Having some tuning questions.
Thread Starter
Safety Car
Joined: Jan 2001
Posts: 4,660
Likes: 3
From: South Central Louisiana
St. Jude Contributor
Having some tuning questions.
OK, here is the current state of tune with my 71.
I get the best vacuum reading with the idle mix screws halfway out, at idle I have 8-9 on the vac gauge.
I go to the distributor which was set with about 20 degrees intial advance and increase till I got about 10-11 on the vac gauge, now my intial timing is 45 degrees. This allowed me to back off a tad on the idle speed screws and gave me a MUCH better off idle response. But above 2500 rpm it doesn't sound to healthy.(too much advance)
What should I do??
What power valve should I run with an idle vac of 8-11 which goes to 20 or better on rev?
For all you guys running A/F meters, I have tried 2 new O2 sensors and even after a jet change down to 78/84 the meter steadily climbs to rich within 30 seconds of cold start up and stays there.
[Modified by LAvetteman, 9:25 PM 10/11/2003]
I get the best vacuum reading with the idle mix screws halfway out, at idle I have 8-9 on the vac gauge.
I go to the distributor which was set with about 20 degrees intial advance and increase till I got about 10-11 on the vac gauge, now my intial timing is 45 degrees. This allowed me to back off a tad on the idle speed screws and gave me a MUCH better off idle response. But above 2500 rpm it doesn't sound to healthy.(too much advance)
What should I do??
What power valve should I run with an idle vac of 8-11 which goes to 20 or better on rev?
For all you guys running A/F meters, I have tried 2 new O2 sensors and even after a jet change down to 78/84 the meter steadily climbs to rich within 30 seconds of cold start up and stays there.
[Modified by LAvetteman, 9:25 PM 10/11/2003]
Melting Slicks
Joined: Dec 2000
Posts: 2,041
Likes: 0
From: Wichita, KS
Re: Having some tuning questions. (LAvetteman)
Start looking for a vacum leak.
You should be using about a 6.5 power valve.
Try using heavy advance springs while you set your mixture and timing. Then swap in the set of springs that you will be using and do a final check.
What type of O2 senser are you using? My one wire sensor works great after it has a couple of minutes to warm up. If you are using a 3 wire it may be wired wrong.
You should be using about a 6.5 power valve.
Try using heavy advance springs while you set your mixture and timing. Then swap in the set of springs that you will be using and do a final check.
What type of O2 senser are you using? My one wire sensor works great after it has a couple of minutes to warm up. If you are using a 3 wire it may be wired wrong.
Thread Starter
Safety Car
Joined: Jan 2001
Posts: 4,660
Likes: 3
From: South Central Louisiana
St. Jude Contributor
Re: Having some tuning questions. (1979toy)
99% sure of no vac leaks, thank goodness.
I do believe 6.5 is whats in there, check tommorow.
But don't you think I have too much advance?? Does idle and off idle better with that much though.
I am using the heated sensor. First one was Nordsk or whatever summit sells. The new one is a GPS.
I do believe 6.5 is whats in there, check tommorow.
But don't you think I have too much advance?? Does idle and off idle better with that much though.
I am using the heated sensor. First one was Nordsk or whatever summit sells. The new one is a GPS.
Le Mans Master
Joined: Feb 1999
Posts: 6,807
Likes: 0
From: Hoosier
Cruise-In III Veteran
Re: Having some tuning questions. (LAvetteman)
Can you describe your engine a bit? Do you have a pretty lopey cam?
First thing I always think of, is the transistion slots on the carb. If the idle adjust is set so that too much slot is exposed, your idle adjust mixture screws do little or nothing.
Another thought is to increase your idle rpm, and back down on the timing. Once again, making sure of the transistion slots.
I'm using the 3 wire 02 sensor Autometer sent with their gauge. After about 30 seconds from cold start up, I have a 13.5 - 14.0 reading.
First thing I always think of, is the transistion slots on the carb. If the idle adjust is set so that too much slot is exposed, your idle adjust mixture screws do little or nothing.
Another thought is to increase your idle rpm, and back down on the timing. Once again, making sure of the transistion slots.
I'm using the 3 wire 02 sensor Autometer sent with their gauge. After about 30 seconds from cold start up, I have a 13.5 - 14.0 reading.
Thread Starter
Safety Car
Joined: Jan 2001
Posts: 4,660
Likes: 3
From: South Central Louisiana
St. Jude Contributor
Re: Having some tuning questions. (AlwaysWave)
Always, yes cam is extremly lopey.
Thats what I am trying to do, keep as much transition slot covered, because I know thats how it should be and I can sure tell when they aren't covered enough.(off idle hesitation)
I cant increase rpm with out uncovering the *****.
13.5-14.0 reading, is that all the way rich or not?
Thats what I am trying to do, keep as much transition slot covered, because I know thats how it should be and I can sure tell when they aren't covered enough.(off idle hesitation)
I cant increase rpm with out uncovering the *****.
13.5-14.0 reading, is that all the way rich or not?
Thread Starter
Safety Car
Joined: Jan 2001
Posts: 4,660
Likes: 3
From: South Central Louisiana
St. Jude Contributor
Re: Having some tuning questions. (AlwaysWave)
I have heard this talked about alot lately. You have me conviced, thats what I will do. Thanks Always.
BTW, our pair is one year off. 72&95 71&94 :thumbs:
BTW, our pair is one year off. 72&95 71&94 :thumbs:
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
Re: Having some tuning questions. (LAvetteman)
Lavetteman
I have a Holley carb tunning book that I can scan the section on tunning for a large cam and send to you. I have tried both drilling the hole in the primary plates and setting the secondary plates open to allow more air in at idle, I prefer to open the secondary plates over drilling a hole in the primaries. It also talks about altering the idle air bleeds to richen the idle mixture so the idle screws will allow more fuel into the motor allowing you to have some control over the idle mixture.
Neal
:)
I have a Holley carb tunning book that I can scan the section on tunning for a large cam and send to you. I have tried both drilling the hole in the primary plates and setting the secondary plates open to allow more air in at idle, I prefer to open the secondary plates over drilling a hole in the primaries. It also talks about altering the idle air bleeds to richen the idle mixture so the idle screws will allow more fuel into the motor allowing you to have some control over the idle mixture.
Neal
:)
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Race Director
Joined: Mar 2001
Posts: 11,322
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From: Living in the Hartland
Re: Having some tuning questions. (chevymans 77)
Lavetteman
I have a Holley carb tunning book that I can scan the section on tunning for a large cam and send to you. I have tried both drilling the hole in the primary plates and setting the secondary plates open to allow more air in at idle, I prefer to open the secondary plates over drilling a hole in the primaries. It also talks about altering the idle air bleeds to richen the idle mixture so the idle screws will allow more fuel into the motor allowing you to have some control over the idle mixture.
Neal
:)
I have a Holley carb tunning book that I can scan the section on tunning for a large cam and send to you. I have tried both drilling the hole in the primary plates and setting the secondary plates open to allow more air in at idle, I prefer to open the secondary plates over drilling a hole in the primaries. It also talks about altering the idle air bleeds to richen the idle mixture so the idle screws will allow more fuel into the motor allowing you to have some control over the idle mixture.
Neal
:)
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
Re: Having some tuning questions. (LAvetteman)
Large cams with excessive overlap need a richer mixture at idle than the stock carb can supply. By putting a .015 piece of wire in the idle air bleed you will reduce the idle air restriction area and allow the idle fuel circuit to draw more fuel at idle. This along with drilling the primary butterflies or opening the secondary butterflies with the setscrew will allow the needed air so the transition slots can remain covered reducing the chance of causing an off idle stumble by having the primaries open to much. This will alllow you to turn your mixture screws in some and give you some control.
Neal
:)
Neal
:)
Race Director
Joined: Sep 2003
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From: Database Error Indiana
Re: Having some tuning questions. (LAvetteman)
I believe mixture screws should be at least 1 turn out but no more than 2 1/2 turns out. Last one I worked on had to idle at 850 RPM . 16 degrees initial advance and light springs on mechanical. Very high initial timing makes for hard cranking. Did you ceck and be sure the outer ring on the balancer has not slipped ? good luck.