Here is a guide to tune six packs for street engines. Stroker or non stroker, big block or small block
This is not the final word, but close enough
Remember six pak engines run on the center carb and idle on all 3
The car must idle and run like a normal car before attempting any secondary action or wide open throttle passes.
A Good ignition system is required., MSD, Mallory or FBO mopar box, NO orange boxes or chrome boxes unless you know for sure it is early 80’s vintage. Anything made after 1988 is questionable.
Quality distributor cap & rotor
High quality spark plug wires like Firecore 50s
Spark plugs of the proper heat range.
Vacuum adv distributor
The distributor phasing has been checked and corrected as necessary
THIS IS IMPORTANT
Distributor vacuum port on carb disconnected and plugged at carb
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 rotor with the cap-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 manifold vacuum source.
ATTENTION -195 degree thermostat- ATTENTION
THIS IS IMPORTANT
If you run a lower temp thermostat, raw fuel will collect in the intake
That fuel then burns off in the cruise mode.
Unless you have a wideband air fuel meter you will not see this happening, but you will have problems getting it to idle and see the mixture go leaner in the cruise mode.
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.
Set the outboards idle adj screws out 1/8 turn ccw THIS IS IMPORTANT
Be careful when seating the idle screws to set them, gently is the word
Install the BLACK springs – Just do it, ignore everything else you have read
Set the center carb idle adjustment screws at 1.5 turns out [ccw] THIS IS IMPORTANT
Check the center carb and 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 would 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 The outboards will have the lead plugs covering the idle adj screws.
Starting point for center carb jetting stock 340 use 62’s, highly modified or stroker use 64’s,
Leave the outboards alone for now unless you have the jetable metering plates, if so read their instructions and follow them
You must know what power valve is in the center carb. Typically a 6.5
Fuel level adjustment THIS IS IMPORTANT, this is best done idling at 1000-1200 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.
You must let the car run a 3 or more minutes so the fuel levels off before rechecking level
Center carb the fuel level is at the bottom of the sight plug hole
Secondaries it is just starting to come over the bottom of the sight plug hole
This is critical so get it right.
Set idle to 950 rpm and allow engine to reach operating temp.
Reminder 195 degree thermostat required or fuel will puddle in the intake. .
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.
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.
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 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. If you do not have control over the idle mixture you have issues that need to be taken care of before proceeding.
Typically if you have the center carb idle mixture screws between 1 to 2 turns ccw
you do not have to adjust the outboard idle mixture any further. Starting with the front carb, adj the mixture screws one at a time 1/16th 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.
Beware of 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 etc.
Recheck idle rpm and set to 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.
Most times you are over jetted.... Do not over jet!
Over jetted carbs will have poor idle control.
Now reset the initial timing again. 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 15 degrees BTDC.
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
Recheck idle rpm and set to 850
How do you know when you are "there”?
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.
The engine when cold starts easily, runs and drives smoothly from the get go.
The engine when hot restarts immediately without touching the throttle.
When the engine is rev’d and the throttle released it immediately returns to idle.
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.
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.
Notes:
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.
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.
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