Big cam, stinky rich idle

[dave at hsp]

Howdy! I'm new here so forgive my mistakes.
I talked to Cliff about this a few days ago and he gave me some info of what to do, so this is sort of an update. First some back ground. This is an '80 Camaro, 305 automatic, 350 turbo with a 2500 stall converter, 3.73 gears. The cam is too big for this application, I know. But, it's in there and that's what I'm gonna make work.
 I rebuilt the engine. It's .030 over with flat top pistons, so the compression is now 10.2 to 1. Stock heads. New Comp Cams Thumper cam, lifters, springs, retainers, locks, timing set, installed at 102° intake CL. Its .479 and .465 lift and 227 and 241 @ .050. The carb so far is as follows:
 idle tube: .042
 Idle down channel restriction, brass thread in : .063
 Lower idle air bleed, brass thread in: .052
 upper idle air bleed: .070
 Accelerator pump discharge holes: .028
Main air bleeds ( base plate) .140
Main air bleed (air horn) .120
 I have the lightest spring in the power valve that comes in the Edelbrock 4 spring kit.
 It will idle warm at about 1000 in park, pull it in gear and it drops to between 500 and 800 with a big chop, which is OK, but the idle is stinky rich. Wing the throttle and it rev's right up. No hesitation or stumble. The vacuum at idle in park bounces a bit between 5 and 8 inches. In gear it's barely 5 inches.
 My question is, where do I go next to back the idle screw off about 1 turn to get the throttle plates to where I'm not exposing so much of the transition slot, and still maintain about 800 rpm at idle, not so stinky rich? How big is too big on the main air bleed in the base plate?
 Thanks for any comments, ideas, answers.

Dave

[ejowrench]

You did not mention if you have idle bypass air, Are you familiar with this?  With a cam like that, you will most likely need it.

Are you experiencing nozzle drip at idle?

[dave at hsp]

Yes, I am using idle bypass air. The main air bleed in the base plate is at .140. That is my main question; how big is too big on the main air bleed in the base plate? (for the idle bypass air)
I also didn't mention the timing. The timing is set at 14° initial, 12° vacuum advance, 16° centrifugal, all in at 3500 rpm. The vacuum advance is full vacuum, with a can that comes in at 3 inches and is all in at 8 inches of vacuum. That is part of the problem. Trying to carry enough vacuum to hold the vacuum advance at idle, so the idle won't drop below about 800 rpm. Once I loose the vacuum, I loose the rpm being held by the vacuum advance. If I turn up the main idle screw, the butterfly goes into the transition slot and pulls fuel from the booster.
 I don't see any drip from the booster at the current setting.
 I did check the AFR this afternoon with my LM-2 and it was 13.0 in park at 750 rpm and 13.5 to 13.9 in drive at 550 to 600 rpm.
That was fully warmed up, and it did have a slight, very slight stumble on acceleration. No problem with a quick wing of the throttle.

Thanks.
Dave

[ejowrench]

Dave, I always like to plumb my vacuum advance to a ported vacuum source, and then set the initial timing to suit. I believe Cliff recommends this in his book. I'm not sure if this is your primary problem, but it is worth trying.

[Marx3]

the idle afr sounds pretty good? like a computercontrolled car almost...
What about afr at cruise and wot?
on a sidenote: I have the same LM-2, great little instrument. How far up the tailpipe do you mount yours ? I homemade a bracket for mine, it goes about 2" up the tailpipe, so I often have trouble getting trustworthy readings in idle. ( 25:1 ). No problems once driving though.

[77cruiser]

I'm thinking the idle is too lean, try setting idle to max. vacuum, & see how it is. O2 sensors don't work very well at idle with a big cam every misfire reads excess oxygen & reads it as lean.

[dave at hsp]

ejowrench, thanks for the reply.
 I have not tried ported vacuum. I always set up a big cam engine with full vacuum. Let me tell you why. I try and set the initial at 12 to 16, then run full vacuum and add another 12 to 14, then pull in the centrifugal with another 12 to 14, so I end up with 40 to 42 total at 3500 in the end. I really shoot for the end result as long as I can get my initial good enough to start hot, and the full vacuum at idle so it WILL idle.
 I think if I run ported at idle, it will idle even slower because of not enough advance. Make sense?

Dave

[dave at hsp]

Marx3, I haven't driven it and watched to see where it's at cruising and wot. The roads have been wet and I really haven't had time. My dad was diagnosed with stage 4 cancer 4 weeks ago, so I've been pretty busy with him.
 As far as the sensor mount, I tig welded a bung in the collector on the passenger side, just past the end of the header.
Got my 02 sensor screwed in there. This is the first time I have used the LM-2, so I'm still learning.
 I ran the car down the road Sunday with the kid that owns it, but like a dummy, I didn't have the LM-2 plugged in. It runs really good though. Good throttle response and smooth at wot. I hope to get some readings this weekend, if it ever stops raining.
Thanks!
Dave

[dave at hsp]

77cruiser, by idle too lean, do you mean the idle tubes? .042 was the biggest long bit I had, so I had my daughter order me more bits up to .050 today. (she is a sales rep for MSC industrial supply) So, once I get those, hopefully Friday, I was gonna go up .002 at a time to see what happens. How big is too big on the air bleeds in the base plate? I'm at .140 now and was gonna go another .002 at a time.

Thanks!
Dave

[ejowrench]

Dave, so what you are saying is you set the initial timing at 12degrees, the vac advance at idle adds another 12 degrees, so at idle you have a total of 24 degrees?

When you slowly open the throttle with this set-up, the vacuum drops down, retarding your timing when it should be advancing. On ported vacuum, as soon as you start to open the throttle, the vac advance starts to quickly advance the timing, and then the centrif advance comes on, a nice smooth fast advance curve.  Make sense?

[dave at hsp]

Something like that, yes.
The centrifugal advance starts to come in early and is all in at 3500 rpm, so the timing never really backs up much.
What you say does make sense and I'm gonna give it a try this weekend. I'll let you know what happens.
 I've been doing it this way so long, maybe it's time for a change.
Thanks!
Dave

[77cruiser]

Not necessarily the idle tubes, but could be. How far out did you have the mixture screws? Did you open up the holes for the mixture screws?

[dave at hsp]

Jim, I had the mixture screws out about 3/4 of a turn. 
I opened the screw holes to .106 from .096.

UPS just dropped off a box of long drill bits from MSC.
So I guess I'll be drilling the idle tubes some more this weekend. I'm gonna go a little bigger on the bypass holes in the base plate too. Unless anyone has a better idea.
 It's pretty close, but could still be closer.

Thanks!
Dave

[77cruiser]

3/4 of a turn was thinking should be out 3 or 4 full turns. Maybe more try that first.
Set idle to best idle or highest vacuum.

[dave at hsp]

3/4 of a turn WAS the best idle and highest vacuum.

Dave