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  #16 (permalink)  
Old 01-02-2019, 08:13 AM
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When you readjusted your idle mixture screws just what exactly how did it make it worse and also what is your idle rpm at? Also do you have your secondary butterflies open up at least a 1/4 of a turn open and do you know how much transfer slot exposure you have on the primary side if you know what I mean?

The reason I ask is transfer slot exposure can affect off idle performance if not showing enough or if it shows to much. I have a hunch that you might need to open up your primary idle feed restrictor just a hair as every time I used that carb with cams that size the idle circuit out of box was always on the to lean side and I had to always richen it up just a hair to get better idle mixture adjustment and also eliminate a lot of issues with hesitation or faint bogs here and there in certain situations.

You want you secondary side to be open up enough to allow extra air to work with the primary side and you want at least some of your primary transfer slot showing but have it show no more then .040 at the most. If it is not showing at all then it can create issues with off idle stumble or hesitations in relation with the idle mixture screws. Its all about getting everything adjusted to work with each other.

But that is something you don't want to do unless you know exactly what your doing and why and how the whole idle circuit works and this is advanced knowledge beyond turning screws and changing jets. I would check those other things first and then maybe go from there.

On your idle mixture screws I normally don't use a vacuum gauge but use engine response. I turn one screw in till the engine starts to stumble just a hair and then I turn it back out a 1/4 of a turn and do the same with the other side. i also will also put it in gear with the emergency brake on and check to make sure the idle mixture screws are still not to lean and do a slight adjustment if needed but never rev the engine when it is like that.

I will use a vacuum gauge at times just to see the overall condition on how things are running but I don't go for the highest reading as that is always to lean for anything but a bone stock engine.

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  #17 (permalink)  
Old 01-02-2019, 08:26 AM
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I adjusted the idle screws with vac and rpm. I turned them out .25 of a turn past highest rpm and vac. It would hesitate in park when I reved it up with that. And was really bad on the road.
Those slots under the card on the primaries are showing enough to make them look square. If that makes since to you.
I don't think the back butterflies are open at all. I will check
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  #18 (permalink)  
Old 01-02-2019, 08:37 AM
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JMHO, change the power valve....
https://www.summitracing.com/parts/hly-125-45/overview/
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Old 01-02-2019, 09:12 AM
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I almost guarantee you will need to richen up your idle circuit calibration at this point as all points with my experience with this carb which I have over ten plus years with and am running one myself with a 228/[email protected] 547/547 lift 110lsa and out of the box that carb as is would never work with my cam size without re calibration of the idle circuit. The downside is holley brand carbs don't come with the option to change idle feed restrictors out on the metering blocks as quick fuel brand carbs on the other hand comes with all these options.

The idle circuit is to lean for this size of a cam as in my experience with all the cams I have used over the years any cam at 215[email protected] or more and a tight 110 or less LSA will have issues like your talking about as no matter how much you adjust the idle mixture screws or set the idle and jetting has no effect on this part while cruising at low 40's or below your still on the transition circuit and that is effected by the idle mixture screws at idle and the idle air bleeds and the idle feed restrictors in the metering blocks and in your case the secondary metering plate will dictate the idle calibration at idle and lower cruising speeds before the mains pick up where the jetting comes into play at about 45 to 50 plus mph when enough velocity and throttle opening comes into play to start pulling fuel from the boosters.

My previous small chevy build I ran a 219/[email protected] 515/530 lift 112 lsa cam and ran a 600 on it just like yours as well and mine did all the stuff yours did and some other things as all builds differ but the idle feed restrictor size in the primary block is .028 and the rear metering plate is .031 as long as know one as messed with it. I swapped out my secondary metering plate to a metering block just so I could have secondary jets but you don't need to do that with yours as regardless of a metering plate or block as what all I had to do was take a .031 size drill bit and drill out the primary side idle feed restrictor from .028 to .031 and just that slight change was all my carb needed and I got rid of my off idle hesitation or slight bogs here in there while cruising around town.

Years ago before I learned about tuning the idle circuit on this carb and the holley 670 street avenger cousin which is the same exact carb just with a secondary metering block and center hung fuel bowls but same main body and primary metering block with the same calibration out of box and I changed shooter size and also pump cams and could never get rid of the stumbles and pops here and there. After I got some tuning books on how holley carbs work and doing research on how the idle circuit works and when and how far they come into play before the mains come in I then realized through other people having the same issue I then got me a gauge drill bit set and learned that by changing the idle feed restrictor size by .02 up will make enough of a change to make my idle circuit richer.

The thing you have to be careful when doing these mods as once you drill bigger you can't go back smaller unless you drill out the old restrictors and then tap them 6/32 and then use brass allen screws that you can drill to size and then you can always change out and make what ever size you need.

But i won't get into writing a book on how to do that as that is another post there but if I bet if you increase the primary idle feed restrictor from .028 up to .031 most of your problems will go away. When I did this for a build I had years ago it fixed 99 percent of that minor but frustrating issue you and many thousands of people have had with this carb and its 670 street avenger cousin.

There is a sticky on the holley 670 street avenger on the top of the engine forum page and read all through it and you will get an idea on what I am talking about and for your case you don't have to buy a new metering block like the quick fuel one or anything. I took my holley 600 like yours and for my particular build I am running .033 primary and .033 secondary idle feed restrictor sizes just to show you as an example from the stock out of box which was .028 primary and .031 secondary.

Idle feed restrictor tuning is an advanced area of tuning and one needs to know exactly what it does and how it functions and must be warned before making changings as just a .01 is a double percentage increase or decrease on the idle fuel curve and all one has to do is make a .02 change at a time to make a noticeable difference on the tune.
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Old 01-02-2019, 01:01 PM
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I run a 600 dp on an engine combination with a similar cam (331 ci).

Dyno testing with tail pipe sniffer showed that I also had this bog and it was caused by an over-rich mixture from the squirters. I put in the smallest squirters offered by Holley, and it substantially helped the situation.

I'm thinking that maybe I also need to reduce the pump shot volume a bit. Shaving the cam could accomplish this. The A/F mixture in teh second graph is 1.1 point lower than actual as verified by prior dyno runs as well as a whole bunch of C5-C6 guys with on-board A/F readouts who also noted this discrepancy.







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  #21 (permalink)  
Old 01-02-2019, 03:15 PM
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A 600 double pumper out of the box will be richer on the idle circuit compared to the holley 600 vacuum secondary by quite a difference in the idle feed restrictor size and I understand your info of what you posted on the pump shot but I am certain he needs to richen up his idle circuit as I had to do it with mine and have had to do so on at least four different small block builds using mild cams around this size or small down to the [email protected]

I even had a previous build 350 with just a 211/[email protected] 112 LSA and when I had this carb on it was just borderline on the idle circuit and I did it and it ran so much better and easier by going up to .031 on the primary idle feed restrictor to give the engine a slightly richer calibration to run better across the idle transition.

Every vehicle is different though but I believe this is beyond a pump shot as I believe its in relation on the idle circuit. Carburetors just like anything else have to be within certain settings to works with said engine builds and it will be that way across the board. No engine with a performance cam will work with a lean idle circuit that will be lean on that particular build.

Holley and other carb companies always say ready out of the box and that is about as true as saying this carb will run perfect and no adjustment required. There is a lot of stuff on adjusting idle mixture screws and changing jets but nothing about the idle circuit. Pump circuit must work in relation with the idle transition circuit just as much as the main circuit must work with the secondary side in wide open throttle etc.

It has to be done across the whole board and the pump shot is not always the problem but a combination of the two that has to be fine tweaked between the two. Just my information and experience and hopefully will help out and this gentlemen can get his issues fixed and gone and have a nice transition all the way through his carb with no more slight hesitation etc.

Last edited by eric32; 01-02-2019 at 03:23 PM.
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