
Ls6 dual plane aluminum intake
10.25 CR
MSD Dist vacuum advance disconnected
Initial timing 14-deg, total timing 35-deg
10 in Hg at 1100 rpm
5 in Hg at 700 rpm
Running 2.5 in power valves
Thanks
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Chasing the idle on 454 (not stock) engine.
I can't get a steady idle.
I adjust the idle mixture screws to get maximum vacuum (5 in Hg). Then when I rev it up it release throttle it idles about 1100 rpm and won't come back down. I back off the throttle adjustment screw and lower the idle then it settles back down.
Here's engine details (close to old LS6 specs)
468 cu in
Holley 850 mechanical secondaries (recently rebuilt)
990 rect port heads
Ls6 dual plane aluminum intake
10.25 CR
MSD Dist vacuum advance disconnected
Initial timing 14-deg, total timing 35-deg
10 in Hg at 1100 rpm
5 in Hg at 700 rpm
Running 2.5 in power valves
Thanks
I can't get a steady idle.
I adjust the idle mixture screws to get maximum vacuum (5 in Hg). Then when I rev it up it release throttle it idles about 1100 rpm and won't come back down. I back off the throttle adjustment screw and lower the idle then it settles back down.
Here's engine details (close to old LS6 specs)
468 cu in
Holley 850 mechanical secondaries (recently rebuilt)
990 rect port heads
Ls6 dual plane aluminum intake
10.25 CR
MSD Dist vacuum advance disconnected
Initial timing 14-deg, total timing 35-deg
10 in Hg at 1100 rpm
5 in Hg at 700 rpm
Running 2.5 in power valves
Thanks
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I'm trying to figure out why you're running without vacuum advance. Your timing is terribly retarded without it (I agree with Driveshaft), and your vacuum levels are much lower than they need to be. There just seems to be several items in your tune that need to be changed.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
With idle vacuum levels as low as they are, trying to run manifold vacuum to the vac advance can would be a nightmare.
The only way to use it would be to run ported vacuum, which wont affect idle anyway (unless the throttle is adjusted too far open).
The only way to use it would be to run ported vacuum, which wont affect idle anyway (unless the throttle is adjusted too far open).
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Is the idle timing steady or jumping around at 1000 rpm?
Check to see that the springs on the advance are stiff enough to not allow the timing to advance below 1000 rpm.
Neal
Check to see that the springs on the advance are stiff enough to not allow the timing to advance below 1000 rpm.
Neal
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Thread Starter
Pro
Joined: Nov 2008
Posts: 680
Likes: 5
From: Denham Springs LA
Just hooked up vacuum advance and tried it. Engine was idling at 1100rpm and 34-deg. When I back off throttle screw about 1/4 turn Roma drop so low engine sputters and wants to die. Inhave to keep working throttle to keep it running. I made a dew small adjustments to idle mixture screws but they didn't make much difference.
It's interesting that either way (with or without vacuum advance hooked up) the throttle position screw has a make or break position. 1/4 turn gives me a high idle of about 1100rpms or drops down to about 700rpm, no in between. I don't understand.
Roger
It's interesting that either way (with or without vacuum advance hooked up) the throttle position screw has a make or break position. 1/4 turn gives me a high idle of about 1100rpms or drops down to about 700rpm, no in between. I don't understand.
Roger
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Whatcha runnin' for a cam? And with 5" of idle vacuum, it better be absolutely ridiculous.
Have you tried the usual tricks, like cracking open the secondary stop just a hair?
Scott
Have you tried the usual tricks, like cracking open the secondary stop just a hair?
Scott
Thread Starter
Pro
Joined: Nov 2008
Posts: 680
Likes: 5
From: Denham Springs LA
Neal,
I think you nailed it. I just taped the mechanical advance weights in my dist to stop timing advance. Was able to adjust throttle screw and get 850 rpm, 900 rpm, etc. I am going to get stiffer springs for the distributor so it doesn't advance until about 1100rpm. I hope I can find spring kit here in Baton Rouge tomorrow.
It's supposed to rain for the next 6 or 7 days so I'll have to wait for an actual road test.
Thanks again!
Thanks everyone!
Roger
I think you nailed it. I just taped the mechanical advance weights in my dist to stop timing advance. Was able to adjust throttle screw and get 850 rpm, 900 rpm, etc. I am going to get stiffer springs for the distributor so it doesn't advance until about 1100rpm. I hope I can find spring kit here in Baton Rouge tomorrow.
It's supposed to rain for the next 6 or 7 days so I'll have to wait for an actual road test.
Thanks again!
Thanks everyone!
Roger
Thread Starter
Pro
Joined: Nov 2008
Posts: 680
Likes: 5
From: Denham Springs LA
Scotty,
Don't know cam specs. Bought a used motor that was in a local mechanics shop sitting around. Was in an old Chevelle running 12's in 1/4 mile.
I tried crAcking open secondaries, still was chasing idle. Also plugged all vacuum ports and checked intake to make sure no vacuum leaks.
I plan to run my car at the strip, hoping for high 11's. I dynoed the engine it made 470hp and 620 ft-lbs torque.
Thanks
Roger
Don't know cam specs. Bought a used motor that was in a local mechanics shop sitting around. Was in an old Chevelle running 12's in 1/4 mile.
I tried crAcking open secondaries, still was chasing idle. Also plugged all vacuum ports and checked intake to make sure no vacuum leaks.
I plan to run my car at the strip, hoping for high 11's. I dynoed the engine it made 470hp and 620 ft-lbs torque.
Thanks
Roger
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Neal,
I think you nailed it. I just taped the mechanical advance weights in my dist to stop timing advance. Was able to adjust throttle screw and get 850 rpm, 900 rpm, etc. I am going to get stiffer springs for the distributor so it doesn't advance until about 1100rpm. I hope I can find spring kit here in Baton Rouge tomorrow.
It's supposed to rain for the next 6 or 7 days so I'll have to wait for an actual road test.
Thanks again!
Thanks everyone!
Roger
I think you nailed it. I just taped the mechanical advance weights in my dist to stop timing advance. Was able to adjust throttle screw and get 850 rpm, 900 rpm, etc. I am going to get stiffer springs for the distributor so it doesn't advance until about 1100rpm. I hope I can find spring kit here in Baton Rouge tomorrow.
It's supposed to rain for the next 6 or 7 days so I'll have to wait for an actual road test.
Thanks again!
Thanks everyone!
Roger
Scott
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Neal,
I think you nailed it. I just taped the mechanical advance weights in my dist to stop timing advance. Was able to adjust throttle screw and get 850 rpm, 900 rpm, etc. I am going to get stiffer springs for the distributor so it doesn't advance until about 1100rpm. I hope I can find spring kit here in Baton Rouge tomorrow.
It's supposed to rain for the next 6 or 7 days so I'll have to wait for an actual road test.
Thanks again!
Thanks everyone!
Roger
I think you nailed it. I just taped the mechanical advance weights in my dist to stop timing advance. Was able to adjust throttle screw and get 850 rpm, 900 rpm, etc. I am going to get stiffer springs for the distributor so it doesn't advance until about 1100rpm. I hope I can find spring kit here in Baton Rouge tomorrow.
It's supposed to rain for the next 6 or 7 days so I'll have to wait for an actual road test.
Thanks again!
Thanks everyone!
Roger
Let us know if you get it resolved with the springs
Neal
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Chasing the idle on 454 (not stock) engine.
I can't get a steady idle.
I adjust the idle mixture screws to get maximum vacuum (5 in Hg). Then when I rev it up it release throttle it idles about 1100 rpm and won't come back down. I back off the throttle adjustment screw and lower the idle then it settles back down.
Here's engine details (close to old LS6 specs)
468 cu in
Holley 850 mechanical secondaries (recently rebuilt)
990 rect port heads
Ls6 dual plane aluminum intake
10.25 CR
MSD Dist vacuum advance disconnected
Initial timing 14-deg, total timing 35-deg
10 in Hg at 1100 rpm
5 in Hg at 700 rpm
Running 2.5 in power valves
Thanks
I can't get a steady idle.
I adjust the idle mixture screws to get maximum vacuum (5 in Hg). Then when I rev it up it release throttle it idles about 1100 rpm and won't come back down. I back off the throttle adjustment screw and lower the idle then it settles back down.
Here's engine details (close to old LS6 specs)
468 cu in
Holley 850 mechanical secondaries (recently rebuilt)
990 rect port heads
Ls6 dual plane aluminum intake
10.25 CR
MSD Dist vacuum advance disconnected
Initial timing 14-deg, total timing 35-deg
10 in Hg at 1100 rpm
5 in Hg at 700 rpm
Running 2.5 in power valves
Thanks
Scott
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Are you really running 2.5" power valves? Ya know, the power valve supplies fuel to the main circuit, if your car is idling on the idle circuit, the power valve can be as open as it wants to be, it won't affect your air/fuel ratio. You should rig up a vacuum gauge thru the firewall so you can see what vacuum is while cruising, then you can decide what powervalve to use based on that.
Scott
Scott
Neal
Thread Starter
Pro
Joined: Nov 2008
Posts: 680
Likes: 5
From: Denham Springs LA
Neal and Scotty,
I wasn't sure if the power valve affected the idle or not. I know that the power valves open when the vacuum drops below the power valve vacuum rating. I had 6.5's in the carb last week, and since at idle I have about 5 in hg, I thought they might be opening and causing/contributing to my up and down idle.
With the 6.5 power valves I had to back the idle mixture screws out about 5 turns to get a decent idle quality and not smell loads of unburnt gas coming from the tail pipes.
With the 2.5 power valves the idle mixture screws are backed out maybe 3 turns. Does that make sense.
I am working on vacuum issues today (headlight actuators and ac/heater) and found that my AC vacuum selector switch is leaking. I just ordered a new one. So with these 2 items fixed maybe I will pickup a little more vacuum from the intake.
Soon as I get the new parts installed I will take it for a spin with a pressure gage so I can see the manifold vacuum while driving.
Thanks for the help guys.
Roger
I wasn't sure if the power valve affected the idle or not. I know that the power valves open when the vacuum drops below the power valve vacuum rating. I had 6.5's in the carb last week, and since at idle I have about 5 in hg, I thought they might be opening and causing/contributing to my up and down idle.
With the 6.5 power valves I had to back the idle mixture screws out about 5 turns to get a decent idle quality and not smell loads of unburnt gas coming from the tail pipes.
With the 2.5 power valves the idle mixture screws are backed out maybe 3 turns. Does that make sense.
I am working on vacuum issues today (headlight actuators and ac/heater) and found that my AC vacuum selector switch is leaking. I just ordered a new one. So with these 2 items fixed maybe I will pickup a little more vacuum from the intake.
Soon as I get the new parts installed I will take it for a spin with a pressure gage so I can see the manifold vacuum while driving.
Thanks for the help guys.
Roger
