LAvetteman

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]

1979toy

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.

LAvetteman

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.

AlwaysWave

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.

LAvetteman

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?

AlwaysWave

I've never done this. But I've read about drilling small 1/16 holes in the throtle plates to pass more air at idle.

LAvetteman

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:

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
:)

lars

Joey -
Don't go drilling your throttle blades. Give me a call.

Fevre

Actually you want to restict the fuel flow at idle, holleys are rich at idle and the idle screws are insensitive. You put a wire about .015" in an l shape in the idle passages and that reduces the fuel flow and makes the idles screws much more sensitive to changes. I drilled holes in all 4 plates to allow more are flow at idle. I have been running high vac cams so I use stagered pv's, I have a 13.5 in the main and 6.5 in the sec pulling 16-18 at idle. I found that I can run leaner at idle and on the mains by having the pv open earlier with the 13.5 pv then still have the sec pv provide fuel under heavier load. I came to that conclusion when I had a hest off idle, I used a vac gauge to see where it was hesitating at (as soon as the 6.5 pv's opened the car would jump) and tried a pv just about at the hest point, problem fixed.

chevymans 77

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
:)

JackDidley

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.