Tuning

This article will show the most common adjustments and repairs of Autolite 2V carburetors and is not intended to answer all fixes or problems. It will however give you some insight into the most common adjustments and repairs Autolite and Motorcraft 2100 series carbs require hopefully saving you plenty of grief and a few bucks.

Several Autolite carbs are available for our Cats from 1.08 venturi to 1.23 venturi found in 69 351 and later applications. Running a larger venturied carb from a bigger engine is an inexpensive way of gaining HP.

1.08, 1.14, 1.21 and 1.23 are common sizes.

How to dial an Autolite carb

There are basically only four external settings

1) air fuel

2) choke

3) idle

4) fast idle

Set Air Fuel Mix

The most common carb setting we need to adjust is the air fuel mix.

This is accomplished by running engine up to temperature. 20 minutes at fast idle is how we achieve operating temperature. Make sure you have a good thermostat 195 in winter and cool climates and 180 in hotter and summer climes.

Once the engine has reached operating temperature its time to set the curb idle which is the setting on drivers side. Reduce or increase RPM to about 550 rpm.

With a good ear or a vacuum gauge hooked up to manifold vacuum, turn one of the two air fuel mixture screws in till the engine rpm decreases and out again till engine runs smoothest and fastest. Move on to the other air fuel screw and use same procedure. Make sure that the idle is about 600rpm. Check your shop manual for specifics for your application. The air fuel mix screws should be 3 turns off bottom as a reference or starting point. Do not tighten them down or the needles will be damaged and possibly the carb as well. Just bottom and back off three full turns and then do the hot air fuel mix setup.

The air fuel mix is only for idle and does not affect any other speed range. There are three circuits in a carb, Idle, Midrange and Hi or Load circuit. Midrange is calibrated by float level and the size of the jets. Hi speed and load conditions the power valve opens and allows fuel to venture booster.

Power Valve

The power valve is vacuum operated and located on the underside of carb and is the only part we need to take carb off engine to access. If you take the cover off the power valve and fuel is present on the cover side the power valve needs replacement and is the second most common problem found in Autolite carbs.

Accelerator pump

The most common problem is a worn out accelerator diaphragm. This comes in the BlueStreak HyGrade carb kit #586. Simply remove the cover and replace the diaphragm. Make sure the spring is on carb side of diaphragm and that the red one way check valve is not damaged. If so pull it off and from inside of carb body (lid off) you pull the new one thru. Part of the accelerator pump is in the Venturi Booster and requires that there be a ball bearing as well as the cylindrical weight above it.

Needle and Seat

A new needle and seat is also supplied and can be replaced using a screwdriver or a nut driver depending on the kit. Be sure to use the shroud for the seat. To remove the float assembly with the needle to access seat simply pry back on the spring retainer and the float will free. Lift out. The needle needs to have the clip properly affixed to float and float assembly needs to be clipped back onto the seat.

Float

The float should be set level with top line of carb base. Check your manuals and directions in kit for specific values. The higher the float the richer the carb will run. Jetting also plays a part in this rich lean condition.

Clear Passage

Always, when a carb is being kitted, blow out every single passage way with compressed air and then use a fluid like carb cleaner, brake cleaner or penetrant to visually see that the passage is clear. Spray with plastic tube into orifices and be sure that fluid is passing thru and out its respective place.

PCV Spacer

The PCV Spacer will require a new gasket as well as carb to spacer requires a new gasket. Clean scrape and remove all pieces of old gasket from intake, spacer and carb before installing new gaskets and the refreshed carb.

CHOKE

The choke settings are not all that tricky. Usually this is done twice a year as major seasons change. Set leaner in winter, richer in summer by about one maybe two notches of the settings shown on top of the housing. Richer means the choke stays on longer or comes on sooner at a lower ambient temperature. This prevents the summer setting of one richer from staying on too long in winter and vice versa. Warmer temps means the choke is already coming off so we need to richen it slightly in summer to get the fast idle out of it. In winter w/cooler temps the stat has a chance to cool down considerably more thus providing a more richer condition naturally..

Fast Idle

Fast idle is accomplished after all other settings are done. Turn engine on, run up to temp and set the fast idle screw on side of rear of choke to 1500 rpm. You will have to lift the fast idle cam into place to set hi rev.

Fast idle is set by having engine warm and setting the fast idle screw on the second highest setting of fast idle cam. You will have to open the throttle slightly to allow the fast idle cam to be set on the fast idle screw if carb is warm. Set this to 1500 rpm warm running on second step. Use the fast idle screw to accomplish 1500 rpm warm on second highest step of fast idle cam.

Operation

When the choke first comes on in AM the highest step is achieved when pedal is pressed one full length of travel to floor. This closes the choke flap and sets the choke to high step. Release foot and start engine. The engine should now start and rev up. It will require that you rev the gas a little to step the high idle down to second step which when warm will be 1500 rpm but cold about 1100rpm. If the choke does not rev off hi step then you will need to decrease the choke by the thermostat on side that has the index markings and says leaner on side. Clockwise is leaner or less choke and counterclockwise is Richer or more choke. Typically one or two notches from center or index are what your car will see throughout the four seasons.

If you have had the thermostat off be sure to renew the gasket. The spring must align into the tang for the choke to work. With thermostat cap loose turn choke lean and rich. The choke flap should open and close respectively.

Hot Air Feed

Most often times are the hot air tube from exhaust manifold to choke stat housing has rotted away. I use a ¼” drill bit to clear the hole in exhaust manifold and then I cut and fit a ¼” brake line to the hole and bend to fit (as Ford did) to the choke thermostat housing. You will need a tubing cutter and a flaring tool in most cases but costs of tube is $2. Be sure to salvage and reuse the insulating fiber shield Ford issued with the original hot air tube and quite possibly the nut to secure the tube to thermostat assembly.

Do not use copper ¼” line. It does not carry the heat like steel does.

Electric Chokes

If you are one who wants to use an electric choke the ONLY place to collect current for an electric choke is off the STA terminal on back of Alternator. This ensures that current is only directed to choke stat when engine is running. If it stalls when cold choke does not come off prematurely. There really is no reason to use an electric choke on stock applications. Only when headers are used is an electric a “good idea” as there is no hot air feed that works adequately for the chokes. The “L” on some headers is not adequate IMO.

Taking current from ANY other location other than STA is a draw on that circuit drawing away from what it’s supposed to be feeding and can hinder purrformance especially if you place the pickup on coil or some other wrong location.

I can tell you that the stock hot air chokes work every bit as good as and possibly better than electric chokes. I have run both over the years and have repaired my stock hot air chokes to work flawlessly.

4300 Notes

Many complain 4300s leak at front and bog under hard acceleration.

The leak is easily fixed.

Over time the metal changes profile and begins to warp in an upside down U shape on the tops. I remove top and carefully while supporting the top safely reset the front flat again using a mallet and a straight edge.

The bog is also easy to fix but may take some time in trial and error. The rear upper flaps that look kind of like a choke on the rear secondaries is a choke of sorts. It chokes off air to secondary and when pedal is depressed suddenly begins to open slowly. This slow opening set of flaps calibrates the air and fuel mixture rushing to the cylinders thru the secondary circuit. If the rear flaps open too soon then a lean condition will be present, too late and the secondaries will make the engine run rich. A bog can be associated with either rich or lean but lean is more common problem I experience.

Setting the secondaries up can be tricky depending on year

You will have to look on passenger side of the carb body where the upper plates pivot for a dial and a Y type clamp. Inside there is a spring that effects to opening and closing of the rear upper flaps. On other years there can be a spring under the brass piston on drivers side of carb. If you’re experiencing a bog then try setting the flaps to open slower. Calibrate as required by retesting on several test drives.

The other bog experienced in off idle conditions may quite possibly come from the lack of travel in the accelerator plunger. Measuring from top of the carb top itself where the acc plunger sticks out from to the bottom of the lever must be .500” travel. If the plunger is not sticking out a full half inch from the carb there is not enough travel for the accelerator pump system to work. Bending the round rod from primary throttle shaft up to the acc lever is how the full ½” of travel is achieved.

Another off idle falter or bog usually stems from a bad vacuum advance pot on distributor. ALL street engines benefit from a vacuum advance.

TIMING W/VACUUM GUAGE

With a vacuum gauge you can set your timing. Once the curb idle and air fuel mix has been established turn distributor clockwise (advance) to get as high a reading as possible from the vacuum gauge. This is usually 19-21”Hg in a stock application. Drop the distributor back counterclockwise till about 1” of vacuum is decreased and lock the distributor down. Test drive uphill under load and listen for pre-ignition. If none then you’re good. If a knocking is evident then back timing off another half inch of Mercury on the vacuum gauge scale.

Vacuum Advance

Be sure the vacuum advance works properly. While there are many ways to do this the cheap and dirty way is to simply take a clean hose to the vac advance pot and to your mouth. If you can draw air the pot is bad and needs replacement. If you cannot draw air then the vac pot is good. Some are calibratable. To determine if yours is you can insert a 1/8 Allen key into the pot and see if the vac pot will turn. If so 3-4 turns is what’s called out for off bottom. Again check your data sheet that comes with the calibratable vacuum advance pot. Two brands I am familiar with that have adjustable vacuum pots are Kem Brand VC10A and Standard/Blue Streak VC31. Each costs about $25.

The purpose of the vacuum advance is two fold. It helps to smooth out off idle response as well as give better gas mileage. ALL street cars benefit from having vac advance. Only Marine and real race applications do not require vacuum advance.

Vacuum advance makes the transition from idle to load smoother. As we load the engine it slows so adding a few degrees of vacuum timing keeps the curve where we want it in relation to principles of air fuel compression detonation and where the crank piston and valves are in relation to this and at what speed. Complicated I know but remember the internal combustion engine was not perfected in a weekend.

By setting the initial timing to somewhere between 6 and 12 BTDC w/vac advance when the pedal is pressed off idle this sends a vacuum signal to the engines spark system via the vacuum advance and the engine “wakes up” as we add timing to this sleeping idling beast. In load conditions i.e. racing or marine applications the engine is always run at or near Wide Open Throttle. When in WOT the carb venturis does not send a vacuum signal to vac advance. Therefore initial timings on non vac advanced engines can have more initial timing say 14 BTDC. Both are limited to about 37* total but some fuels allow for a few more degrees of total advance.

For stop and start driving you want the vacuum advance. If WOT all time is your application then you don’t need one.

Shown above is the adjustable pieces found inside a Ford adjustable vacuum advance. The weight slots refer to where the weights fit below the breaker plate.

The Ford issued vacuum advance pots are different and have a removable snout on the vacuum control. This type is also adjustable but requires shims and the procedure is well documented in the Ford Factory Shop Manuals. The manuals will tell you how to set the vacuum advance at specific rpm as well as show you how to dial in the mechanical advance. Taking the time to read the shop manuals and learn how to set a Ford distributor can save you hundreds of needlessly spent dollars. Unless you are running a racing engine pulling more than 6000 rpm consistently the stock distributor properly setup will be as reliable as any $400 aftermarket distributor setup.

Shown are the Duraspark pieces you can adapt to your stock dist. You will need to elongate the slot for wires as well as undo the clip in cam to fit the inducer.

Duraspark

The Duraspark system Ford offered after 1973 will bolt directly into your engine and is also completely calibratable. New springs for the mechanical advance are available from Ford. You might benefit by using the 13 or 15 slots for the weights if you are experiencing too much total timing and getting ping or pre-ignition. Later lower compression era distributors have 18 21 slots. You may have to locate a couple parts distributors to get the better weight cams. Again the Ford shop manuals will guide you thru setup process. All Ford distributors are good to 6000 rpm. If you use the 18 21 weight cam you will have to run a lower initial timing more like 6BTDC.

Mechanical Advance

Mechanical advance consists of a pair of springs and a pair of weights with stops that allow the inducer or point’s cam to run ahead as the engine speeds up. Calibrating the weights by using springs is one method or bending the tabs or posts the springs attach to will change the mechanical advance curve. Pretty much rule of thumb though is you want 10* initial with a total of about 37 @ 4000rpm in a stock application. Check your shop manuals for specifics.

You'll need to ID your dist # and the values for rpm are set in the shop manuals.
The values cover a wide range in the shop manuals.
There are 3 dists for 351W. Std trans auto trans and each has different values.
C90F-M std and auto 21.5-31* w/vac 10.5-15@ no vac @ 2000 rpm
C90F-N std trans 28.5-37.5* w/vac 12.6-16.5* no vac @ 2000
C90F-T auto trans 36-45.5* w/vac 16-20.5* @ 2000

The good step by step on dialing FoMoCo distributors in
the FORD shop manual every enthusiast needs. This is the manual Shop techs referred to for guidance when your car was new. Your shop manual will list specific amounts of timing at specific RPM. You calibrate using a timing light and either shims for the original style or an allen key for the newer adjustable style offered for both electronic and point ignition systems. I use STANDARD brand or KEM brand ignition vacuum controls when FoMoCo is not available.

adjustable vac pots (point dists) from

KEM VC10A
Bluestreak Standard brand VC31

Ford Engines of the 60s tend to like 10ish initial and have the slots in mech adv set to
either 13 or 15. That gives a total of 36 or 40 depending on which slot plus
there is a few degrees available thru vac adv. A vac adv is critical to
smooth off idle acceleration as well as better gas mileage and over all
performance.
If someone tries to tell you a street car doesn't require a vac adv
probly best to walk away. Engines w/o vac adv are marine or industrial or
race apps. The reason none of those have vac adv is the engine is always
under load and no vac is present to run the unit and therefore of no use in
marine industrial or race applications.

Road or Load Timing aka Power Tuning

To set your timing for max HP and mileage select your ½” wrench your favorite hill and head up it aggressively. The engine should not ping. If it does, retard the timing a degree or two depending on how much pinging you are experiencing. If no pinging is present then advance the timing clockwise a degree or two and head up that hill again. You want to hear a slight tinkle of pre-ignition and then back timing off just a degree or two so that it does not ping. This is where your best gas mileage will be attained and is set by you under load to your specifics cam compression rear end etc.