idle when hot
Thread Starter
Instructor
Joined: May 2023
Posts: 104
Likes: 10
From: Paris
idle when hot
Hi All,
I'm a new comer and happy to join this forum.
My 69 L71 gets issue to idle smoothly once hot. when the engine is cold, all good but after several kilometers, no way to keep it idle. in addition,
i get a K66 TI, ignition coil is new with the right resistance (0,5 ohms and 8 ohms)
timing @ 750 is 4° as per GM specs.
any ideas ?
thank you.
I'm a new comer and happy to join this forum.
My 69 L71 gets issue to idle smoothly once hot. when the engine is cold, all good but after several kilometers, no way to keep it idle. in addition,
i get a K66 TI, ignition coil is new with the right resistance (0,5 ohms and 8 ohms)
timing @ 750 is 4° as per GM specs.
any ideas ?
thank you.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
a good performance tune will help but more info is needed.
car new to you or what?
auto or stick?
have you turned idle screw?
welcome
pics when you can post after some post count increase
car new to you or what?
auto or stick?
have you turned idle screw?
welcome
pics when you can post after some post count increase
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
Lars frequents the C3 forum, but is much quicker to respond if emailed at <V8FastCars@msn.com>
Do a forum search on "lars timing" and you'll be overwhelmed with threads to read. That said, he is a, no exaggeration, carburetor expert, so ask for his Holley carb papers too and mention it's a tri-power that you're having issues with.
Do a forum search on "lars timing" and you'll be overwhelmed with threads to read. That said, he is a, no exaggeration, carburetor expert, so ask for his Holley carb papers too and mention it's a tri-power that you're having issues with.
Last edited by barkingrats; May 20, 2023 at 10:12 AM.
Thread Starter
Instructor
Joined: May 2023
Posts: 104
Likes: 10
From: Paris
thank you for your quick answer. I will try to find these tuning paper. having increase a bit the timing once hot, it works better. any reason why GM specs are not accurate ?
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
If you think about it, best performance can't be a set of single settings. Especially after 50 years. At this point, you really don't have an engine that was just like every other L71 when they were made. Things wear, replacement components installed, fuel formulation changes, driving altitude, etc. all affect the outcome. Proper settings are really a custom diagnosis and treatment plan for your specific setup.
Last edited by barkingrats; May 20, 2023 at 10:43 AM.
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Verdad Gallardo
Race Director
Joined: Mar 2017
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From: Virginia
The GM specs are 100% accurate for their day. The goal then was NOT performance, driveability, or low underhood temperatures.
Follow @lars' papers, and time your engine "correctly", based on current understanding of what works best. You'll be surprised how much better it performs.
Follow @lars' papers, and time your engine "correctly", based on current understanding of what works best. You'll be surprised how much better it performs.
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
Your problem might be as simple as adjusting the throttle stop screw on the carb linkage.
When cold, the idle speed is increased by the cold run cam on the choke linkage on the pass side of the Q Jet carb. When warm, the idle speed is controlled by the screw on the linkage on the driver side of the carb.
Also, as stated above, some GM spec's are for emissions, not performance. I think you probably do need more innitial timing.
When cold, the idle speed is increased by the cold run cam on the choke linkage on the pass side of the Q Jet carb. When warm, the idle speed is controlled by the screw on the linkage on the driver side of the carb.
Also, as stated above, some GM spec's are for emissions, not performance. I think you probably do need more innitial timing.
Thread Starter
Instructor
Joined: May 2023
Posts: 104
Likes: 10
From: Paris
The GM specs are 100% accurate for their day. The goal then was NOT performance, driveability, or low underhood temperatures.
Follow @lars' papers, and time your engine "correctly", based on current understanding of what works best. You'll be surprised how much better it performs.
Follow @lars' papers, and time your engine "correctly", based on current understanding of what works best. You'll be surprised how much better it performs.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,247
From: US-PNW
Your problem might be as simple as adjusting the throttle stop screw on the carb linkage.
When cold, the idle speed is increased by the cold run cam on the choke linkage on the pass side of the Q Jet carb. When warm, the idle speed is controlled by the screw on the linkage on the driver side of the carb.
When cold, the idle speed is increased by the cold run cam on the choke linkage on the pass side of the Q Jet carb. When warm, the idle speed is controlled by the screw on the linkage on the driver side of the carb.
Thread Starter
Instructor
Joined: May 2023
Posts: 104
Likes: 10
From: Paris
Is my vette stock?? HAHA
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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I wouldnt just start turning screws..
YOu need to get your car hot enough that the choke releases the high idle and the car idles down. disconnect the vacuum advance to the distributor, plug the line and bump the timing up to 14 degrees BTDC. This will raise the rpm of the motor. It will also give you some better performance. After you get Lars timing papers you can learn to set the timing curve. If it will idle around 900rpm leave it, reset your idle mixture screws for best vacuum at an idle your car likes which may be 800 to 1000 rpm..If its higher back the idle set screw out. You dont want to turn that screw in to much because it will open the throttle plates and if they open too much you will move past the idle circuit and be idling in your transition circuit which will give you more problems and make the idle mixture screws not work. If your car stalls when you are coming to a stop after these adjustments you need to raise your float levels to give the idle circuit more head pressure to help keep fuel flowing through the idle circuit
YOu need to get your car hot enough that the choke releases the high idle and the car idles down. disconnect the vacuum advance to the distributor, plug the line and bump the timing up to 14 degrees BTDC. This will raise the rpm of the motor. It will also give you some better performance. After you get Lars timing papers you can learn to set the timing curve. If it will idle around 900rpm leave it, reset your idle mixture screws for best vacuum at an idle your car likes which may be 800 to 1000 rpm..If its higher back the idle set screw out. You dont want to turn that screw in to much because it will open the throttle plates and if they open too much you will move past the idle circuit and be idling in your transition circuit which will give you more problems and make the idle mixture screws not work. If your car stalls when you are coming to a stop after these adjustments you need to raise your float levels to give the idle circuit more head pressure to help keep fuel flowing through the idle circuit
Thread Starter
Instructor
Joined: May 2023
Posts: 104
Likes: 10
From: Paris
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
runs rough, stalls.
min spec idle rpms and mixture screws settings, timing work together to fight that.
at this point you are going to have to check everything,
plugs, wires, spark intensity
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
If the timing is at 4 degrees at idle....you are probably leaving about 75hp on the table when you drive.......
The L-71 has a relatively long overlap camshaft that wants a lot of initial timing......
You will not know for sure until you are done.....but if you set timing per Lars papers.....with the total at 36-38 degrees......your initial will be in the 15 degrees range.....not only that, you need to run the vacuum advance off of full manifold vacuum....this will help idle quality a lot too and keep underhood temps down during traffic. Retarded timing specs cause elevated EGT's (exhaust gas temperatures) and raise underhood temps.....
Lawyers and the Gov dictated the original timing specs.......all the way into the 90's until cars lost the distributor...even after engine timing was computer controlled it is on the light side.......just about any car produced can be tuned for optimal timing and pickup power and efficiency......
I have tuned my wife's Equinox and my Silverado and the difference is very noticable.....
Jebby
The L-71 has a relatively long overlap camshaft that wants a lot of initial timing......
You will not know for sure until you are done.....but if you set timing per Lars papers.....with the total at 36-38 degrees......your initial will be in the 15 degrees range.....not only that, you need to run the vacuum advance off of full manifold vacuum....this will help idle quality a lot too and keep underhood temps down during traffic. Retarded timing specs cause elevated EGT's (exhaust gas temperatures) and raise underhood temps.....
Lawyers and the Gov dictated the original timing specs.......all the way into the 90's until cars lost the distributor...even after engine timing was computer controlled it is on the light side.......just about any car produced can be tuned for optimal timing and pickup power and efficiency......
I have tuned my wife's Equinox and my Silverado and the difference is very noticable.....
Jebby
Thread Starter
Instructor
Joined: May 2023
Posts: 104
Likes: 10
From: Paris
If the timing is at 4 degrees at idle....you are probably leaving about 75hp on the table when you drive.......
The L-71 has a relatively long overlap camshaft that wants a lot of initial timing......
You will not know for sure until you are done.....but if you set timing per Lars papers.....with the total at 36-38 degrees......your initial will be in the 15 degrees range.....not only that, you need to run the vacuum advance off of full manifold vacuum....this will help idle quality a lot too and keep underhood temps down during traffic. Retarded timing specs cause elevated EGT's (exhaust gas temperatures) and raise underhood temps.....
Lawyers and the Gov dictated the original timing specs.......all the way into the 90's until cars lost the distributor...even after engine timing was computer controlled it is on the light side.......just about any car produced can be tuned for optimal timing and pickup power and efficiency......
I have tuned my wife's Equinox and my Silverado and the difference is very noticable.....
Jebby
The L-71 has a relatively long overlap camshaft that wants a lot of initial timing......
You will not know for sure until you are done.....but if you set timing per Lars papers.....with the total at 36-38 degrees......your initial will be in the 15 degrees range.....not only that, you need to run the vacuum advance off of full manifold vacuum....this will help idle quality a lot too and keep underhood temps down during traffic. Retarded timing specs cause elevated EGT's (exhaust gas temperatures) and raise underhood temps.....
Lawyers and the Gov dictated the original timing specs.......all the way into the 90's until cars lost the distributor...even after engine timing was computer controlled it is on the light side.......just about any car produced can be tuned for optimal timing and pickup power and efficiency......
I have tuned my wife's Equinox and my Silverado and the difference is very noticable.....
Jebby
your recommandation for initial timing around 15° is for the cameshaft right....not for the distributor ?