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Six Pack Idle Mixture Screws Don't Do Much???

Can you say vacuum leak......

Yeah, that's the first thing I worked on for about a month before I got in to the carbs themselves. I do have a vacuum gage on the car and it's steady as a rock, low as hell (9-10 in. Hg), but steady. It also passes the "carburetor cleaner sprayed everywhere that could leak" test with no idle change.
Very interesting reading!!! I've just put toghether a 340 Six PAck engine and every bit of info about Six Pack tunning is of great help!!! Hope you sort the bugs out!!!
I'm just about ready to **** can this holley 6 pack set up my self....I got a vacuum gauge and when it reads 15-21 my timing is way off the chart. Not on the timing tab at all.
I'm just about ready to **** can this holley 6 pack set up my self....I got a vacuum gauge and when it reads 15-21 my timing is way off the chart. Not on the timing tab at all.

i agree. please pm me and i will provide an address that you can send the setup to for disposal.
i agree. please pm me and i will provide an address that you can send the setup to for disposal.

they will be smashed to bits......holley carbs can piss up a rope......
they will be smashed to bits......holley carbs can piss up a rope......

This is a mile long but if you pay attention it will solve your problem,patience is a virtue !!

It's called copy and paste...do you guys on here own a computer? Try saving this and sharing the information provided. I can't believe the bad advice you give. Six paks require a procedure or they don't work. That's it.

Mr OP if you want help this is it:

This is a guide to tune six packs for street engines.
Revised 10-2012
Stroker or non stroker, big block or small block.
Never the final word, but close enough for now, it gets updated from time to time.


Gasoline is flammable and will burn you

Carbon monoxide is produced from running cars
Esp. improperly tuned six pak cars.


This is not for the faint of heart. If you wondered why people shy away from six paks just read on.

When tuned properly Six pak cars turn on! faster & run far better than single 4 barrel cars exc a thermoquad. If you want your six pak car to run like it should do the dance and have the right tools to make it happen. Patience must be used throughout the process. Much of the information here can be used on a 4 barrel as well.

Some theory of operation:

Things to Remember:

Six pak engines run AND idle on all three carbs at all times.

The outboard are always contributing fuel…always

Only the center carb has an acceleration pump

The car must idle and run/drive like a normal car before attempting any secondary action or wide open throttle passes.

Whacking the throttle in neutral to see if the outboards open is not a legitimate test!!!

Over jetting will not allow you to get the idle mixture correct as the jet size does contribute to what happens in the idle circuit. Please review the theory of operation in the Mopar Performance Engine book.

Tools needed: A good vacuum gage, quality tach in the car and dial back timing light/digital tach [snap on timing light with numeric readout].

A good ignition system.

MSD, Mallory chrome mopar box, NO orange boxes unless you know for sure it is early 80’s vintage. Anything made after 1988 is questionable.

Quality distributor cap & rotor
Firecore spark plug wires
Spark plugs of the proper heat range. Clean and gapped
0.040 mopar box
0.050 msd cd type ignition
Engine well grounded to the body & battery.
Vacuum adv distributor with heavy enough springs to hold advance until 1200 rpm.
The distributor phasing has been checked and corrected as necessary IMPORTANT
Distributor vacuum port on carb disconnected and plugged at carb

ATTENTION -195 degree high flow thermostat- ATTENTION

60%water-40% coolant with a bottle of water wetter
Stewart components has the best thermostats

Pay attention here: If you run a lower temp thermostat, raw fuel will collect in the intake. That fuel burns off in the cruise mode and the air-fuel mixture goes lean.
This is transparent unless the a-f ratio is being monitored with a wideband a-f meter
There will be problems getting it to idle and drivability.

Make sure the timing is 15 - 18 deg btdc [advance] at idle. THIS IS IMPORTANT
Set the timing marks at 15 btdc and align the leading edge of the rotor with the LEADING EDGE of the cap contact-this is one reason the phasing was checked.
Car in neutral-auto or 4 sp, emergency brake set.
A good quality vacuum gage is required
Connect vacuum gauge to direct manifold vacuum source.

The heat crossover should be blocked on big & small blocks
Note: in temps below 40 degree it will take a good while to get the car warmed up. Block heaters will eliminate the long warm ups.

Automatic cars: be sure there is enough stall in the torque converter or the car may be a real pig idling in gear and have poor get-up & go.

Beware of mopar orange ignition control boxes that retard the timing etc. Orange boxes built after 1988 tend to have issues.

Preparation: on the work bench
Outboards: Remove the lead plugs
Set the outboards idle adj screws out 1/8 turn ccw THIS IS IMPORTANT
Be careful when seating the idle screws to set them before the 1/8 setting. Gently is the word. If you look inside the carb bore you will see the needles poking in ever-so-slightly. They should be equal.

Install the BLACK springs – Just do it, ignore everything else you have read.
Install the BLACK springs – Just do it, ignore everything else you have read.
Yes I repeated that, explained way below

If you have the jetable metering plates, If so read their instructions and follow them.

Center carb
Set the center carb idle adjustment screws at 1.5 turns out [ccw] THIS IS IMPORTANT
Be sure to adjust the idle screw until the throttle blades are closed and the transfer slot is exposed no larger than a square. [Carb will have to be off the car to see this] You only want about .040" of the transfer slots exposed below the throttle plates. If the idle screw is adjusted too high, you will be into the transition circuit, exposing too much of the vertical rectangular slot. Many times the idle screw is adjusted incorrectly to compensate for other issues. This puts the carb into the transition circuit and at that point you have no mixture control on the center carb.

If you have new carbs (untouched) they will have 62 jets in the center carb & a 6.5hg power valve. Starting point jetting stock 340 use 62’s, highly modified or stroker use 64’s, big blocks start with 64, stroker 65.

You must know what power valve is in the center carb. Typically a 6.5

Reminder 195 degree thermostat required.

Temporarily change out the brass sight plug on the fuel bowls (all 3) with clear sight plugs, to see the float level without any gas spills. See thru sight plugs deteriorate quickly so use only as a tuning aid. Do not leave them installed on the carbs.

Do not use an idle solenoid to set idle rpm.
A properly tuned car will have no “run on” issues

Factory style linkage, no progressive/ mechanical linkages!
Installation: use the gaskets made by oh company spec p/n
Do not over torque bolts.
Make sure the linkage is set properly. The rods should fall into the hole on the carb lever at the idle setting position.
Check the linkage for any binding, manually open the center carb to wot and see if the secondaries will rotate open.
Have an assistant floor the gas pedal and check for wide open throttle

Fuel pump: Carter street pump only.
Factory style fuel lines only.
Use rubber hose only for tuning purposes, typically on the front carb as this is the carb you remove to rejet the ctr. .
Fuel filter should be in the stock location.

Ready set go

Start car & allow engine to reach operating temp. Set idle to 1000 rpm

Fuel level adjustment THIS IS IMPORTANT, this is best done idling at 900 - 1100 rpm
The slotted screw on top of the needle n seat is just a lock screw.
To adj the float level loosen the lock screw to rotate the seat nut.
Turning the adjuster nut counter clockwise will RAISE fuel level in the bowl,
Clockwise will LOWER it
Make only small 1/2 turns and wait 3 or more minutes so the fuel levels off before rechecking level. Patience is a must!!

Center carb the fuel level is at the bottom of the sight plug hole
Secondaries just starting to come over the bottom of the sight plug hole
This is critical so get it right.

Set idle for 900 rpm
If the car won’t idle:
Is engine vacuum reading at least 2 hg higher than the power valve rating? If ok proceed, if not correct power valve issue and proceed.
Note some engines only pull 5 hg of vacuum so use a 2.5 power valve.
Advance the initial timing a bit to see if it helps idle.
Be sure operating temp is 195-210
Be sure there are not light springs in the distributor.

Now set the initial timing to where it wants to be. Somewhere between 10-20 degrees BTDC. The engine will tell you by increasing vacuum and rpm at this point. In some applications the engine does not care, so set it to 12-14 degrees BTDC.

Rule of Thumb Chart:
Cams with 106-degree ctrlines seem to like initial timing set at 16-22 BTDC
Cams with 108-degree ctrlines seem to like initial timing set at 12-18 BTDC
Cams with 110-114 degree ctrlines seem to like initial timing set at 8-14 BTDC

Re-Set the idle rpm for 8-900-See if you have "control" over the idle mixture screws on the ctr carb.

Using a good vacuum gage adj center carb mixture to highest reading of vacuum.
This is where the digital [numeric readout] tach is better than the vacuum gage as you can see the instantaneous rpm. If you do not have control over the idle mixture between 1-2 turns out ccw of the mixture screws there are issues that need to be taken care of before proceeding. Over jetting contributes to this problem.

Typically if you have the center carb idle mixture screws between 1 to 2 turns ccw and the idle mixture/rpm properly set you may not have to adjust the outboard idle mixture any further.

If you are 2 turns out on the ctr carb idle mixture screws and the idle is still too lean - the outboards need to contribute more fuel to the idle. Open the idle mixture screws on the outboard carbs another 1/8 turn ccw. Now they will be out a total of ¼ of a turn ccw. Now go back and reset the idle mixture and rpm.
If you need richen the idle mixture-set the idle mix to 1.0 turns out ccw

Starting with the front carb, adj the mixture screws one at a time 1/8 turn ccw, after turning each screw wait and see what the engine vacuum and rpm do. Obviously if you have a wideband a-f gauge you will see what is happening. It’s a balancing act, just remember about the ctr carb and it’s proper settings. Also remember you have ½ turn ccw left in the ctr carb to richen the overall mixture. The end spark plugs will indicate of the out boards are to lean #s 1&7 / 2&8.

If the idle is too rich no matter what you do…Most times you are over jetted or you have other issues. Over jetted carbs will have poor idle control. At idle fuel flows from the float chamber thru the main jet then into a the small angular but horizontal passage that leads across to a vertical passage and onto the idle feed restriction where it is mixed with the air coming in from the idle bleed. Remember this. Do not over jet!

Beware of other issues such as poor intake sealing, carb gaskets backwards, the wrong pcv valve, a vacuum leak from the brake booster or other places, wrong pwr valve, wrong thermostat etc.

Recheck idle rpm and set to 8-900

Drive car like a normal person, no wide open throttle. Is the car rich? Jet down 2 steps until you find the min jet size. You will know when you are lean as you will have no power.

Now reset the initial timing again. Somewhere between 10-20 degrees BTDC. The engine will tell you by increasing vacuum and rpm some point and then falling off. In some applications the engine does not care, so see chart.

Re-Set the rpm for 900-See if you have "control" over the idle mixture screws. Using a good vacuum gage adj each mixture screw to highest reading of vacuum. If you have a wideband afr meter set to 14.7. If you don’t like this number set it at your number reading. See how close you are between the vac gauge and af meter and digital tach.

Recheck idle rpm and set to 800-950 depending on engine build, hook up vac adv and make sure car still runs/drives properly.

How do you know when you are "there”?
If the car gets up and goes seamlessly you are there
The engine when hot soaked restarts immediately without touching the throttle
The car will idle at 700-900 rpm in neutral and the response is crisp.
There is no smell of raw gas in the exhaust.
The bottom of the intake is not soaked with fuel. Open a carb and look in
The spark plugs are clean and white.
The engine when cold starts easily runs and drives smoothly from the get go.
When the engine is rev’d and the throttle released it immediately returns to idle.
The vacuum advance is hooked up and the car drives well.

OK if you made it this far it’s time for the Secondaries

The reason you put the black spring is to delay the opening of the secondaries until the engine is ready for it. The engine will run fine on just the center carb till at least 3000 rpm. The air fuel mixture spikes lean when the secondaries open, but at higher rpms this is transparent and has no affect on performance. The opening of the secondaries should be seamless, but very apparent and usually scary to the uninitiated.
Some cars may enjoy a lighter spring.

The secondaries rods should be disconnected and removed and the vac signal blocked.

Please do this safely and with regard for others….
Go out and drive the car on the center carb and determine what rpm the car starts to fall off in power. Take note.
The car should have a ton of power just with the center carb.
Be sure to several wot runs.
Please do this safely and with regard for others…

Reconnect carb linkage and vac lines; be sure to set the length of the rods properly.
Now go for a drive and see what rpm the six pak hits.
Please do this safely and with regard for others….
Hold first gear or 2nd gear, run up to 2500 rpm, and floor it. What should happen is the secondaries open without any hesitation and the cars gets up and really goes.

The long vacuum hoses for the outboard carbs need to be exactly the same length.

Pulling a vacuum on the hose should make the vacuum pod open the throttle blade and hold a vacuum

The best way to dial in the secondary air fuel ratio is with a wide band air fuel meter.
A fine tuned seatofthepantsometer and spark plug reading will work for the more experienced.

If you made it this far and the car is bogging when the six pak opens you need to go back and recheck starting at the top. Bogs are usually from the secondaries opening too soon!!

Automatic cars with too tight of a converter will cause significant idle rpm drops when in drive, the car will not run at it’s full potential so be sure to use the correct converter for the application.

Some cars like staggered jetting.

Reminder 195 degree thermostat required.

If car spits fuel out of the vent it means the o ring on the needle seat is bad.

It’s always easier to remove the front carb for rejetting.
Tape over intake and make sure there is never any unaccounted for hardware.

If you are using a wideband O2 meter you will see a lean spike when the secondaries open. It should be small and you should not feel it.

Do not use Teflon tape or any other sealers on the flare fittings. A drop of light oil on the threads is a good ides

If the initial timing exceeds 12 degrees BTDC with a MP distributor typically the advance curve will need to be modified so the total timing is not more than 34 degrees BTDC.

Chart to shorten the slots if you have a non adjustable mopar distributor.
Distributor degrees X 2 + initial= total
18 initial plus 14
32-36 total advance typically, every car is different.
There should be no advance until 1200 rpm
Then the advance should increase slowly until it is “all in” at 2400rpm for lighter cars - 2800 rpm - heavier cars

Dist. degrees / slot size
6.............. .340
7............... .355
8............... .375
9............... .390
10.............. .405
11.5 ........... .420
12.............. .435
13.............. .445
14.............. .460
15.............. .475
16.............. .490
17.............. .505
18.............. .520

There are 2 basic plates in the distributor one that has 11deg advance and one that is 17. The slot length on all is .480.

Back to top
how can you tell if the orange box is made before 1988?
Thanks 62Max. I already did about 1/2 that list and I've bought all the parts now to go back to 100% stock and work from there, line-by-line down this exact list....probably sometime next week if we get some good weather.
Hello 62max,

could you please send this to me private email.

After reading all the post and problems that some of you have with adjust ability of end carbs,if the car will run with the screws completely seated I would check the amount of throttle shaft/base plate wear, if significant that is likely to be your problem.
After reading all the post and problems that some of you have with adjust ability of end carbs,if the car will run with the screws completely seated I would check the amount of throttle shaft/base plate wear, if significant that is likely to be your problem.

Hey 62Max, please explain what you mean? I had considered throttle shaft/base plate wear, but the guy at Chicago Carburetor told me that in 30 yrs. he had never seen a six pack carb that needed to be bushed. Plus, back when I was searching for vacuum leaks, I smeared some wheel bearing grease on the ends of all 3 carbs' throttle shafts just to make sure there were no leaks.

Also, the throttle shafts would leak air... which would make the car run BETTER since I'm running very obviously rich (black smoke out of tail pipes at idle). If it matters, I did replace the rear base plate when I went to the ProMax base so I could adjust the rear carb easier.

So, I may be missing something or am way off base.... can you please explain?
If you have enough wear on the base you can not get a correct idle adjustment on the mixture screws,these base have anywhere from .004 to .010 shaft side play and on the car they are all different,some you can adjust the mixture screws and change the idle characteristics,others will do nothing,it is a vacuum leak same as if you had a warped base which also shows up when using the thin black base gasket rather than the correct 3/32' brown O.E.M.gasket.


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Those low initial timing recommendations might work if you plan to retain the vacuum advance. Otherwise, low initial timing, aftermarket performance cams, and no vacuum advance never play well together. 18 degrees initial is still low. 8 degrees is just stupid.
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I have my six pack 440 set at 20 degrees initial and 38 total advance, and it works great! .510 lift solid cam, 5400 feet elevation.
Here's an update on the idle mixture screws on the center carb.... while rebuilding the carbs I sonicated them & blew them out with compressed air and big glob of oil mess shot out of the metering body when I sprayed compressed air into the hole where the driver's side idle mixture screw goes. That's one mystery solved. Thanks everyone!
So is it running and idling properly now? Pulling the PCV hose and all that air coming into the mix indicates pig rich if the idle goes up. When properly tuned, pulling the PCV hose would drastically reduce idle speed or kill the motor.
Try buying some NEW carbs, bolt them on and go. Thats what I did, and the set up works great! I set the end carbs mixture screws at 1/2 turn out. Centers at 1 1/2 turn out. after awhile the rear carb started dripping fuel out of the booster venturi 's, so had to change the needle and seat. Now it's working good. In 1/4 mile testing, The car picked up .4 sec and 5 MPH over the Eddy Performer RPM and the Holley 780.
I found that my base plate outboard carbs idle mixture screws were not capped. Adjusted the front carb until I can get to the back. The good news is that my idle mixture screws on the center carb actually effect the idle now. Yea!
Hey Purple, any good results yet? One thing you can do (and maybe you already have) is look down each carb throat at the booster venturi's while the engine is idling. Sometimes fuel will drip out of the booster causing an over rich condition. This can be caused by too high a float level or a needle and seat that isn't sealing completely. I have seen this happen on a few Holley 4 barrels, and on my NEW six pack rear carb. It's a quick easy check. Good luck!