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Discussion Starter · #1 · (Edited)
Got a strange one. for me anyway.

Brawler 670 VS carb, Holleys copy of a Holley...lol

Small block chevy. 9.5 compression.
Edelbrock Alum RPM heads.
Comp 284 Cam ( 240/246 @.050 )
RPM air gap intake. 1" spacer.

The idle is giving me fits.

Im only seeing approx 5" of vacuum at "idle" around 700-750 rpm. And its a little jumpy at that too low rpm.
No vacuum leaks that I can find.

When I adjust the idle screw the idle doesnt really change from 750 ish for like a half turn and then it jumps up to 1400-1500 rpm.
If I adjust the idle screw down from there there really isnt any middle ground, 1400 then down to 750. It usually wont stay running in gear that low. I was expecting to need to be at around 900-1k.
Also, When the idle gets anywhere near where I want, as soon as you rev it up a little the idle with hand for quite a while before sometimes settling down.

The idle screws do make some change but not as much as I'm used to seeing but those have been pretty stock motors with much smaller cams.

I already swapped the power valve to a 2.5 because the 6.5" valve the carb came with was dumping fuel with only 5" of vacuum.

Ive tried the timing at 10-12 initial and up to 22 and it clearly makes a difference but the idle issues remain.

I thought the valves may be a touch to tight so I re-adjusted and im getting a little clattering now so I know they are slightly loose if anything currently. I went 1/4 turn past "zero lash" this time.
This didnt affect the vacuum or the idle issues.

Once above 1200 rpm it makes around 10-12" of vacuum and runs perfect...
Revs clean and super crisp.

It smells rich when attempting to mess with idle.

The only thing I can think of is that Im right at the very edge of opening up the blades too much for the transfer slots and then it idles way up once Im past that spot ?

My next step is to maybe drill a 1/8" hole in each primary blade unless Im not thinking correctly.


I ordered an air/fuel guage so once that shows up Ill at least know if its rich, or how rich anyway.

Am I on the right track with drilling the blades or what am I overlooking here ? lol

Im guessing the timing as starting to advance at this higher RPM as well so Im not sure if im on the right track or not... Ugh...
 

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Discussion Starter · #2 ·
I just went and set the timing to 33 initial and it seems like it will idle there at 1000 with a ton more vacuum.
The idle mixture screws are also way more responsive to changes at that setting.

I have a MSD with 10 or 20 degrees of start retard I could set if just locking the timing out is the answer.

Lastly, I may need some tighter springs possibly if it doesnt get locked out as I just found the "hanging idle" seems to totally timing related.
I set it and checked it at 33 then revved it out and when the idle hung at 1300ish I grabbed the timing light and it had more than 33 for sure. Seems like the timing isnt falling all the way back to initial cleanly after it gets revved up.
 

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Discussion Starter · #3 ·
I read a bunch of other threads and did another experiment out in the shop and now I’m pretty sure I don’t have a carb problem…lol

it’s all in the timing.
I checked timing with and without the vac advance connected and then with my little vac pump. It’s not even starting to advance until 7” of vacuum so the advance I was counting on for idle just isn’t happening unless I have a bunch of initial timing and at least 1000 idle speed.

I pulled the distributer out and it had light springs and a red stop bushing so it is definitely advancing at 1100 and up and the light springs arent bringing it back to initial reliable and quickly.

I think the plan is to put in a 10 degree stop bushing and 2 heavy springs in it.
Then set the initial at 24 degrees. That should give me a solid 24 at idle and 34 at 2600-2700. The vac should also be high enough for the vac advance to give me more at idle and light cruise.
in theory it all makes sense…lol
 

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One thing I’ve always done when troubleshooting timing is block off the vac advance completely. Eliminate it until I’ve got everything else working better.
do you have a 4 hole or open spacer?
where do you live?
Ambient temps and elevation can effect your tune.
‘And another don’t do thing.
Never drill holes in your carb plates.
 

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At that elevation you’re on the right track thinking it’s all about timing.
thinner air needs way more initial.
‘After you get close on timing you might need to lean your carb a bit.
 

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Discussion Starter · #7 ·
Its been a LONG time since I had anything with a cam this size and a carb, and its not even super racy...

I was really worried initially about the low vacuum, it just seemed way to low at the 750-800 idle speed but comp says it should have 9 - 9.5" at 1000 and Im right there once I finally advanced it enough to get it to actually hold 1000.

Im gonna try the 10 degree stop bushing and heavy springs and hope the little extra I get from the vac advance will get it idling good.

If not, Ill lock out the distributor at 34 and call it a day. Ill just set the start retard on the box and let her rip.
At some point Im gonna hit it with a 100 shot as well so Im a little worried if I back down the total even a couple degrees a I wont have enough initial...lol Locking it out is starting to sound better and better...lol

The new air/fuel gauge should help me get the tuning and jets correct from there.
Once it actually idles Ill have to go back up on the power valve as well. At that point the 5.5 or 6.5 may be pretty good.
 

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With bigger cams like that, its a very fine window to find the sweet spot on what your engine will like. First thing it sounds like your trying to do one to many things and changes at once and it will leave you lost in the shuffle. I have tuned Holley carbs for over 14 years now from stock to wild up there like your cam size and believe me even after all this time you can still get a wrench thrown in the works to throw you off even with a lot of experience.

I would start from scratch and the very first thing you want to do is get a basic timing curve to start off with. I can tell you with my previous build which had a 268/272 220/[email protected] 510/510 lift hydraulic roller cam which gave me around 17 inches of vacuum would only idle about 800 to 900 rpm in park. Anything lower and it would have slight issues at idle.

The thing I do regardless how much I know is write down the specs of the carb from the power valve size, jets, air bleeds etc and have all that written down. I then have my carb off and will undo the primary idle screw and then slowly turn it in to where it starts to open up the butterflies. I count how many turns I can go to get the transfer slot exposure to be square or about .020. I then turn it some more to see how much is about .30 to .040. That is the max amount of turns you can go on the primary idle before the transfer slot would be exposed to much and thus leave your idle mixture screws unable to work or barely work at all.

I then close the secondary side and then do the same with them and just make contact to where the secondary throttle plates are opening up and then go half a turn starting off. I write all this down and every turn I make I jot down all changes and adjustments to keep track of things. I will then if it being a four corner idle carb will have all four mixture screws out about 3/4 of a turn as a starting point.

I will have it installed and proceed to have the vacuum advance unhooked and adjust the distributor to where you can hear a good turning of the engine over to start off with in case your distributor has never been timed before. On a side note you would want to have an advance curve for your distributor as most likely you might have to change it especially with a radical cam like yours. I then get the engine fired up and warmed up and adjust the primary idle as needed in order to get it running. Also if you can get a fuel pressure gauge and if needed a fuel pressure regulator to help keep the pressure in check and about 5.5 psi is all I try to run and it does well.

After I have it running enough I proceed to check my timing and adjust it to give it what it needs to start off with a good idle and its stable. I will adjust the idle speed to what I need. In your case when I had cams that size in the past it was always around 1200 rpm as they like a good amount of rpm to idle vs something way smaller.

After adjusting the timing and getting it in the ballpark, I then proceed to the carb. I will hook up a vacuum gauge to help aid in the setting and getting an idea on what the engine is liking and not liking but also listening by ear as well.

I start off with making sure my primary idle screw is within the right range. For example if I have a .020 transfer slot at say 1/2 a turn past contact and I can go to a max of 3/4 of a turn and if I needed to go past that max amount then I would proceed to open up the secondary side another 1/4 of a turn open to help keep it in balance with the front. Trying to have the front transfer slots equal in the front and the rear don't always work depending on the base plate.

On some Holley carbs the base plate rear transfer slot sits up about .020 higher in the rear versus the front and it can throw things off a hair but not always. Its just besides the rear side having transfer slots, it also besides having the idle discharge ports, has two very tiny discharge ports as well to help keep the fuel fresh in the rear bowl so that little bit kind of adds to the mixture but I can't say how much.

After getting the correct idle setting and adjustment set and the timing locked in on what it needs to start off with, I will then proceed to start to adjust the mixture screws and on the four corner idle you want to just go an 1/8th of a turn at a time starting in the front. I go in a 1/8th of a turn on all four and look at my vacuum gauge and look for when I can get higher vacuum and once I see that I know I am in the good range but I don't set it for highest vacuum as it will always make it slightly to lean with anything other then a stock or rv sized cam.

I also listen to how the engine is running and listen for a smooth as can be consistent idle (not meaning stock sound smooth idle) and go in to where it will start to studder a hair but sometimes you can't always get that. If an automatic transmission is used I look for the leanest setting I can get going into gear and to where it struggles a hair then I will back out the idle mixture screws enough to where it drops into gear yet its not out to far on the mixture screws while also having a near best vacuum reading and a good consistent idle.

If I am unable to get very much range adjustment at all on the mixture screws and no matter how I try to get the butterfly setting to be square on the primary side then at that point you might have to recalibrate the idle circuit with the idle feed restrictors in the metering blocks. If the idle circuit is not in the correct range to get a good idle for your engine and its one too rich or to lean it will make it impossible to get a good adjustment on your carb and end up with a bunch of band aid adjustments to keep it running one way but suffering in another area. Air bleeds can come into play as well.

If after I have done what is necessary to get the right idle setting and adjustment and know its the right calibration and know the timing is correct for the initial go out and drive I will then test it in different situations and make changes as needed. I would post more but that is about getting the idle setup and there is a lot one can do and many adjustments that can be done. Basically I do everything on the idle circuit first and get my timing changed accordingly to what is needed and then proceed to try town driving and slow acceleration, then faster acceleration and then half throttle acceleration and check how the pump shot circuit operates in different conditions and make changes.

I start off with a middle of the ground pump cam that gives a quick shot and then go from there and it to a lesser of an initial shot if its too much and vice versa. Then when you get a proper pump cam I will change the shooter size but sometimes you can change back and forth between them. After I get that all sorted out then I proceed to the main circuit and jetting. Then I also check the secondary side of things and get an idea if there is a stumble on the secondary side when they are opening, is it the secondary coming in that is causing the stumble or is it the primary side or a combo of both.

I normally work on the primary side to get it as good as possible before getting into the secondary side. Once the primary side is very close and then you get into the secondary side then its a lot easier to determine what direction you need to go. A vacuum secondary is a little bit tricky on getting the right opening up rate but with your quick fuel carb you can adjust the screw on the rear to adjust the sensitivity of it and get it to where you need.

Sorry for the long post but just trying to give you a general idea on what is involved and I don't know how much you know. There is still a lot more left out but that should get you started
 

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Discussion Starter · #9 ·
With bigger cams like that, its a very fine window to find the sweet spot on what your engine will like. First thing it sounds like your trying to do one to many things and changes at once and it will leave you lost in the shuffle. I have tuned Holley carbs for over 14 years now from stock to wild up there like your cam size and believe me even after all this time you can still get a wrench thrown in the works to throw you off even with a lot of experience.

I would start from scratch and the very first thing you want to do is get a basic timing curve to start off with. I can tell you with my previous build which had a 268/272 220/[email protected] 510/510 lift hydraulic roller cam which gave me around 17 inches of vacuum would only idle about 800 to 900 rpm in park. Anything lower and it would have slight issues at idle.

The thing I do regardless how much I know is write down the specs of the carb from the power valve size, jets, air bleeds etc and have all that written down. I then have my carb off and will undo the primary idle screw and then slowly turn it in to where it starts to open up the butterflies. I count how many turns I can go to get the transfer slot exposure to be square or about .020. I then turn it some more to see how much is about .30 to .040. That is the max amount of turns you can go on the primary idle before the transfer slot would be exposed to much and thus leave your idle mixture screws unable to work or barely work at all.

I then close the secondary side and then do the same with them and just make contact to where the secondary throttle plates are opening up and then go half a turn starting off. I write all this down and every turn I make I jot down all changes and adjustments to keep track of things. I will then if it being a four corner idle carb will have all four mixture screws out about 3/4 of a turn as a starting point.

I will have it installed and proceed to have the vacuum advance unhooked and adjust the distributor to where you can hear a good turning of the engine over to start off with in case your distributor has never been timed before. On a side note you would want to have an advance curve for your distributor as most likely you might have to change it especially with a radical cam like yours. I then get the engine fired up and warmed up and adjust the primary idle as needed in order to get it running. Also if you can get a fuel pressure gauge and if needed a fuel pressure regulator to help keep the pressure in check and about 5.5 psi is all I try to run and it does well.

After I have it running enough I proceed to check my timing and adjust it to give it what it needs to start off with a good idle and its stable. I will adjust the idle speed to what I need. In your case when I had cams that size in the past it was always around 1200 rpm as they like a good amount of rpm to idle vs something way smaller.

After adjusting the timing and getting it in the ballpark, I then proceed to the carb. I will hook up a vacuum gauge to help aid in the setting and getting an idea on what the engine is liking and not liking but also listening by ear as well.

I start off with making sure my primary idle screw is within the right range. For example if I have a .020 transfer slot at say 1/2 a turn past contact and I can go to a max of 3/4 of a turn and if I needed to go past that max amount then I would proceed to open up the secondary side another 1/4 of a turn open to help keep it in balance with the front. Trying to have the front transfer slots equal in the front and the rear don't always work depending on the base plate.

On some Holley carbs the base plate rear transfer slot sits up about .020 higher in the rear versus the front and it can throw things off a hair but not always. Its just besides the rear side having transfer slots, it also besides having the idle discharge ports, has two very tiny discharge ports as well to help keep the fuel fresh in the rear bowl so that little bit kind of adds to the mixture but I can't say how much.

After getting the correct idle setting and adjustment set and the timing locked in on what it needs to start off with, I will then proceed to start to adjust the mixture screws and on the four corner idle you want to just go an 1/8th of a turn at a time starting in the front. I go in a 1/8th of a turn on all four and look at my vacuum gauge and look for when I can get higher vacuum and once I see that I know I am in the good range but I don't set it for highest vacuum as it will always make it slightly to lean with anything other then a stock or rv sized cam.

I also listen to how the engine is running and listen for a smooth as can be consistent idle (not meaning stock sound smooth idle) and go in to where it will start to studder a hair but sometimes you can't always get that. If an automatic transmission is used I look for the leanest setting I can get going into gear and to where it struggles a hair then I will back out the idle mixture screws enough to where it drops into gear yet its not out to far on the mixture screws while also having a near best vacuum reading and a good consistent idle.

If I am unable to get very much range adjustment at all on the mixture screws and no matter how I try to get the butterfly setting to be square on the primary side then at that point you might have to recalibrate the idle circuit with the idle feed restrictors in the metering blocks. If the idle circuit is not in the correct range to get a good idle for your engine and its one too rich or to lean it will make it impossible to get a good adjustment on your carb and end up with a bunch of band aid adjustments to keep it running one way but suffering in another area. Air bleeds can come into play as well.

If after I have done what is necessary to get the right idle setting and adjustment and know its the right calibration and know the timing is correct for the initial go out and drive I will then test it in different situations and make changes as needed. I would post more but that is about getting the idle setup and there is a lot one can do and many adjustments that can be done. Basically I do everything on the idle circuit first and get my timing changed accordingly to what is needed and then proceed to try town driving and slow acceleration, then faster acceleration and then half throttle acceleration and check how the pump shot circuit operates in different conditions and make changes.

I start off with a middle of the ground pump cam that gives a quick shot and then go from there and it to a lesser of an initial shot if its too much and vice versa. Then when you get a proper pump cam I will change the shooter size but sometimes you can change back and forth between them. After I get that all sorted out then I proceed to the main circuit and jetting. Then I also check the secondary side of things and get an idea if there is a stumble on the secondary side when they are opening, is it the secondary coming in that is causing the stumble or is it the primary side or a combo of both.

I normally work on the primary side to get it as good as possible before getting into the secondary side. Once the primary side is very close and then you get into the secondary side then its a lot easier to determine what direction you need to go. A vacuum secondary is a little bit tricky on getting the right opening up rate but with your quick fuel carb you can adjust the screw on the rear to adjust the sensitivity of it and get it to where you need.

Sorry for the long post but just trying to give you a general idea on what is involved and I don't know how much you know. There is still a lot more left out but that should get you started

I think the carb may be OK afterall.
It will need the powervalve swapped back out and then of course tuned/jetted normally but Im thinking I just didnt have nearly enough timing for this cam and motor combo.
Once the 10 degree bushing gets here Ill set it at 24 initial and 34 total. The stiffer sporings should keep it from bouncing around and hanging as well.
Im hoping the B26 vac advance can on the MSD will have enough vacuum to add some more at idle and it will be perfect.
Ill set the MSD start retard to 10 degrees and it should even start like a mild motor that only has 14 in it.

After I get the timing squared away Ill start working on the carb but my quick test has me thinking Ill be good.

More to come once parts get here..
 

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It will be interesting to learn what you find. At first read I thought you were loading up, and curious where your mixture screws are set, but then you mention 6000 ft up and makes sense.
 

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Discussion Starter · #11 ·
Got the bushing this morning so I threw the dist back together and back in the motor.
I set the idle screws back at 1 turn out and fired it up.
I set the initial at 23 degrees @ 1000 rpm
At that setting with no vac advance connected it sits at 32-33 @ 3500.
The idle came back down and was now consistent.
I fiddled back and fourth with the idle screws and idle setting a bit watching the vac gauge and AFR.

It returns back to idle now every time and timing isnt jumping all over.
It will also still idle around 850 in gear and not die.

Im sure it will need jetting, Vac secondary tuning and a larger size powervalve ( I had a 2.5 in it for testing ).
Its also pulling 8" now pretty steady.

I think she is good to go until get it out of the shop and play with it.

It does sound Rowdy and that is clearly the most important part...lol
 
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