Very good idea.!

 

idle feed restrictions LIMIT the amount of fuel..

idle air bleeds adjust the curve.. of how much fuel at what speed..

go larger on your idle feed restrictions..

the idle transition circuit is just like the primary main circuit..


IFR's limit the max amount of fuel..

IAB's control the fuel curve.

does that model have screw in idle feed restrictions..

get this right and you are going to go thru rear tires really fast.

the 080670 carb you have.. uses this primary metering plate on some of the versions.. 134-57 https://www.holley.com/products/fue...ucts/fuel_systems/carburetor_components/metering_blocks_and_plates/parts/134-57

i cannot find any thing listing the size of the idle feed restrictions in that metering block..

if they are screw in.. they are easily changed.. if they are press in.. replacements are available thru carb shops that sell walker products..

page 18 part number 85-607..
http://www.walkerproducts.com/wp-content/catalogs/oxygen-sensors/high-performance/compressed.pdf

 

I have done many different fuel curves on several of my builds and on my mild small block 350 with dart 180cc aluminum heads and 9:1 and a lunati voodoo hydraulic roller cam 272/280 219/227 515/530 lift and with correct timing curve to what my engine likes I have around 17 inches of vacuum in park. Out of the box I had the famous holley street avenger bog and hesitation during low rpm use.
Out of the box the primary metering block has around .028 for the idle feed restrictor and the rear block has .031 on the idle feed size. The primary idle feed as you stated is .078 but I can't remember right off hand what size the rear are without getting mine out of storage since I am currently not using it.

On my idle I have my primary idle set around 800 rpm and it drops to 700 rpm in drive and anything less it does not like regardless of what carb I put on my motor. My idle mixture screws would not adjust too much as it was just to lean and from my experience its better to go up or down .002 sizes at a time as it allows more of a change compared to air bleeds which on average need around four sizes up or down just to see a difference.

I took my primary metering block and drilled out the idle feed size from .028 to .031 and left the rear the original size and also left the air bleeds alone as to what they were out of the box. By doing this I was able to get my carb to finally have the adjustment it needed on the idle mixture screws and also I had my primary throttle blades open to where a square transfer slot was shown and I had the rear cracked open to allow some extra air and to balance out with the front.

Just that one mild change on my primary idle feed restrictor was all I needed to get my carb to run pretty mean and also I used a quick fuel adjustable vacuum pod to where you turn the screw and you adjust how quickly the spring responds as it comes with a purple spring. The easiest thing you can do is drill out the pressed in restrictors and then use a 6/32 tap and then you can buy some 6/32 brass allen screws 3/16 length size and then just drill them to what size you need and if your to much you can just drill another pair of them and go back in size and you can hit it right to where it needs to be.

I have had about 4 different small block chevy combos with several different cams and I have never had to change the air bleed size as it was always better to change out the ifr size then the air bleed size as it makes a world of difference just by a minor change compared to air bleed size. The street avenger carbs mainly the 670 one is very lean out of the box and I also upped the secondary jet size from 68 to a 73 size which gave me 66 primary jet size and 73 secondary and that put my carb pretty much were it needed to be for my engine combo but each engine is different on what they will like and not like.

Tuning idle restrictors is the same like changing out jets and you only want to make very minimal changes as each step up or down should make a difference unless your trying to use one on a very big came and yours is not much more then my lunati voodoo cam and I run around 16 inches of initial timing and I have my vacuum advance set to give and extra 12 to 14 degrees of advance timing and run it off of full manifold to have the extra at idle and it made it run very good and way better then running the vacuum advance on timed spark port on the side of the primary metering block.

Try just a little at a time and you might also need to change shooter size and pump cam color but its different on each engine as all will react differently.

 

Also forgot to mention make sure your timing curve is right for you engine as well as it can make a difference especially while its in park and in gear along with lower speed rpm. Having your timing not being enough can cause a lot of the issues of popping through the carb and exhaust as well.

 

Good info.

Before you go and drill out the Idle air bleeds and tap them, I go and get a junk carb and practice on it. A mis step on a 5-600 dollar carb is costly. There is not much meat around the stock ones. Doing it with your DeWalt 1/2 in cordless drill just isn't going to work. You need a new quality tap too.

It's easier to just purchase an aftermarket metering block and a handful of jets.
I've got 4 of each of ten sizes. I think all of them have been in my carbs at one time or another.

As noted the distributor curve is equally important. Forget ported vacuum.

So is having a note book to keep accurate records.

 

As noted the distributor curve is equally important. Forget ported vacuum.

So is having a note book to keep accurate records.

This ^

I get the small spiral bound note pads, one for each vehicle. I write down all my baseline numbers. Jets, air bleeds, ifr, timing and everything. Every time I make a change, I date it, and put down the reason behind it. Such as attempting to fix off idle stumble. If it doesn't work, it's easy to go back.


Sent from my XT1254 using Tapatalk

 

replacement push in idle air bleeds are available thru some sources..

FYI..

 

Ditto on the notebook... I've been doing that.

I'll be getting my LM-2 AFR meter in a couple weeks which should help. I'm going to hold off on messing with things until I get it. That way I have some hard data to base my changes off of.

I'm betting that I'll need to enlarge the Idle Fuel Restrictor on the primary side.

Question... I've got a 2 corner idle carb (only on the primaries). My secondary metering block does not have idle screws. And the secondaries don't have idle discharge ports beneath the butterflies. I'm finding that I need to crack my primaries too much in order to get a good idle. If I want to get the primary transitions slot square, I need to crack the secondaries a bit to get more air.

So my questions are:
- Is that OK that I'm letting more air through the secondaries even though they don't have any fuel at idle? I find that I need to crank up my primary idle screws when I crack the secondaries.
- When should I stop opening the secondaries and resort to enlarging (or adding) holes on the primary butterflies?

 

there are actually idle feed passages and idle transition slots in vacuum secondary holleys carbs.. have a look thru this album.. enlarge the pictures..

complete ALBUM
Holley R1850-3 - Album on Imgur

secondary 4160 metering plate.
http://i.imgur.com/1xZv2Lt.jpg


4160 base plate top
http://i.imgur.com/0O2wLa6.jpg

4160 secondary idle feed port and shared passage secondary idle transition slot
http://i.imgur.com/UPK2cVQ.jpg

with the air fuel ratio display... do the idle transition testing at 1900 RPMs.. cruising on a flat level road at a constant throttle and speed..

do the primary main circuit testing same thing.. at 2500 to 3300 RPMs steady throttle.. level road.. constant speed.. so you are not on the idle transition circuit.. and you have not gotten on it to open the secondaries.

the jet size effects the highest RPM lean or rich for each circuit. if its lean just before the next circuit comes in.. then increase the jet size.. then play with the air bleeds if its not stable thru the RPMs under load for each circuit..

 

If you want to drill out your idle feed restrictors on the metering blocks its actually quite easy. I used a regular standard drill and a set of drill bits from harbor freight and started small and went bigger each time till I got to the right size then usually it will either pop out on the drill bit or you can use a screw extractor which will grip it and allow you to pull it out.

After you get them out I just used a tap set from harbor freight as well and I took my time and just did so many turns at a time and used a tooth pick to mark down on how much is to far and I would do just a few turns at a time with my tap till I got it at the right depth. On my very first try I did not mess any up at all and I am not the best with a drill. On the air bleeds especially on holley brand carbs the front side on the idle feed ones it has more metal to work with and to drill and tap but on the high speed air bleeds on the front and rear you don't have much material to work with so you have to be careful on that one. I used 8/32 for the primary idle air bleeds and 6/32 for the high speed air bleeds. Then on the rear is where you have to really watch things as there is not much to work with and I drilled the idle and high speed air bleeds and tapped them at 6/32.

I just drilled a few sizes smaller on the drill bit size recommended to tap an 6/32 size hole to tap it and they all came out good but you have to mark your drill bit and also your tap on the maximum depth you can go. Since its a through hole you don't want to go to far or the brass allen screws will then just go down to far and not have any way to be tightened up.

As far as doing the mod I honestly would not even mess with the air bleeds as in all of my tuning I have never had to change them and just changing out the idle feed restrictors in the metering blocks is all it took to get it right on the money on the idle circuit. Just changing out the air bleeds four sizes up or down did not effect to much so that is why I stick mostly to the idle feed restrictors only.

As far as the advantage of a 4 corner idle system vs a 2 corner only with engines with really high duration camshafts has it really made that much of a difference. On my wildest small block with a lunati hydraulic roller with 282/290 231/239 @50 535/550 lift I have had both 2 and 4 corner idle carbs on it and the 4 corner worked a little bit better but not by to much.

On my milder small block with a 219/227 @ 50 515/530 lift cam the 4 corner did not help out at all and the 2 corner worked just fine. Its usually on the 230 @ 50 range and up that you would want to use a 4 corner idle setup vs the 2 corner as the 4 corner idle system is way more sensitive to even just to an 1/8 of a turn on the idle mixture screws.

I have been through about 4 different camshafts over the last ten years and after learning more and more on tuning a holley style carb I found that the main thing to change if need be on the idle circuit is to change out the idle feed restrictors on the metering blocks vs any air bleeds. I have pretty much just left my air bleeds that came in the carb alone. Putting in 10/32 screw in air bleeds on a holley brand carb is not really a good idea as there is not enough material to tap into and thus is why I used the 8/32 for my primary idle air bleeds and everything else 6/32. Using a junk metering block is very good advice and the same for the tower if you want to have changeable air bleeds as it does not take much to ruin it which is why I just replace the metering blocks.

Don't always think that your idle circuit is to lean cause of having the primary butterfly's open to much and also needing the secondary ones open up as well. On my old holley 670 street avenger I would always have the issue to where no matter what it was just to lean and always needed too much throttle opening but you have to be careful as the rear ones have the transfer slot up higher about .020 compared to the front and trying to make them equal you end up with too much open in the rear and not enough in the front.

At that point and after many different adjustments I finally after trying many adjustments I just went up .002 on my primary metering block idle feed size and I then with the rear being set up to where it needed to be I finally got a nice steady idle and better adjustment out of my idle mixture screws. I had some issues with having the secondary being open so far and then having the front closed so as to not have too much transfer slot showing and an off idle surge and pop would get me here and there.

It takes a lot of time and experience and trial and error to learn what your engine is wanting and just minor adjustments at a time is how you want to do things and eventually you will get it done and it will run very good.

Usually the 670 street avenger/600 holley vacuum secondary carbs work out of the box just fine minus jet size on anything with a cam of about 215@50 and less duration on mild small blocks. Its when you get into the entry level of performance cams that is in the 220 @50 and up range that you then have to do advance tuning and changes in order for the carb to work well and run with its setup.

Even though I can drill and modify my metering blocks to take brass allen screws to which I can drill and make it what ever size I want I honestly still just go ahead and buy the quick fuel billet metering block kits which you can just use regular 6/32 screw in bleeds for the idle and also on the power valve channel restriction. Its a little over a hundred dollars for the kit but with it being billet and being able to change everything out its easier for me to just go that route.

 

Good post!

 

Have you read this thread yet?

https://www.hotrodders.com/forum/holley-670-vs-street-avenger-hesitation-fix-507033.html

I've got a 670 SA as well and I had all kinds of problems with the factory configuration. Replacing the metering block was the best thing I did. Something to consider.

 

Adjustable metering blocks are a big help. The tunable metering well jets is a lot more capable for dialing in the fuel curve than being limited to just messing with the main and air correction jets.

Keep in mind that the HOT cam has timing that without fuel injection is getting a bit radical for only making 1700 turns on the crank. Also, be aware that 1700 without a load on the engine is not the same metering as 1700 when the crank has to pull the vehicle and that will change the throttle position and metering with the topography and altitude/temperature curves.

Bogie