Maybe there is no simple answer as every engine is different but according to every thing I can find on the internet an advanced engine needs, wants (maybe not the correct word?) a more rich mixture and if you retard your timing the mixture should be leaned out. And vice versa.

In theory, in a perfect world, with the stars aligned, is that a true statement?

Have you ever heard anyone say something along the lines of "Oh, you want to change your timing by 2 degrees don't forget to turn your idle mixture screws in/ out 1/8 of a turn"

 

This might shed some light:

 

Setting the idle mixture screws is the LAST adjustment to make to the engine. Idle mixture has NO EFFECT on timing.

Make sure you have NO vacuum leaks anywhere in the system FIRST. Then set timing & dwell (if you have points). Rough-set idle speed. Follow good procedure for adjusting idle mixture screws (gradual changes to one, then the other) and re-adjust idle speed after each change. Continue until mixture screws find their 'sweet spot'.

Now make any carb rich/lean changes required to get A/F ratios as you wish. Repeat adjustment series with idle screws and idle speed. Done.

 

Yes it will affect your A/FR depending on it if was set properly to start with and how much more vacuum you gain

1. I set mine so the engine will idle.
2. I get the timing where I want it
3. readjust the throttle linkage so its idleing at the desired speed
4. Then set the idle mixture screws so either you get the highest vacuum or if you have an A/FR gauge, set them for the 14.7 reading
5. reset the throttle linkage for idle speed
6. recheck A/FR for any change

 

It is absolutely silly to think of timing causing rich or lean A/F. Regardless of where the timing is set at.......when the piston goes down the bore it is going to "signal" and pull the same amount of fuel out of the booster.
The confusion lies in that charge being burned or not burned thoroughly.....advance is there to light the charge slightly before TDC so the flame front can get a head start as everything is happening so quickly.....but there is a sweet spot of how early to light it off......too early and it will light off and try to push the piston back the way it came, which is what folks call detonation. Too late and the charge does not get completely burned......both of these situations can affect the reading of an A/F meter......but it does squat as far as tuning the actual carb is concerned. You set the timing first, then adjust the A/F to the place where the engine is happiest......not the other way around. Because timing can throw off the A/F meter......but it is not changing how much actual fuel is being pulled by the booster. So you set the timing then adjust the A/F.
when setting timing, say....per Lars papers.....the 36 degree total number is strictly a baseline, this may not be where the engine makes peak power....but you set it there because we know it is damn close and will get you into a zone where you can now adjust the carb........you can tweak timing and jetting from there in small increments......
You might say "Well, get it right the first time and you will not need to tweak it...", and the blowhard geniuses of the internet have told me this many times. But if somebody could get a carbed engine right the first time then they are on a higher plane of existence than myself or say Warren Johnson and Austin Coil. If you get it close, you are already ahead of 75% of Vintage Vette's that are set improperly. It is amazing how important timing is and the problems it causes when it is wrong but it is the most misunderstood and neglected part of an engines tuning.
Anyway.....set timing, tune carb. You do not specifically have to set idle screws last.......I like to set them before I jet so the thing isn't blubbering in my garage or trying to stall out.

Jebby

 

Watched the video. Looks like timing does not affect the AF ratio that much that the idle screws need adjusted if I decide to go up or down a couple degrees of timing.

I've always wondered because when I took my car into a dealership he found the distributor was a tooth off and reset the timing to 8 degrees and he said he adjusted the idle screws.

I've now got it 16 initial and it looks like about 37 at 2650 rpm (52 with advance hooked up) so close enough without getting into weights or springs. I think I recall the guy that restored my car saying the guy he had do the engine set it up for best performance but I didn't know what any of that meant 7 years ago.

Anyway, when trying to find a sweet spot I don't see a difference on the vacuum gauge anywhere between 2.5 and 4.5 turns out on the screws (and I forgot where it was set - rookie mistake) but when I test drove it at those settings I had sputtering at WOT at 2.5 and flooding/stalling while braking at the 4.5 so now I have them at 3.5 and it seems to have eliminated those issues.

That brings up another question in my brain- if there is negligible difference in performance in that 2.5-4.5 turn window would you "lean" toward the lean side or the rich side? Or keep it in the happy medium knowing I'm not doing any harm?


If you can't tell I have time on my hands

 

Error to the side of rich. You may have the vacuum gauge on the wrong port....what does it read? Does the engine respond to the idle drop method?

Jebby

 

If no vacuum leaks in your system, you should be able to adjust the idle mixture screws to where 1/4 turn in either direction will cause the engine to drop a few rpm with each screw. You indicated that you don't see/hear any difference with two turns of your idle screws. Something else is going on or you have some difficulties with detecting change. If detection is your issue, you may want to connect a GOOD tachometer to the ignition system while doing your adjusting. Then you can see small changes in rpm as you are working.

 

I have the vacuum gauge hooked up to the hose from the rear of the manifold that goes to the filter and check valve.

With the advance hooked up it just kisses 15 for the most part between 2.5 - 4.5 turns then above 4.5 it gets close to 16 but that's where it would flood out and die. With advance disconnected it's more like 14.

I don't know the cam specs but it's what would have been in a 69 or 70 L46 I BELIEVE. Unless the base engines would have had a bigger cam than mine in those years then that might be what's in it. (I remember telling the guy I did not want this thing sounding like my dad's base engine 75 he had when I always 5)

I use my timing light as a tach. The Rpms do change more than the vacuum reading does but most of the things I read said go by vacuum when adjusting the screws.

I might put together a little chart for fun.

 

Checking my notes it was actually at 4 7/8 turns when it stalled so I took my second test up to 5 turns.

So according to this I should go ahead and run it at 4.5 turns and if it doesn't flood I'm set.

 

Seems like a lot..friend called yesterday had a similar issue and after fooling with the air bleed sizes nailed it.
Doesnt answer much but recall Joe Sherman on Speedtalk used to harp to first get the ignition curve/timing perfect. Then attack the carb...if using an 02 sensor get close to the proverbial perfect 02 readings at idle cruise and WOT. From there adjust to give it what it wants regardless of #s.

 

Just drove it on the highway to get dinner and it wanted to die pulling into the drive thru. This was at 4 turns. Had to give it gas to keep it running a couple times. Took my screwdriver with me and turned it in to 3.5 and it would still bog down just doing starts and stops in the parking lot.

Finally started to hold idle on braking and left to go home. One stoplight had to do the brake/ gas to keep it running and it popped a few times getting back on the highway but by the time I got home it was all good.

I guess I could try a 3.75 but it pretty much looks like the decision has been made for me as ti where to set my screws.

 

I wonder if you need to adjust the idle? Or was it idling smoothly after setting the timing...and when you started on the mixture screws it idled roughly..?

 

I'm not sure where you got the idea that you use engine vacuum to adjust the idle mixture screws....but that's not how it's done. You adjust to engine rpm, either increasing or decreasing. Adjusting to max. vacuum is what some old timer's (and I'm 75!) used to do with their flathead Fords. Can you do it that way? Maybe. Is that the best way to do it? No.

Use Google to get a better idea of how to [properly] adjust idle mixture screws. Or you can e-mail Lars for his write-up on tuning/timing. Send an e-mail request to V8FastCars@msn.com and he will send you his latest version of that paper.

 

He rebuilt the carb by the way.

Here's one of the places I read it.

 

But there is a correlation between RPM and vacuum so it's still high score wins regardless of what column you look at.

 

Do you have a vacuum gauge with 0.1 in. Hg increments on it? No. So using a vacuum gauge with 1.0 in. Hg increments is useless to adjust your idle mixture screws. Sorry. Just the way it is.

 

Is this an automatic and if so are you adjusting the screws in drive, parking brake on and wheels chocked?

 

It's a gauge with a needle so I guess it would be as accurate as however many needles can fit between those increments, right?

In that case I'd say it's readable to .2.

 

Manual, in neutral of course, parking brake on / and it works/ and I always keep a front tire chocked in front so I don't drive into my kitchen wall.