Hey I am one of those dare to be different people with an AFB. Anyway I have an off idle lean spot (Air/Fuel gauge tells me) that I can't tune out. It comes from slowly pulling from a stop. If I hold the throttle just right once I hit the "spot" it will stay off scale lean on the Air Fuel gauge. So that seems to rule out the accelerator pump, which I have tried tuning with nozzles and shot volume. And a possibly related porblem is the idle mix screws don't really do anything. I can turn them all the way in and it doesn't make the engine die. The best idle setting (still not very good) is with the screws roughly two turns out, but there isn't much difference in idle quality throughout the range of adjustment. Is my LT4 Hot cam radical enough to have forced me to have the throttle blades open too much, exposing the transfer slot? I plan on pulling the carb soon to put on a spacer and will look at the transfer slot. Otherwise I have run all the different metering rod springs and have messed with richer power and cruise settings with the rods/jets, and all that does is change when the spot occurs. Ideas?

 

Yes it sounds like you are idleing on the transition slots.
Edelbrocks don't idle on the primaries and secondaries like a holley does so if this is the case (after looking at the throttle bores) then allowing more air through the PCV and or more initial timing will allow you to close the throttles a little an maintain the same idle speed. The idle screw should deffinatly be responsive
and the motor should die if ya screw them all the way in.

A spacer will probabily make it worse.

Try blowing out the air bleeds with compressed air.
Could be junk in there.

 

.......

 

Hmmm....maybe I need to stop being a weeny and run more than 10* initial advance! I'll check tomorrow. I guess I will shoot for 16* base for around 36* total. Will bumping up the advace 6* allow me to close the throttle blades a bit?
Thanks,

 

Yes.. Do you have the PCV hooked up? it will allow some additional airflow too. The carb is actually designed for this.
If the PCV is not hooked up the carb will have to be adjusted too
far open to idle at a good speed.
The bigger the cam the more this is nessessary.
You can even run two PCV valves if nessessary.
Be sure the valve cover is vented too so the motor breathes.
Pull the carb off and see where the throttle blades are.

 

Yeah I have one PCV valve on there already. I will do the timing and see if that cures it. If not I will pull the carb.

 

If you are not already, you should be running the distributor vacuum advance canister to full manifold vacuum to get more than 10deg advance at idle. Don't use the ported vacuum tube (no vac advance at idle) with a ZZ4 cam. This will increase your idle speed dramatically (from 10deg advance) and allow you to close your throttle blades a bit, alleviating your problem. Hopefully?

The ZZ4 would like about 20-25deg of advance at idle.

Also, sounds like you (at least) need a stronger spring (allows metering rods to raise under higher vacuum levels, providing enrichment at higher vacuum level) in your primary metering rods to allow a quicker transition to richness or, posibly, a entirely new jet/metering rod/spring combination altogether.

Do not give up. Easy to fix.

 

Well I bumped the initial to 16* which did raise the idle some. So after resetting the idle speed and mixture, it runs better but that dang lean spot is still there full force. I wonder if the gauge might have a glitch because it doesn't seem to fall on it's face when it shows the off scale lean. Hmm. I put in the strongest springs again and didn't cure it either. I am running a vacuum advance but I have it on the ported vacuum port of the carb. I guess in a few days I will pull the carb and see if the transition slots are too exposed.

 

Do not pull the carb until:

1) Reset timing back to 10deg, with the vacuum advance disconnected, of course. Then, hook the vacuum canister to a full manifold vacuum port on the carb. This should help DRAMATICALLY with a LT4 Hot cam. I thought you had a ZZ4 cam, oops! You MUST try the full vacuum!!! That cam will like 26-32deg of idle advance... Please, just try it, trust me. Don't cost nuthin'.

2) read the Calibration intstructions on the carb to get rid of the lean spot. You probably have to get more flow in the non-enriched (needle down) mode, if the heaviest spring did not help. I would put a bigger primary jet in AND/OR put in a rod with a smaller diameter at the tip than the one you take out. Again, go the Edelbrock tuning directions. Go the richest you can in the "Cruise Mode" to see if it goes away...

Of course, you could just have a vacuum leak and/or a mismatched/broken PCV valve.

Edelbrock.com has all the info you need.

 

Well I brought the base timing back down to ten. And then I hooked up the vacuum advance to the manifold vacuum port. Yeah, fast_broker I think you made a believer outta me. My first impression is that it feels nicer and idles better. The lean spot is still not cured however. I have the Crane adjustable advance and have it locked out maximum so it adds 12*, so 22*all together. Anyway I am still betting my transitison slots are showing too much. Before I pretty much tuned out the spot for the most part by going two stages richer than I am now with the cruise. But it doesn't seem too wise to go that rich since it kills the fuel economy and it runs fine with a leaner cruise execpt for that spot. I have practically memorized the Edelbrock manual, hehehe. And the PCV valve is new, might not be matched. Don't really know how to do that. Vacuum leak is a posibility but it would have to be at the carb base or intake since I have already tried plugging all of the ports on the carb with no change.

 

Well, 22 isn't really enough for that cam, but enough for us to decipher your problem... If you have vac advance locked out, you should go to 12-14 advance at idle to get a couple more idle deg advance. Do that first.

Sounds to me like you ight be on the right track... You stated you "pretty much tuned out the spot for the most part by going two stages richer than I am now with the cruise" but you didn't want to run rich. If, when you did that, it ran ok and didn't show overly rich or lean on the AFM, you were on the right track...

I/You really need to know the EXACT vacuum level you are having your problem... Are you tuning the carb with a very accurate vac gage in the cab next to the AFM???


To check for vacuum leaks, use an UNLIT (no flame!) propane torch. Turn the propane gas (low level) on with the automobile engine running and pass the torch tip near areas of a possible vacuum leak (carb and spacer areas, ALL hoses, intake manifold, etc). If/when you find a leak, the engine rpm will pick up drastically as it begins to burn the propane.

Couple of more questions for you to answer:

1) how big is the carb???
1.5) Age of carb
2) What is the carb's Carter or Edelbrock PN???
3) Did you buy a tuning kit for that PN carb???
4) From above, exact vacuum level (or areas) taht you are having this lean-spot issue.
5) Type of fuel pump
6) Float level setting on the carb floats (have you checked it?)
7) Fuel pressure at idle and WOT...
8) ALL primary jet and rod combinations you have tried...

 

First on the advance, should I go up on the initial or I can free up alot more vacuum if need be. I can choose incriments of 2* from 12* up to like 24*. Which would be better?

With it two stages richer on cruise the AFM didn't really show it running richer by much, if any. But I don't have the most confidence in the thing. When any electrical load is introduced (e.g. electric fan, headlights, ect.) It makes the reading show leaner as more electricity is in demand. About all I think it is good for right now is seeming things like the lean spot I have.

I don't know the vacuum this is occuring. I guess I will have to buy 8 feet of vacuum hose so I can have the gauge in sight as I drive, but that would be a good idea.

I will check on the propane, I guess I will have to make sure I don't point it at the headers.

Ok I will try and answer all of your questions.

1) 750 cfm
1.5) 3 years, but not many miles. It sat for about two years of that time.
2) Edelbrock #1411
3) I bought the tuning kit for that one and since it didn't have the parts to go as rich as I needed, I got the kit for the #1407
4) Will have to look into this one
5) Mechanical, stock replacement. Have a fuel pressure regulator and fuel pressure gauge. The readings are not consistent but they seem to be within range. I don't know if my fuel press. gauge is the greatest, because I don't see why I can have 3 psi one time at idle and like 6 another. I have recently replaced the pump and the regulator is a new addition too. But the problem existed before and hasn't changed in characteristics becuase of them.
6) I checked the float levels and they are per Edelbrock specs.
7) I guess I answered that in question #5. My press guage is under hood so I don't know what it is at WOT.
8) 104-70x42, 107-73x42, 107-73x37, 110-75x47, 110-73x37, 110-70x42, 113-73x42, 113-71x47, 113-68x47.

Right now I am running 113-71x47. The spot is still there alittle. Before I went with 113-68x47 and that seemed like it took out the spot fairly well. That is as rich on cruise as I went. 113 and 47 works well for power but that was before these timing changes. Will I need to change jetting for the timing changes? Thanks for all the help.

 

Just advance a bit more at idle, to 14BTDC. Leave the vac canister alone, for now.

Why are you using an FPR with a stock replacement fuel pump? Should not be needed. Take it off! It should not help you at all...



What you are running now is the "base" calibration for the 1407 carb...


http://www.edelbrock.com/automotive/eps_1407.html


IMO, you need to go richer in cruise to get rid of the problem. You should also encounter this problem at highway cruise speeds, locked in gear at lower rpm, right? Your other point is 4% richer than what you are running now and you stated it was better. So... keep going richer until you fix it. Or find the vac leak...

Also, make sure your floats really are set right and your carb isn't gummed up after sitting for so long. Make sure the carb is TOTALLY clean. It might actually be worth removing to disassemble/clean if it sat for that long...

Here is what you should do, using the 1407 chart from above:

http://www.edelbrock.com/automotive/eps_sect2.html

 

Well I am going to go out and pull off the carb and check things out. I was planning on giving it a bath while I was at it. I will also up the base advance to 14 and see what that does.
I got the regulator because when I first installed the pump my gauge "showed" 8 psi at idle. Again I have my doubts but I did notice it seemed like it ran better especially through the mid rpm ranges with the reg. on there. I got a stock pump for a '68 Camaro because I don't have a return line.
I don't notice the spot really at cruise speeds. But I have a 3 speed auto so it might not get to low enough rpm to show.
I guess going richer makes me wonder because I am at 4000 feet. The manual says you should have to go lean when raising altitude. Maybe my 400hp 383 just has a bigger apetite.
I'll go and clean the thing up and put in 68x47 rods and advance the base alittle more and see how it goes from there.

 

Just got done with the latest changes. I put the initial at 14*. I pulled the carb, disassembled it completely, shot the crap out of it with gumout carb cleaner and put it back together again. I noticed as I took the carb off the connection to the hard vacuum line that goes to the brake booster was alittle loose (might have been a vacuum leak). I measured the transfer slots and they were exposed borderline too much (.060") but not more than that, or at least not by much. In inlet side float was off. It was like 5/16" versus the spec. 7/16". So I fixed that. I must not have checked both floats or something. I put in my wood 1/2" spacer and threw in the 68x47 metering rods. After I got the idle mix set I took it for a spin. NICE. I didn't notice the lean spot at ALL. I will probably give it a longer test tomorrow and see. Thanks a bunch for all the help. Pulls like a beast.

 

Ok works well, no lean spot. Only issue is that sometimes when I stop at a light my idle rpm will be at 750 (perfect) and a couple of times it idled at 1000 rpm. Anyway to fix that? I assume it has to do with the vacuum advance, but I don't know. Otherwise I am happy with it. Thanks.

 

Vac advance wouldn't do that. Either you have a misadjusted choke or a vacuum leak, still.

 

Well I did notice one time that I came to a stop when it was idling high, I pushed hard on the brake pedal and then the idle rpm dropped back down to 750. So maybe the brake booster or that inline black check valve thing? I hope it would just be that black thing because I don't like the idea of swapping boosters.