Car won't idle
Thread Starter
Instructor
Joined: Oct 2004
Posts: 124
Likes: 1
Car won't idle
I have a stock 71 LT1 with stock Holley carb and A.I.R removed. Car used to idle fine and now will not idle when hot. Idles ok 8-900 rpm until it warms up, then slowly dies at a stoplight. Checked timing - 14 degrees initial, idle mixture and idle rpm set. Even took fuel tank cap off to see if it was not venting properly with no luck. Hotter it gets shorter time it will run. Have also sprayed around bottom of intake and carb for leaks and nothing. Retorqued carb and intake mounting bolts/nuts. New fuel filters installed. Also checked vacuum hoses and no leaks unless I missed it. Plug wires and coil are new in the last year. Choke seems to work ok - butterfly almost closed when cold and open when hot. Also new PCV valve did not help. Checked float levels at bottom of sight plug.
Don't know how to check vacume advance but why would that be different with higher temp?
Any ideas?
Thanks
Neil
Don't know how to check vacume advance but why would that be different with higher temp?
Any ideas?
Thanks
Neil
Instructor
Joined: Sep 2004
Posts: 116
Likes: 0
From: Northern California
.
I had a similar problem.
Turned out the heat was boiling the fuel causing vapor lock.
I switched from the mechanical fuel pump to Holley electric "blue". Fuel has a higher boiling point under pressure - problem solved.
Turned out the heat was boiling the fuel causing vapor lock.
I switched from the mechanical fuel pump to Holley electric "blue". Fuel has a higher boiling point under pressure - problem solved.
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
If the float is stuck it should be stuck and would run lousy cold or hot.
I don't understand how Hinz-73 fix worked since the pressure has a limit. Any more than about 6 psi and your forcing the needle off its seat, regardless of how much fuel is in the bowl and whether the float is trying to close the valve. Maybe he went from 4-6?
I did the same fix but it was for hot soak with the engine off, for the same reason. Electric pump came with a regulator.
Check for vacuum leaks. What does your engine idle at when hot but not dying?
I don't understand how Hinz-73 fix worked since the pressure has a limit. Any more than about 6 psi and your forcing the needle off its seat, regardless of how much fuel is in the bowl and whether the float is trying to close the valve. Maybe he went from 4-6?
I did the same fix but it was for hot soak with the engine off, for the same reason. Electric pump came with a regulator.
Check for vacuum leaks. What does your engine idle at when hot but not dying?
Thread Starter
Instructor
Joined: Oct 2004
Posts: 124
Likes: 1
As the car heats up the idle gets lower. Starts off around 900 when cold, then 800 when warm, then 700 when hot, then drops to 0 over about a 5 second period and shuts off. What is the best way to check for vacume leaks? I have inspected all the hose within easy reach and all looks ok and have sprayed carb cleaner aound the base of the carb and intake. If I plug my vac guage directly into the vacume advance what should it read at idle?
Thanks again
Neil
Thanks again
Neil
Le Mans Master
Joined: Apr 1999
Posts: 6,134
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From: Raleigh North Carolina
With the choke off, so that the primary throttle plates are completely closed, and the throttle spring removed, grab a hold of the throttle lever on the primaries. Wiggle it up & down.... but NOT side to side. Side to side (passenger to driver) movement is normal. If the shaft moves a lot in its bore, then your carb may need bushings in the throttle shaft bore. Typical symptoms are a rough, non-adjustable idle when hot (choke is off). Air goes between the shaft and its bore causing a lean condition at idle... which the idle ports cannot compensate for. The mechanic typically turns up the idel set screw until the carb is running on the non-adjustable transfer slot/ports instead of the adjustable idle circuit. When the choke is set, the engine gets enough gas. When the choke goes off, the system leans out & stalls.
Another symptom is that the engine idle is erratic... as the shaft intermittently seals & unseals between the shaft & bore.
This may not be your problem as there can be several causes, but it is a common problem with old Holleys. I have the tooling to repair this and have done many of these, and also many Q-Jets with the same problem.
Troubleshoot the easy stuff first though.
Another symptom is that the engine idle is erratic... as the shaft intermittently seals & unseals between the shaft & bore.
This may not be your problem as there can be several causes, but it is a common problem with old Holleys. I have the tooling to repair this and have done many of these, and also many Q-Jets with the same problem.
Troubleshoot the easy stuff first though.
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Don't overlook the obvious... Make sure both your idle speed screws are adjusted properly. It almost sounds like once the choke comes off, you die...any chance the primary idle screw is set wrong? Fast (dead cold) idle should be higher than 900, BTW. My HOT idle is 800.
Just a slightly applicable anecdote... One time I replaced the jets in the main meter block and when I reassembled the carb (on the car) I couldn't get it to start when cold. It was fine when warm, though. It just gave me hell trying to get it started when cold. I just couldn't figure out why all of a sudden I had problems. The frustration was kindof blinding me to the real issue. I pulled the plugs and cleaned/gapped them, checked the ignition, fuel pressure, etc. Turns out that when I re-assembled the front bowl, I forgot the accelerator pump arm. Since the carb doesn't have a choke, I wasn't getting any fuel unless I floored it and got the secondary shooter going. Simple crap seems to cause me the most frustration.
Just a slightly applicable anecdote... One time I replaced the jets in the main meter block and when I reassembled the carb (on the car) I couldn't get it to start when cold. It was fine when warm, though. It just gave me hell trying to get it started when cold. I just couldn't figure out why all of a sudden I had problems. The frustration was kindof blinding me to the real issue. I pulled the plugs and cleaned/gapped them, checked the ignition, fuel pressure, etc. Turns out that when I re-assembled the front bowl, I forgot the accelerator pump arm. Since the carb doesn't have a choke, I wasn't getting any fuel unless I floored it and got the secondary shooter going. Simple crap seems to cause me the most frustration.
Last edited by CGGorman; Aug 2, 2005 at 10:21 AM.
Thread Starter
Instructor
Joined: Oct 2004
Posts: 124
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I recently wired the exhaust riser open after finding it was essentially rusted shut. This idle problem began occuring before that and has recently gotten worse. I will take a look at the throttle shaft next.
Thanks
Neil
Thanks
Neil
Thread Starter
Instructor
Joined: Oct 2004
Posts: 124
Likes: 1
Trouble shooting my poor idle today and found something. After removing the top ignition shielding pieces and checking to make sure all plug wires went to the correct plug I started engine to look for any arcing with big garage door closed for darkness. Didn't see anything but-- when I began reinstalling the side shielding with engine still running the passenger side shielding began arcing around the wingnut hole in the orange shielding support arm. Drivers side does not do it. All plugs wires on that side are firing. I removed one at time to see if removing one would stop the arcing - it did not.
I don't have a spare plug wire but will get one today and substitute it for one at a time to see if that will eliminate the arcing and improve idle etc. I am guessing that the 2 or 4 wire is pinched in the engine mount.
Any thoughts?
Thanks
Neil
I don't have a spare plug wire but will get one today and substitute it for one at a time to see if that will eliminate the arcing and improve idle etc. I am guessing that the 2 or 4 wire is pinched in the engine mount.
Any thoughts?
Thanks
Neil
Le Mans Master
Joined: Dec 2003
Posts: 5,727
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From: Fremont CA
Possibility of you idle mixture being to rich. Mixture screws should be 1 1/2-2 1/4 turns out on a stock LT-1 Holley. You may also have a blown power valve on Holley. Is your idle stop soleniod adjusted correctly? another posibilty.
Thread Starter
Instructor
Joined: Oct 2004
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I have checked the mixture screws and they are ok. I don't have an idle stop solenoid - removed long ago by a PO. Car used to idle ok so I don't think the lack of the solenoid is the problem.
What are other symptoms of a blown power valve? How do I check mine?
Thanks
Neil
What are other symptoms of a blown power valve? How do I check mine?
Thanks
Neil
Thread Starter
Instructor
Joined: Oct 2004
Posts: 124
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Now theres a good thought but it idled ok for over a year and now is getting worse. Can the fuel pump gradually deteriorate like that? A good thing to check tho as there might be an obstruction in the line or something. What should the fuel pressure be at idle?
Also read further up this thread to place a palm over the primary side of the carb at idle and listen for a change in rpm. I did this several times today and idle did not change. Does this eliminate a vacume leak?
So noone thinks the arcing on the passenger side plug wires is the problem?
Thanks again for all the suggestions. I am learning alot as I go here.
Neil
Also read further up this thread to place a palm over the primary side of the carb at idle and listen for a change in rpm. I did this several times today and idle did not change. Does this eliminate a vacume leak?
So noone thinks the arcing on the passenger side plug wires is the problem?
Thanks again for all the suggestions. I am learning alot as I go here.
Neil
Le Mans Master
Joined: Jul 2005
Posts: 5,752
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From: Austin Texas
Your hand over the carb. gives no change, then not lean...Yes, a fuel pump can fail over a period of time. And, an obstruction is a remote possibility....Fuel pressure should be 4 to 5 lbs. min. at idle, up to 6 or 7 max.