Poor Idle - Intermittent Problem
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
Poor Idle - Intermittent Problem
Here's the skinny:
1972 LT-1. Visually stock except for Holley Street avenger carb. Found some old receipts and previous owner purchased a carb (presuming it is this one) in late eighties.
Idle speed drops out the bottom. I have the idle set at 800RPM's with no load. No issues and car drives and performs great. Suddenly the idle will fall out to below 400RPMS and runs stinking rich. This problem can disappear as soon as it appears, sometimes it lasts for a day or two but last summer it lasted until I thought I fixed it. I replaced the fuel filter, fuel pump, plugs, adjusted the floats, and checked the choke operation. Checked for vacuum leaks and found none. Miraculously though I pulled it out of the garage and it ran like a champ, until this past weekend. Idle went away on Saturday night, ran fine yesterday afternoon then started running poorly again last night.
This problem comes and goes for no apparent reason.
Carb rebuild time????
1972 LT-1. Visually stock except for Holley Street avenger carb. Found some old receipts and previous owner purchased a carb (presuming it is this one) in late eighties.
Idle speed drops out the bottom. I have the idle set at 800RPM's with no load. No issues and car drives and performs great. Suddenly the idle will fall out to below 400RPMS and runs stinking rich. This problem can disappear as soon as it appears, sometimes it lasts for a day or two but last summer it lasted until I thought I fixed it. I replaced the fuel filter, fuel pump, plugs, adjusted the floats, and checked the choke operation. Checked for vacuum leaks and found none. Miraculously though I pulled it out of the garage and it ran like a champ, until this past weekend. Idle went away on Saturday night, ran fine yesterday afternoon then started running poorly again last night.
This problem comes and goes for no apparent reason.
Carb rebuild time????
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
A Holley? Maybe the power valve has died. They sometimes get really erratic. Either way, I'd almost bet a good rebuild will help- especially since it's been on htere so long.
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
Checked those, all tight. No oil seepage from any either. Double checked by spraying carb cleaner along the intake and cab base while idling, no improvement, no indication of any leaks.
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
update. I installed a new 670cfm Holley Street Avenger last night. Fired right up, smooth idle at 950rpm, backed out of the garage and it runs like crap. Pulls hard, noticably more than the old carb, but flutters and surges under part throttle (2200rpm or so). Came home and checked the timing. Found it at 14*BTDC. Book says 4* for the LT1 but it wont run at that so I set it at 8*. Still fluttering/surging only at part throttle, smooth idle prevails.
This morning I installed new plugs, wires, cap and rotor. Idle is still smooth but the part throttle problem is still there. I get another beer out of the fridge, tell my buddy about the new problem but also how smooth the idle is and how strong it pulls. I fire it up and guess what, the idle is doing the exact same thing as before! Rough idle fighting to stay about 500rpm. Now I'm at a loss, go on a beer run and stop at the parts store thinking that maybe its the points and condenser. Parts guru says I need a new distributor. I say I need another beer. Debating on trying a new set of points tomorrow.
HELP! I am almost out of beer again and my objectivity is running on empty. I can't keep chasing my tail like this.
This morning I installed new plugs, wires, cap and rotor. Idle is still smooth but the part throttle problem is still there. I get another beer out of the fridge, tell my buddy about the new problem but also how smooth the idle is and how strong it pulls. I fire it up and guess what, the idle is doing the exact same thing as before! Rough idle fighting to stay about 500rpm. Now I'm at a loss, go on a beer run and stop at the parts store thinking that maybe its the points and condenser. Parts guru says I need a new distributor. I say I need another beer. Debating on trying a new set of points tomorrow.
HELP! I am almost out of beer again and my objectivity is running on empty. I can't keep chasing my tail like this.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
5 Best & 5 Worst Corvette Daily Drivers
Joe Kucinski
The Headlights of Every Corvette Generation Explained
Joe Kucinski
5 Best & 5 Most Overrated Corvette Track Packages of All Time!
Joe Kucinski
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
update. I installed a new 670cfm Holley Street Avenger last night. Fired right up, smooth idle at 950rpm, backed out of the garage and it runs like crap. Pulls hard, noticably more than the old carb, but flutters and surges under part throttle (2200rpm or so). Came home and checked the timing. Found it at 14*BTDC. Book says 4* for the LT1 but it wont run at that so I set it at 8*. Still fluttering/surging only at part throttle, smooth idle prevails.
This morning I installed new plugs, wires, cap and rotor. Idle is still smooth but the part throttle problem is still there. I get another beer out of the fridge, tell my buddy about the new problem but also how smooth the idle is and how strong it pulls. I fire it up and guess what, the idle is doing the exact same thing as before! Rough idle fighting to stay about 500rpm. Now I'm at a loss, go on a beer run and stop at the parts store thinking that maybe its the points and condenser. Parts guru says I need a new distributor. I say I need another beer. Debating on trying a new set of points tomorrow.
HELP! I am almost out of beer again and my objectivity is running on empty. I can't keep chasing my tail like this.
This morning I installed new plugs, wires, cap and rotor. Idle is still smooth but the part throttle problem is still there. I get another beer out of the fridge, tell my buddy about the new problem but also how smooth the idle is and how strong it pulls. I fire it up and guess what, the idle is doing the exact same thing as before! Rough idle fighting to stay about 500rpm. Now I'm at a loss, go on a beer run and stop at the parts store thinking that maybe its the points and condenser. Parts guru says I need a new distributor. I say I need another beer. Debating on trying a new set of points tomorrow.
HELP! I am almost out of beer again and my objectivity is running on empty. I can't keep chasing my tail like this.
You never said a thing about checking float levels or just a basic general check of the carb. I'd start there. Vacuum leaks, float levels, are all the screws tight in the new carb? air bleeds all clear?What's your vacuum reading at idle? Is the timing staying in one place at idle, or jumping all over?
I've alway had really good luck(?) applying the KISS principle. If it's pulling good, I'd not be very quick to junk the ignition. It'd be on the list, but not very close to the top. Plug wires, plugs, points/condenser will usually all show up under a load before they do at idle.
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
To start, buy more beer.
You never said a thing about checking float levels or just a basic general check of the carb. I'd start there. Vacuum leaks, float levels, are all the screws tight in the new carb? air bleeds all clear?What's your vacuum reading at idle? Is the timing staying in one place at idle, or jumping all over?
I've alway had really good luck(?) applying the KISS principle. If it's pulling good, I'd not be very quick to junk the ignition. It'd be on the list, but not very close to the top. Plug wires, plugs, points/condenser will usually all show up under a load before they do at idle.
You never said a thing about checking float levels or just a basic general check of the carb. I'd start there. Vacuum leaks, float levels, are all the screws tight in the new carb? air bleeds all clear?What's your vacuum reading at idle? Is the timing staying in one place at idle, or jumping all over?
I've alway had really good luck(?) applying the KISS principle. If it's pulling good, I'd not be very quick to junk the ignition. It'd be on the list, but not very close to the top. Plug wires, plugs, points/condenser will usually all show up under a load before they do at idle.
Sprayed carb cleaner around base and also around intake and found no changes in idle quality. (no vacuum gauge)
Carb to intake is tight but not overtight. New gasket installed with carb.
This problem with the idle was the reason for the carb swap. It has happened a couple of time before and went away for no apparent reason. I thought about intake manifold leaks but hate to pull the intake before looking at everyhting else.
Le Mans Master
Joined: Jan 2004
Posts: 8,295
Likes: 30
From: one night in Bangkok you are never lonely
I had the problem with the idle dropping and wanting to stall but only when it is hot out, long traffic lights heavy traffic ect. But when going down the road it was fine also when cold. When I first built the car I changed the fuel pump to a Holley but it did not have a return to tank setup like the stock pump. So I changed to a return line pump plus added a heat spacer under the carb. And the problem went away. Vapor locking in the fuel line.
Le Mans Master
Joined: Mar 2002
Posts: 8,575
Likes: 3
From: On an island surrounded by sharks Hawaii
Did you open up the old carb?
I had a problem like this on my 73. Opened carb and found a fine mud like substance that turned out to be rust from the gas tank.
The rust was so fine that it was able to get right pass the new fuel filters. My mechanic cleaned out the carb, then we drain the gas
tank, took filler neck and boot off and found very fine particles
of rust in the gas and on the bottom of tank. Luckly the rust spots
were not that big and bad, wire brushed them and sanded them down.
Long skinny arms are helpful now.
Blew out the fuel lines and fuel pump with air. Installed two new
filters at the tank outlet and a new one up front. Works fine now.
HTH
I had a problem like this on my 73. Opened carb and found a fine mud like substance that turned out to be rust from the gas tank.
The rust was so fine that it was able to get right pass the new fuel filters. My mechanic cleaned out the carb, then we drain the gas
tank, took filler neck and boot off and found very fine particles
of rust in the gas and on the bottom of tank. Luckly the rust spots
were not that big and bad, wire brushed them and sanded them down.
Long skinny arms are helpful now.
Blew out the fuel lines and fuel pump with air. Installed two new
filters at the tank outlet and a new one up front. Works fine now.
HTH
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
Running down to the parts store for a new fuel filter and vac advance kit. I'll try it and look for other suggestions in the meantime.
So far it sounds as though timing is not the culprit???
So far it sounds as though timing is not the culprit???
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Replace points/cond.
Set Timing according to this post
http://forums.corvetteforum.com/show....php?t=1430339
From what I've read, you've done everything on the ignition system except points/cond. When you are doing this, check the dist shaft for play. Also inpsect the dist. cam lobes for wear. How's your plug wires?
You've replaced the carb and filter. If you have the carb set-up right, then all you have left here is the fuel pump and fuel lines.
IMHO finish the ignition system first. Make sure it is ok and them move on to the carb....but that is my HO.
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
back from Sears where I picked up a dwell meter. Also got a vacuum tester which I will try on the vac advance in a minute.
Initial timing now at almost 14*BTDC. Dwell checked out at a solid 30* without wavering. Curb idle set at 950rpm.
New fuel pump, filter and line to the cab last spring.
As my luck runs, the car fired up on this cold Carolina morning like a champ. Even after if warmed up completely (in the garage) the idle never changed. Held at a good 950 to 1000rpm the whole time once the choke came down. I haven't road tested it yet to see if the bump in the timing to 14* has done anything with the part throttle fluttering. If that is gone at least I' m back to square one again with the intermittent idle problem.
Initial timing now at almost 14*BTDC. Dwell checked out at a solid 30* without wavering. Curb idle set at 950rpm.
New fuel pump, filter and line to the cab last spring.
As my luck runs, the car fired up on this cold Carolina morning like a champ. Even after if warmed up completely (in the garage) the idle never changed. Held at a good 950 to 1000rpm the whole time once the choke came down. I haven't road tested it yet to see if the bump in the timing to 14* has done anything with the part throttle fluttering. If that is gone at least I' m back to square one again with the intermittent idle problem.
Thread Starter
Instructor
Joined: Dec 2005
Posts: 246
Likes: 1
From: Concord NC
Update:
Road test yesterday proved no difference in part throttle. Actually was worse under a load. Came home and put my new Mityvac to the vacuum advance and found it was shot. Would not hold any vacuum at all. Manifold vacuum is not all that good either at 12-14" but I am overlooking that right now due to the 86k miles on the engine. No wild flucuations either just rapid bouncing between 12 and 14.
This morning I pulled the cap off the gas tank. Wow what a lot of crap is settled on the bottom of the tank. I can see the sock and its black. Pulled the inline filter and guess what, it is filled with the same crap. Ran to the parts store and grabbed another inline filter and a vac advance. Put on the filter and the advance.
Initial timing set at 10*
Dwell solid at 30*
Advance now working!
OLD timing light without dial set back so I am not sure what total timing is.
I hope this works!!
Road test: Great idle and throttle response. Most of the flutter or stumble under load is gone. Still there above 3000rpm cruising. Going to pull the tank and clean it out. Remove the sock.
Still looking for comments, advise or similar problems. This whole ordeal has left me feeling like I've been running through the woods with my eyes closed. It hurts.
Road test yesterday proved no difference in part throttle. Actually was worse under a load. Came home and put my new Mityvac to the vacuum advance and found it was shot. Would not hold any vacuum at all. Manifold vacuum is not all that good either at 12-14" but I am overlooking that right now due to the 86k miles on the engine. No wild flucuations either just rapid bouncing between 12 and 14.
This morning I pulled the cap off the gas tank. Wow what a lot of crap is settled on the bottom of the tank. I can see the sock and its black. Pulled the inline filter and guess what, it is filled with the same crap. Ran to the parts store and grabbed another inline filter and a vac advance. Put on the filter and the advance.
Initial timing set at 10*
Dwell solid at 30*
Advance now working!
OLD timing light without dial set back so I am not sure what total timing is.
I hope this works!!
Road test: Great idle and throttle response. Most of the flutter or stumble under load is gone. Still there above 3000rpm cruising. Going to pull the tank and clean it out. Remove the sock.
Still looking for comments, advise or similar problems. This whole ordeal has left me feeling like I've been running through the woods with my eyes closed. It hurts.
