[PRNDL]

So which school of thought do you guys recommend as far as idle is concerned?

Secondaries closed with just a small 1/8 turn to prevent binding. Then set the idle via the primaries only?

or…

Secondaries adjusted to match the exact same level as the primaries. Then make idle adjustments by adjust both fronts and rears the exact same?

Both methods assume the primary transitiin

AED Holley 750 HOHP DP 4 corner idle. Came with 1/8 holes in primaries only. and PCV hooked up. One inch open spacer.

The engine is a freshly rebuilt 383 stroker. All machined, brand new stroker internals, 30 over, zero deck, 6 inch rod Skip White unit with 10cc dished pistons. AFR 195 heads, Performer RPM AIR GAP, dual plane, and Comp Cams XR282HR 230/[email protected] 110LSA CAM. HeadersBreak in has been completed.


I’m noticing that if I set my idle to 900 rpm via the first method, primary adjustment only, my rear idle mixture screws stop responding. I can screw them in all the way and no change.
When I set the idle via the second method, primary and secondary matching, to 900 rpm, the front mixture screws stop responding.

I’m thinking of lowering my idle to 800 rpm.

Any ideas?

 

[eric32]

I already shared most of my method with you on a previous post. I use the method of keeping the primary set to within keeping the transfer slot squared within a certain range to not be open up to much or closed to much. On the secondaries I keep them open up enough to keep the front correct and also not to have them open more then the front as much as possible. Having the secondaries open up more then the front can create issues in my experience and I can' get really into details as its been a while since that happened.

I never had to do the equal transfer slot showing method as on most Holley style carbs the secondary transfer slots sit up higher in the rear and it will off set things with a cam size like yours or smaller. With really big high overlap and low vacuum making cams then things are all out the window and a lot more has to be done. There is no complete etched in stone way on doing that but you have to know your way around them and get the window and adjustment range and what your engine needs in order to not have too much and too little on the idle circuit.

Timing is the first thing to have good and then on the carb you have to have the proper idle speed to start off with and adjusting the idle mixture screws and then determine if your idle circuit needs to be changed or not on the calibration by changing the idle feed restrictors in the metering blocks and at time to final tweak things the idle air bleeds.

A lot of times after you get one spot dialed in good and you start on another part of the carb such as changing the idle feed restrictors for example you have to maybe go back and readjust the mixture screws or at times even the primary idle setting on the butterflies and the rear. Its just a precise thing to get them right and it takes time and experience and also knowing what your doing and if your not careful you can get lost pretty quickly . The key is very small gradual changes made one step at a time and keeping track of what you have already done and then trial and error and seeing how your engine is either improving or getting worse.

The idle circuit is the most complex part of carb tuning on a Holley vs other parts when starting off as with performance cams there are so many variables and different driving conditions that the idle circuit has to be able to transition with the throttle position changing over and over again while operating without it having running and being able to meter fuel correctly and as good as you can get.

Sometimes if you can't get your primary butterflies or rears to adjust correctly along with the idle mixture screws thena t times it can either the circuit is too lean is you have to have them out of a functional range or if its super rich and you have to have them closed too much and you can't have a proper idle rpm setting either way. Its a fine art to see the sings of it being either too rich or too lean and also being able to also go by adjustments to tell what is going on especially without using an air fuel ratio gauge. I have never needed to use one and have always with time get things done really well even on builds like yours.

 

[2001Blazer4x4]

I would call AED and put it back to original for starters.

I just went through this with a Holley Tech on my own Quick Fuel SS-830 in an attempt to get consistent cruise AFR at different RPMs on the street with a milder cam than I have been running for a while (Comp XS274S) Would be 14.0 AFR at 2500 but cruise would get much richer (and richer) as RPMs went up. Never really cared much when racing as full power AFR was OK - but now on the street.......

Talked to a Holley Tech for starters, since I had jacked around with the secondary opening when I first got the carb. Went to secondary blades just barely covering transfer slot. Primary blades set so transfer slot is a square. Put original IFRs, PVRs, and main/secondary jets back in. Went from there but left the secondary throttle opening alone. It fixed the inconsistent cruise AFR and best of 18 mpg so far.

Point is that I've realized that you can't just change secondary throttle openings willy-nilly as you can change the carb calibration completely (and not for the better) by doing so. For yours I question why AED drilled the throttle plates for a fairly mild XR282HR, but they are the carb builder - I'm sure they had a reason (and this would certainly affect other choices that they made). I would suggest you get things back to as-built. After all, it's a custom carb built just for you! (or did you buy it used?)

Doesn't an open 1" spacer defeat the purpose of a dual plane manifold?

Your build should be killer!! This is the sleeper Nova, right?

 

[eric32]

Also on one last note on your AED carb, I don't know why for such a build like yours that if they did built it for you, why they would be using a drilled hole in the throttle blades on the front. That there will change things a good bit versus them having no holes drilled in them and the size of the holes will effect things as well. Its kind of like having an extra idle air bleed for each butterfly drilled. I never had to have drilled plates for any Holley carb I have ever used even on a way bigger cam then what you have. But AED also does certain things on there carbs that an out of box Holley does not have and I have always just used out of box carbs or used ones from ebay.

They might have something modified on the carb and in there theory that is why they are drilled. But even on a custom carb you still have to have things adjusted and changed within a certain degree as all they do is give you a way closer window vs an out of box carb which is more setup as a generic starting range and it will take more time and effort to get dialed in.

 

[PRNDL]

Thanks Eric, I was going to ask this on the other post, but I thought the question was too specific so I thought it would be more proper to keep it simple and start a new thread. On the other post, my timing resolved my issues of running rich at idle. I bumped the initial timing much higher. This seems to be a final fine tuning issue.

My secondary transfer slots sit much higher in the barrels.

Not sure why AED drilled the primaries, but it’s a standard mod with this HOHP 750. I spoke to the owner of AED and he said not to touch the secondaries. They explained that the drilled primaries mean no cracking of the secondaries at idle. Problem is my idle would be too low. My engine seems to prefer the secondaries matching the primaries method. Problem is I lose control of the primary mixture screws. When I close the secondaries, I lose control of the secondary mixture screws.

by losing control, I mean, when I close them, the engine does not stall. Should the engine stall when closing any one of the 4 mixture screws? I guess that is my true question.

 

[BogiesAnnex1]

No if three idle screws are feeding fuel the engine is unlikely to stall. Even closing two are unlikely to stall the engine.

The idle issue is more one of getting air which is why your carb has the drillings in the primary throttles. This to a huge extent negates the need for the secondary idle circuit being there at all.

The open spacer while being an aid to WOT redline power messes everything up else where. Unless you drive this competitively only on a track and nowhere else I’d get rid of it for a 4 or 3 hole, but that said any spacer does not insure any power improvement and can be detrimental, this is the kind of thing you experiment with on a dyno or with timing slips. The latter is also skeptical data because track times get diluted with human and mechanical variations one run to the next so it can take a number of timing slips averaged to see if there is a trend and that trend is repeatable.

My baseline setup on four corner idle is set the primaries to the general rules of primary transition slot exposure then crack the secondary side to supply sufficient air to establish consistent idle. Depending usually on the cam and how much air it requires to get the needed idle speed I might not even open the rear idle mixture screws. I generally only fiddle the rear mixture screws if I think I’m getting the primary screws too open where the air bleed circuit is getting unstable. This is something you just develop a feel and hearing for over the years of doing it.


Bogie

 

[PRNDL]

Thanks guys. The one inch spacer is for clearance issues. I’ll look into a 4 hole.

so with the primaries set and the secondaries opened to match the primaries, if my idle is too high… Do I adjust them both down or just the secondaries

 

[eric32]

I don't completely agree on what the guy from AED said to you on the the reason why the primary holes are drilled. Usually that is for engines that are so high up there in cam size that its normally for something that is not a street cruiser and does not spend much time with very little throttle opening.

I have always had to tune with thee secondary side being adjusted regardless if the front has holes or not. One time I had to drill holes but that was because I did not have a rich enough fuel curve on the idle circuit and had to open up the primary butterflies and secondary side too much and the mixture screws would not work.

After re calibrating my idle circuit by bigger idle feed restrictors and stuff I replaced the drilled butterflies and fixed the base plate back with new plates and the thing would then adjust and work like a charm. Just a lesson to be learned. In my opinion like Bogie said on the seconary side I always adjust the secondary side to allow the right amount of air in relation to keeping the front in the correct range needed for everything to function like it needs to be.

I also use a spacer for clearance issues especially using a double pumper with an edelbrock intake and the vacuum fitting hitting the fuel bowl in the rear. I With the intake you have I recommend going to a 4 hole spacer vs an open spacer as the open spacer will make a weaker signal on idle and also while cruising and cause all sorts of tuning headaches that you will compensate for and on a street engine that is used for mostly just driving around, the four hole spacer has always worked the best for me as it enhances the signal to the carb in most of my experiences with using a dual plane intake. Now the thing I don't like the air gap intake the most is the cut down divider.

I have had an issue some time back within the last two years but my brain is foggy on which build it was, and I was using the air gap intake and I had no problems with the air gap design of the lower runner and the air flow going through it vs the non air gap design intake but did have a lot of problems because of the cut down plenum and it causing the signal to not be as effective and cause fuel distribution issues with certain camshafts and making a worse situation and a lesser tune vs a regular dual plane intake where both sides are separated from each other.

The sharing of the signals getting shared with each side through the firing order and then being pulled back and forth and getting cut around the cut plenum in my opinion ( I can't describe it good enough in words) makes for an unstable signal at certain times on the bottom end and mid range and even using a four hole spacer still does no help in that instance. I swapped out to an Edelbrock rpm intake and even though I still had my issues on said build with the cam I had, just the taking out the air gap cut plenum out of the equation made a big change for the better in trying to get my problem figured out which I was unfortunately unable to get fixed because of another issue that has me needing to take my engine out and checked out by a machine shop for machining error somewhere.

The edelbrock rpm air gap intake with cut plenum and a four hole spacer was worse then using an Edelbrock victor jr single plane intake and a four hole spacer combination on a previous build that was able to pull each style of intake off without effecting the performance and overall tune. In my opinion the cut down plenum is not worth the messing up the bottom end just to get a 3 horse power difference on the top end and maybe a 100 rpm difference in the overall curve of power being made. To me the edelbrock rpm air gap would be better with the plenum left alone and perform better that way.

To me in most cases the dual plane intake is better suited to keep the sides separate from each other in all situations and if you have to use an open spacer to get what you want overall on the upper end of things then going to a single plane intake is better off then using a modified dual plane intake and you will get better results overall but there is always take and give when going anything more then stock type builds.

If that was my carb I would be replacing the front butterflies with new plates with no holes in them and re adjust and tune things from there. To me those holes being there are just making things more complicated then they need to be with that cam. I ran the comp cam big mother thumper cam which has a massive 243/[email protected] duration and only made 6 inches of vacuum in park and I did not have to drill holes in my carbs I used and one was a two corner idle and the other a four corner idle and I had my transfer slots on the front correct and the secondary side adjusted as needed and had a good idle required for what it was and no problems with fuel fouled and black plugs etc except the poor manors that comes with cams like those for just a daily driver type deal.

 

[PRNDL]

Thanks brother. Great info. I set it how you said, ignored what AED said and my idle mixture screws work well. I reduced the idle to 900 by turning both primary and secondary idles down a hair. It’s idling very happy. It absolutely does NOT like the secondaries closed at all.

So here are my stats…
Primaries T-slot at .030. Primary idle screw at 3/4 turn. Secondary idle screw at 3/4 turn. Idle at 900 in park. About 850 in drive.

weird part. Front idle mixture screws want to be at 1-1/4 and the rears at 1 turn for 11hg of vacuum and best idle. I used a vacuum guage and remote tach under the hood. Lastly, all mixture screws respond well, but I can close the rears and no stall. Only the primaries stall the engine if closed.

What are your thoughts on the mixture screws?

 

[BogiesAnnex1]

My preference on spacers are insulating. It’s usually a good thing on carbs to reduce heat transfer into the carb especially these days with pump gas being blended for high pressure fuel injection rather than sloshing about at atmospheric pressure in a float bowl.

Bogie

 

[eric32]

I would say your mixture screws for being a 4 corner idle being that far out and the rear mixture screws not cutting out the engine makes me feel starting off that your idle might be on the tad bit rich side but not too rich but plug reading and test driving and seeing how things look over time will tell tale of the tape and also I would have to actually adjust things and see how they work. In my experience with a proper fuel curve on the idle circuit, the four mixture screw being about 3/4 to an 1/8 more of a turn or 1 turn at the most and anything more would make it too rich and at minimum at 1/2 a turn out on all four and if it stalled when put into gear it would be pretty close but results vary build to build.

Usually if you can't get past a 1/2 turn on all four then its too rich and within normal range and it being good would allow you from half a turn minimum to 1 full turn on all four being equal without being too lean but it can vary a hair. On a two corner idle setup normally if you can't get past 1 turn on each screw its too rich and if you have to have more then 2 full turns its too lean. Normal range is about 1 turn to 1 3/4 on average but factors can come into play and is not etched in stone. There is no concrete way but you want to have an acceptable amount of adjustment not being too little or having to have a lot to get a good mixture setting in relation to the size of the idle feed restrictor and the idle air bleed sizes and the proper range of use from the transfer slot all working together smoothly in operation.

I like too keep all four mixture screws all equal and go down as lean as possible one step at a time since that is the norm that most carb companies have given over the years and such along in the tuning books but it also that if you can keep the primary screws at 1 1/4 and then close the rears and if it idles well and does not stall then I would try turning the rears all the way in and then back out just an 1/8 of a turn and run it like that and see how it does by test driving and see how your plugs look in different driving conditions and see if you have any off idle stumbling when getting into the throttle. Sometimes if the idle circuit is not quite enough on the mixture screws it can cause a slight lean condition and just giving a hair more on the mixture screws can cure it.

It sounds like your front might be more then enough to keep it idling right and run good and if that is the case then check things over time and how it performs and if it is good then leave it alone.I always look back at previous builds and compare what my current build is and use that as reference as a staring point to the cam specs and it does not take long to get it dialed in good and running nice.

Since I have been through so many different cams over the years and kept all my tuning notes in a library I can pretty much start off real close and know what will work without having to do to much tuning and get it spot on in a few changes on the idle circuit along with others.Like I said its all a gradual adjustment thing and sometimes when you get one area real well you have to go back to a previous one and make minor tweaks and get it super dialed in good.

 

[PRNDL]

That’s great info Eric. You know, I read somewhere a while ago, where a few guys turn the rears out just 1/2 and do all the idle tuning with just the fronts.

 

[BogiesAnnex1]

If you open the idle mixture screws too far the fuel flow overwhelms the air bleeds and they fail at their job and at this point get yourself caught up in the air bleed dance where it’s easy to get lost.

I find it much easier to set the primary side as you would to a stable idle with a smaller cam then crack in the secondary side just enough to clean the idle to the proper and needed RPM. Rebalance the mixture screws based on the thoughtful position of the throttle blades primary to secondary. Unless you’re running a huge cam it generally doesn’t take much secondary opening of either the throttle blades or the mixture screws.

If there are already air holes drilled in the primary throttle blades it might not even require opening the secondary blades nor the mixture screws at all. But here you need to be cautious not the crank the primary mixture screws so far open to overwhelm the air bleeds.

Bogie

 

[PRNDL]

That’s my next move this weekend bogie. While trying to fine tune, I learned the following….

If I set it the traditional way with the secondaries closed, my primaries need to be opened way too far. This renders my primary idle mixture screws dead and my secondary mixture screws hyper sensitive.

If I set the primaries to .030 T-slot exposure with the secondaries open to match the primaries, the engine is way happier. The front mixture screws come to life and become very sensitive. However, the rear ones go kind of numb. They allow for adjustment, confirmed via vacuum gauge, but if I close them, the engine keeps running.

Through experimenting, it seems this engine carb combo wants a hybrid of both methods. Seems opening the primarily plates an 1/8 of a turn, showing .035-.040 T-slot, and closing the secondaries and 1/8 evens out the sensitivity of all four mixture screws.

So instead of my primary curb idle being at 3/4 turn and secondary curb idle being at 3/4 turn, I’m now at 1 turn primary and 1/2 secondary.

 

[johnsongrass1]

I always set them with around 60 percent idle rpm on the rear so the fronts are on the idle circuit a little longer at tip in. It works well and the preferred method of many custom carb builders.

 

[PRNDL]

I always set them with around 60 percent idle rpm on the rear so the fronts are on the idle circuit a little longer at tip in. It works well and the preferred method of many custom carb builders.

That pretty much is what took care of my rich idle issue.