Six pack secondary paper clip test?

-

Kllrbee

Well-Known Member
Joined
Jul 3, 2018
Messages
141
Reaction score
67
Location
Richmond IL
I just got the car back on the road after a long overdue suspension rebuild.
After doing some carb tuning, I had a suspicion that the outboards werent opening. Honestly, I never knew if they were in the last couple years cuz I didnt do this test.
I read to get it up to about 2500 rpm in 1st, then stab it to the floor. After doing this, I went out and noticed the clips were both off of the diaphragm "levers" and lying underneath in the intake valley. So, successful test right?
If so, is that what it takes to get these to open on a regular basis? Cant be right? I know it depends on the springs and the kill bleed, but dont they partially open at lower RPMs...say 4k for instance?
I guess what Im asking is if the stabbing it to the floor and getting up to 7-8k is mainly just for testing purposes to see if they are opening?
I know, kind of dumb question, but havent figured out any other way to test.
Thanks.
 
They open based on vacuum and the secondary springs. The lighter the spring, the lower the vacuum signal (RPM) they open. It's not really RPM based, it's vacuum based, but the two are related. You could get a GOPRO and stick under the hood. Aren't they pretty affordable now?
 
They open based on vacuum and the secondary springs. The lighter the spring, the lower the vacuum signal (RPM) they open. It's not really RPM based, it's vacuum based, but the two are related. You could get a GOPRO and stick under the hood. Aren't they pretty affordable now?
Haha, thats not a half bad idea.
I pulled my springs but the color at the tip has worn off. I tried to match them up with the pics of the different springs, counting the rings, and they look to be either the purple or short yellow. Purple is mid point, yellow is light. Anyways, I guess what Im saying is that when regular driving, the paper clip didnt move. Only when I stabbed it at 2500 in first.
I have read that along with others multiple times. its incredible info for tuning. Doesnt cover testing of secondary opening.
 
Haha, thats not a half bad idea.
I pulled my springs but the color at the tip has worn off. I tried to match them up with the pics of the different springs, counting the rings, and they look to be either the purple or short yellow. Purple is mid point, yellow is light. Anyways, I guess what Im saying is that when regular driving, the paper clip didnt move. Only when I stabbed it at 2500 in first.

I have read that along with others multiple times. its incredible info for tuning. Doesnt cover testing of secondary opening.

Remember too, there has to ba a load to test. Revving in neutral or park won't do it. Have you hooked a vacuum gauge to manifold vacuum and taped it to the windshield to see where vacuum is at various speeds? That will be a helpful key to tuning.
 
I said in my OP my testing procedure. In 1st up to 2500rpm, then stab it to the floor. Kind of hard to do that in neutral...haha
 
When the end carbs come in, there is no doubt that they did or did not open. They kinda hit like a freight train on mine when they open.
 
I said in my OP my testing procedure. In 1st up to 2500rpm, then stab it to the floor. Kind of hard to do that in neutral...haha
.
I assume that everybody has tried stabbing it to the floor sitting still just to see if they can tell if the end barrels come in, that's just a fact of nature.
Personally I use mechanical linkage so I don't have that problem.
 
@RustyRatRod Rob, isnt the vaccum secondaries velocity based in a Holley? ie, velocity across the small vacuum pot sense orifice in the venturi? It generated a vacuum in the pot and opens the secondaries, or outboard carbs in a 6-pac. stiffer spring keeps em closed for a later in the RPM opening to reduce bogging. Still cant open them revving it as there is not enough velocity.
 
@RustyRatRod Rob, isnt the vaccum secondaries velocity based in a Holley? ie, velocity across the small vacuum pot sense orifice in the venturi? It generated a vacuum in the pot and opens the secondaries, or outboard carbs in a 6-pac. stiffer spring keeps em closed for a later in the RPM opening to reduce bogging. Still cant open them revving it as there is not enough velocity.

Yeah, that's how I always though about it. In other words, when manifold vacuum DROPS OUT, the secondaries open. The venturi vacuum takes over. That's how it works.
 
I think the manifold vaccum has a minor effect, I think its the velocity across the sense orifice. I could be wrong....But there are vacuum pumps that are simply a tube stuck in the path of a compressed air stream (draft tube, this carbs "primary venturi pickup", header evac systems....Heck, I was always a Carter guy.
holley-vacuum-secondary-operation.jpg
 
I think the manifold vaccum has a minor effect, I think its the velocity across the sense orifice. I could be wrong....But there are vacuum pumps that are simply a tube stuck in the path of a compressed air stream (draft tube, this carbs "primary venturi pickup", header evac systems....Heck, I was always a Carter guy.
View attachment 1715761059

Yeah, that's it. It all kinda works together. That's why getting the primaries right is important to secondary operation too.
 
Yes Pishta you are correct. It's the velocity in the venturi and sometimes there is (or should be) a tube sticking out a little to amplify the signal. It does what Rob say when the carb size is somewhat normal. But it still works if the the carb is small (restrictive) and the manifold vacuum rises with rpm.

To the OP's questions.
Not sure how you had the paperclips. I generally use a twist tie.
First thing I'd want to check is whether they are opening at low throttle high speed cruising, like 60 mph.
The reason I want to know that is for setting and tuning in the primary main jets. If there is fuel air coming in from the secondaries (outboards in your case) then then changes to the primary won't have as much effect as they should.

Here's the hights and coils I measured for some of the Holley vacuum secondary springs.

Originally posted here Vacuum Secondary Diaphragm Springs
there's also an update there showing the short yellow spring (1.5").

Tuner discussing how the cross passages work and more. On the next page he discussing the check ball.
Vacuum Secondary Diaphragm Signal Tube
(If calling a 6-pack a "tri-power" gets your pants in a wad, don't read. I think he does that just to get a rise out of the overly sensitive. LOL)

Vacuum Secondary Springs - Heights.jpg
 
Thats great info right there Mattax. Good stuff in those links too. I measured and compared my springs to your pic and I have the black springs. Heaviest.

I was able to do some testing today and the outboards are definitely NOT opening. I tried the paper clip test again and they didnt move. At first I tried just low throttle crusing at 60ish with nothing. Then I did what I tried the other day. Getting up to about 2k RPM in 1st and flooring it. Nothing. Paper clip in same spot.
Then I disconnected the secondary linkage and vacuum and plugged the hoses to just run on the 2bbl. Felt the same as it had. So that cinched it for me that the outboards were definitely not opening. Actually runs really strong on just the center so thats good.

Something I tried just to see what would happen is I took a little off the mechanical linkage rods thereby having the outboards slightly open...just a little. But that was a bad idea cuz they started to bind holding the outboard throttle plates open and it wouldnt return to idle. So I put the rods back to where they were before...just short of binding. Cruze mentioned above that there would be no doubt if they were opening and I believe it, cuz it was very noticeable even when the throttle plates were only partially open due to the mechanical linkage binding.

Also manifold vacuum is at 11-12 at idle. Climbs up to higher teens with lighter throttle RPMs. When I stab it, drops close to 0 (low single digits at least) for a split second, then up to mid 20s. That seems normal to me. When it drops low, I should be getting a rapid increase in venturi vacuum carried to the ouboards correct? But Im not.

Anyways, my thoughts on this are
1) I could try lighter springs. I do have the heaviest springs installed, but even so, shouldnt they open at some point. I mean when I floored it in 1st, I really tore loose.
2) The kill bleeds have been enlarged at some point. I doubt this very much, but it could cause this right? Also, when I removed the diaphragm to check the springs, I checked the bleeds to make sure they werent clogged or something with a very small paper clip. No problem, came right out into the body.
3) Something is blocking the venturi vacuum from the center carb. A gasket shifted or shrunk over time plugging up the port? Either way, Ill get in there and rebuild them, they are due anyways.

Thoughts?
 
1) I could try lighter springs. I do have the heaviest springs installed, but even so, shouldnt they open at some point. I mean when I floored it in 1st, I really tore loose
Take a look again at the snip from Holley about how much difference in opening rpm the springs made in their tests of two engines.

My experience with a .060 over 340 and a modified 3310-3 was the black spring prevented opening until very high rpm (for that engine).
I'll dig out the screenshots in minute



2) The kill bleeds have been enlarged at some point. I doubt this very much, but it could cause this right? Also, when I removed the diaphragm to check the springs, I checked the bleeds to make sure they werent clogged or something with a very small paper clip. No problem, came right out into the body.
3) Something is blocking the venturi vacuum from the center carb. A gasket shifted or shrunk over time plugging up the port? Either way, Ill get in there and rebuild them, they are due anyways.

If you have shop air, you can direct it past the signal tube in the center carb.
Or if you just want to check for good connection in the carb, shoot some carb cleaner through. Just keep it away from rubber parts.
 
There are two different ports on the ctr carb, off the main body is for the end carbs. The port from the metering block is dist vac. You say there is no vac for the end carbs, with the engine running, pull the hose and see if you feel any vac with opening and closing throttle.
There are actually 7 different springs, not positive, but brn is stock. Usually the tall yellow is what you want, there is a short yellow. I have run the purple and yellow.
The spring sets are available from Chicago Carburetor. You will need two sets. They each come with a chart of values for large and small cu in engines. Having gone through a couple sets of these, they may be due for an overhaul. Most of the gaskets are different, the ends are the same, the center is different, possibly someone has been in there and crossed something up. If you do them yourself, the air/idle adj screws on the end carbs will probably be stuck, soak them with blaster for a day or two . Will save you some grief from breaking them. If the carbs are full of a white looking substance, it's from alcohol gas, vinegar will dissolve it, but will also take the gold plating finish off. Sometimes you can't win! Those screws go back in 1/8th of a turn out.
Keep trying!
 
I'll dig out the screenshots in minute
Ignore the AFR tuning issues being worked on at the time.
Look at the rpm where the horsepower increases over the black spring. First with a plain spring and then with a yellow spring.
Wide open throttle begins between 3750 and 4000 rpm.
upload_2021-7-6_21-35-10.png

Based on this data and also the manifold vacuum and Innovate WBO2, I don't beleive the secondaries were significantly contributing to airflow with the black spring until the engine exceeded 5200 rpm.

Here's two runs on the datalogger with the black spring installed. You can see the manifld vacuum continues to rise all the way to 2"Hg or so.
The sudden drop in AFR (goes richer) of the mixture around 5200 was when the secondaries were opening.
upload_2021-7-6_21-42-50.png



We can see the AFR move down in rpm with the plain and yellow springs.
upload_2021-7-6_21-45-38.png


Wide open throttle begins around 3800 rpm.
Thats about as low of an rpm as can generally be done on a chassis dyno with an automatic (lacking full manual valve body)
 
Last edited:
Wow Mattax, Im kind of surprised the blacks are that stiff. That really could be it then. Ill definitely start with getting 2 spring kits and go from there.
Good idea, Ill remove the hose and see if I can get some air or cleaner thru there.

Thanks Cruze, Im sure its all due for a rebuild. As far as the end A/F screws, I just recently removed the plugs and found they were pretty easy to adjust. I was shocked. Set the front to where it just ever so slightly falls off when cover the air bleeds, then swapped front to rear and vice versa, then set the front the same.

I plan to rebuild them regardless, but I just got the car back on the road after a long layover, so I just want to enjoy driving it for the summer. Runs perfectly fine on the center for now. Ill wait until I put her away for the winter.
 
Google Chicago Carburetor Six-pack parts, they have an extensive parts list available.
 
An engine that was pulling harder would open the secondaries sooner with the same springs.
At least I think so.
With the AFR going up and down so much - especially when the secondaries begin to open - the engine just wasn't calling for more air.
In fact we can see with the yellow spring that the vacuum drops above 4750 rpm. Its was actually going closer to atmospheric than when the throttle is floored (around 3800-4000 rpm). So that suggested the engine wasnt using that extra airflow effectively.
I've got it much better now but don't have any logs comparing the various springs. Almost did. Brought the springs with me to the dragstrip last fall but only get one pass in. :( Time to go home. Maybe this year.
 
Good stuff Mattax. Seeing a graph really helps put things in perspective.
I just checked and shot some carb cleaner, then air thru the venturi port. No blockage whatsoever. Im convinced now after seeing your graphs that its got to be the springs.
I havent purchased anything yet from Chicago Carburetor, but Im familiar with them and was planning on getting anything I needed in the future from them so thanks for the suggestion Cruze.
 
Any questions, feel free to pm me. I am by no means a tuner. Have had several bumps in the road with my 418/six-pack. My carbs have Promax end carb baseplates and metering plates. Presently have a crack in my manifold, but I can get you into the ballpark.
 
I like them. Obvious reason for the rear, did the front also, radiator hose gets in the way of the stock base, and then there is the whirling fan blade right there! They are pricey, but quality. The drivers side housing for the air bleed gets into the bottom of the float bowl, I file it flat to clear. They would like you to die grind a groove for it to fit? Pass side OK. You have to transfer your old throttle shaft and blades.
 
Quick update for Cruze and Mattax. Thanks to all for your help, but especially you 2.
I realize this was almost a month ago, but I went on a 3 week backpacking trip. Just got a chance to tune with new springs.
Turns out it was the springs as suspected. I had the black (heaviest) in there which werent opening at all. Decided to do a go all the way on the other end and try the white (lightest) first. I expected these would be too light and I would get a noticeable bog. But they were perfect, which surprised me. Did the paper clip test on both outboards and they did open. I still had to somewhat mash on it, but I could also feel them open. Although it wasnt as noticeable as I thought it would be. Maybe cuz the transition was relatively smooth with no bog. At any rate, outboards are opening and functioning properly with the white springs. Thanks for the help again.
 
-
Back
Top