six pack vaacuum throttle question

[Jeff Erwin]

440 6-pack, running well. I am about to replace the vacuum diaphragm springs on the two outer carbs, but before I did anything I performed the very scientific 'suck test' to make sure the diaphragms were responding as they should. Both move when I suck on the tubing, but I can hear air escaping and the vacuum doesn't hold. Is this normal? Is there some path inside the outer carbs that 'leaks' vacuum? It's not the tube and it's not the connection to the carb, and I tightened all the screws around the diaphragms, no change. Should this be a really tight vacuum seal or is some leakage normal?

I also noticed that pulling vacuum on one carb moves both diaphrams (of course it does, they are connected on the other side...) but I never knew that was the way it worked.

 

[R413]

Yes normal, there is a bleed hole into the Venturi of the outboard carbs. Don’t plug it either.

 

[Jeff Erwin]

Yes normal, there is a bleed hole into the Venturi of the outboard carbs. Don’t plug it either.

Thanks! Now I can quit sucking.

 

[RJRENTON]

440 6-pack, running well. I am about to replace the vacuum diaphragm springs on the two outer carbs, but before I did anything I performed the very scientific 'suck test' to make sure the diaphragms were responding as they should. Both move when I suck on the tubing, but I can hear air escaping and the vacuum doesn't hold. Is this normal? Is there some path inside the outer carbs that 'leaks' vacuum? It's not the tube and it's not the connection to the carb, and I tightened all the screws around the diaphragms, no change. Should this be a really tight vacuum seal or is some leakage normal?

I also noticed that pulling vacuum on one carb moves both diaphrams (of course it does, they are connected on the other side...) but I never knew that was the way it worked.

As you may know or not, the initial opening signal originates with center carb venturi, as increasing venturii velocity pressure. This pressure signal is distributed to the end carbs, via hoses. As the end carbs start to open, a similar velocity pressure signal develops via a small drilled orifice at the vrnturii point and that developing pressure signal adds to velocity pressure developed by the center carb's venturii and applied to the end carb's diaphragms to open the end carbs. IMO....a very simple and very EFFECTIVE way to control additional CFMs by the engine. Be careful with your selection of the springs in the diaphragms as too light a spring may result in a bog or stumble in the acceleration curve.
BOB RENTON

 

[Jeff Erwin]

Bob, thanks for this. I have ordered new springs from Holley and I've been told to just use the black spring, the heaviest one. You agree with that?

 

[RJRENTON]

Bob, thanks for this. I have ordered new springs from Holley and I've been told to just use the black spring, the heaviest one. You agree with that?

Jeff,
The black or the heaviest spring, will DELAY the opening of the end carbs until the center carb's velocity pressure signal approaches its maximum. This MAY result in slower overall 1/4 mile times or what ever measurement you are using....but will be influenced by other factors: engine's state of tune, distributor advance curve and total timing, cam, rear end ratio, traction, weight transfer, vehicle weight, etc. Personally, the "ideal" spring MAY be one step above standard or one step below the heaviest spring.....but only you can make that detetmination. Curiosity question: why do you feel the need to change diaphragm springs? Have you considered carb jet changes? Just asking.....
BOB RENTON

 

[Jeff Erwin]

Jeff,
The black or the heaviest spring, will DELAY the opening of the end carbs until the center carb's velocity pressure signal approaches its maximum. This MAY result in slower overall 1/4 mile times or what ever measurement you are using....but will be influenced by other factors: engine's state of tune, distributor advance curve and total timing, cam, rear end ratio, traction, weight transfer, vehicle weight, etc. Personally, the "ideal" spring MAY be one step above standard or one step below the heaviest spring.....but only you can make that detetmination. Curiosity question: why do you feel the need to change diaphragm springs? Have you considered carb jet changes? Just asking.....
BOB RENTON

No real reason for wanting to change the springs, the car is more of a garage queen and won't be seeing the track in this lifetime. I am just one of those guys that is trying to get everything correct. When I bought the car a year ago it barely ran. The carbs were full of gunk from sitting for 10 years in a warehouse, brakes shot, vacuum wasn't even hooked up under the hood etc. It's taken a year but the engine runs smoothly now and doesn't catch fire any more. It seems like every little change I make to get it all back to original makes it run a little better. I recently replaced the throttle and kickdown springs back to factory and even that made a little difference in driveability. I completely rebuilt all three carbs a while back, making sure jetting and idle adjustments were all correct.

I am currently working on a high-idle issue. The idle set screw is all the way in on the throttle yet it idles a bit high. I can get it down a bit by altering the advance and playing with the fuel-mix screws and I know the outer carbs are closed and adjusted correctly for idle, just haven't found the magic bullet to bring the idle down.

 

[RJRENTON]

Jeff,
If your trying for origionality, perhaps you should investigate the high idle RPM issue first. Re, the high idle rpm issue, is the engine totally warmed up, choke open and off the fast idle cam? Most six barrel cars have a hot idle solenoid, on the driver's side of the manifold, that, when energized, allows the engine to idle at ~ 950 RPM, and when de-energized, retracts, allowing the engine to idle at ~ 700 RPM. Does your car have this feature?
The front and rear carbs have idle mixture screws but are sealed by lead plugs directly in the carb's baseplate. The front carbs screws are accessable directly, the rear carbs screws are not. These screws sre adjustable but a little difficult to access (rear carb ). Search the site for a more in depth explanation, use the sesrch function. BTW.....do you have a shop manual (Factory Service Manual) for your year and model car)?.....its invaluable tool ? This subject is detailed....perhaps we can continue via PM? Additionally, revising the spark advance curve will do wonders as well. Remember to document your changes vs the shop manual specs....BTW...what year and model is your car? An origional six barrel car....a "V" code ? I'll help as much as you want....
BOB RENTON

 

[Jeff Erwin]

Thanks for the assistance. It's a 71 Charger SuperBee. Originally a 383, somewhere in the early 90's it went through a complete restoration and a 440 6-pack was put in. Automatic. Generally they did a great job on the car and engine, but the 10 years in storage did a number on almost everything so I've spent the last year replacing/restoring everything mechanical. Even had to replace the entire wiring harness bumper to bumper.

I do have the hot-idle solenoid but it isn't powered. Engine shuts right off, no dieseling so not sure it's needed. Starts cold, choke working correctly.

Shop manual is the FIRST thing I purchased, actually on my second one as the first one kinda wore out.

The carbs are now in pristine condition, all linkage adjusted and vacuum stuff all working. Black springs. As I mentioned, it fast idles when all warmed up and the choke is full open. I'm down to the advance and air/fuel mix. I'm kinda hesitant to mess with the outer carbs and fuel/air mix, I did adjust both outer carbs so the right amount of the fuel inlet was showing on the manifold side of the throttle plates, and I've made sure the blades are completely closed at idle.

So the only issue right now is the fast idle (~1300-1400 rpm) with no adjustment available at the idle screw. Doesn't seem right. I can bring it down with the fuel/air screws and I can bring it down with the advance (10 BTDC with vacuum advance disconnected, about 25 when the vacuum is reapplied). Just not sure which is preferable overall or if there is something else going on.

 

[R413]

You need to make sure the end carb are closed completely. They do stick. Also get rid of any vacuum leaks. These will keep the idle high. Try plugging the PCV and brake booster lines for a test.

I don’t know why but the word is getting passed around to use the black springs. This is really not good. The carbs will open very late. I guess it so they can avoid problems if the outboards never open. LOL

The black is the stiffest springs, all the way on the end of the spectrum. They never came with those from the factory, why use them now?

 

[RJRENTON]

Jeff,
As suggested previously by others, check for vacuum leaks, as this will contribute to high idle speeds. The hot idle setting is done with the solenoid energized (plunger extended) using the screw that contacts the extended solenoid plunger to service manual RPM setting; because of my cam my hot idle is ~ 1200 RPM. De-energizing the hot idle solenoid (just unplug it) and using the idle speed screw on the center carb's throttle body assrmbly; Set RPM to ~ 750 RPM. I re-energize the hot idle solenoid, then adjust the center carb's idle mixture screws to the highest RPM. Some guys use a vacuum gauge to set these screws to the highest vacuum....your choice. BUT the distributor vacuum advance connection is disconnected and plugged at the source. Then set the ststic spark advance to ~ 12° - 15°. Stock Mopar distributors like the vacuum advance connected yo the ported vacuum source (carb) rather than the manifold vacuum source.....but...this subject is controversial....it depends on who you talk to or believe. I use a recurved Prestolite dual point distributor to provide 22° total at 2400 RPM and 13° static = 35° total. But this is what works for me.
As noted, insure that the end carbs throttle plates are closed and the external closing linkage is adjusted correctly. Jetting the end carbs is difficult because of the metering plates. I've installed the Promax jet plates to be able to stagger jet and use screw in jets. The center carb uses #64 jets as standard; i use #66 for my application. Make sure the plastic fast idle cam (part is on the choke pulloff side) is not stuck and is not holding the throttle open.
Let me know how you make out.....
BOB RENTON

 

[Jeff Erwin]

You need to make sure the end carb are closed completely. They do stick. Also get rid of any vacuum leaks. These will keep the idle high. Try plugging the PCV and brake booster lines for a test.

I don’t know why but the word is getting passed around to use the black springs. This is really not good. The carbs will open very late. I guess it so they can avoid problems if the outboards never open. LOL

The black is the stiffest springs, all the way on the end of the spectrum. They never came with those from the factory, why use them now?

Regarding the black springs, I've read several detailed write ups regarding the tuning of the six-pack, and all have said to use the black springs. I have no idea why and while I understand the science behind slower opening of the secondaries, I haven't seen any other consensus regarding the use of the lighter springs. I have had several people, you included, question the black spring usage recently so now I'm not sure what to think. I am going to leave the black springs in until I can get the car back on the road in a week or so and see how it drives. As I've mentioned, it's a bit of a garage queen so I'm not out racing it. My previous car was a 2015 Hellcat I bought new and drove for 5 years so I've kinda lost the need to go fast anyways. The car was doing the 'brief pause' when jumping on it off the line which is what caused me to start investigating this stuff. If the pause is gone as a result of using the black springs, I may leave them in or maybe back off one spring just to see.

 

[Jeff Erwin]

I have added vacuum leak testing to my list. Good advice. Can I just plug the port on the back of the intake manifold and see what changes?

 

[Jeff Erwin]

As a followup, two things. I did indeed have a small vacuum leak, the manifold vacuum port had a third nipple that I missed. Very small but a leak. Got it plugged and my idle issues really settled down, so thanks for that.

I started with the black springs just because of a couple of sources recommending it. Ran the car a bit and felt, while it was running very smoothly, it took a bit of work to get the outer carbs to open up, which is what R413 said was going to happen. I then replaced the black springs (heaviest) with the 3rd heaviest (marked as 'plain', no color) and things dramatically improved in terms of getting the outer carbs to open up without getting involved too early and causing a pause in acceleration.

Thanks for everyone's help!