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Thread: Autolite/Motorcraft 2100 and 2150

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    Autolite/Motorcraft 2100 and 2150

    I've started the tumble down the rabbit hole on these simple 2 barrels with the addition of one to my project CJ7 and wanted to get the information I have gathered in one spot to hopefully help others. Was going to add it to my project thread but it turned into a novel and seemed more general.

    Carb Tuning Info: (Warning, if you don’t want to go down the rabbit hole of carb tuning please skip this post)

    My Autolite/Motorcraft 2100 has been a challenge to get running 100% where I want it so I thought I would jot down what I am doing. Just for reference this info is not guaranteed and is mostly applicable to a 2100 on non-standard engines say an AMC 258, 232, odd fire V6 etc.. 2100’s are dirt simple which is why they are used in so many applications.

    They are important to us because in the off road world can be set up for awesome running at most angles, some of which is inherent to the design since they have low mounted main jets centered in the bottom of the float bowl, the float is side hung so it is less affected by incline/declines and the bowl vents don’t have an easy path when flooded into the barrels of the carb. All of these can be exploited further with some slight changes covered on Pirate4x4
    https://www.pirate4x4.com/forum/jeep...ool-jeeps.html
    I learned a lot going through that thread and pulled information from their and multiple other locations to put this together. I am in no way claiming all this information as my own, just gathered it and typed it out.

    2100/2150s are sized by the diameter of the bore of the carb where the throttle plates are. The sizes of these bores affect how much air can flow through the barrels of the carb. Small engines will require lower CFM (Cubic Feet per Minute of air) and bigger will require more. Sizing the carb to an engine will change the efficiency and drivability drastically since a carburetor works off the principal of pressure differential through an air speed change which drives the fuel and air through the carb. Generally you want to keep the air speed through the carb up to improve low RPM throttle response and the increased air speed will keep fuel atomized for better efficiency (smaller fuel particles, avoid pooling etc..). There are also calculators online to figure out the required CFM of an engine size based on engine size and desired RPM within a given volumetric efficiency. (I warned you about rabbit hole)

    Bore CFM
    0.98 190
    1.01 240
    1.02 245
    1.08 287
    1.14 300
    1.21 351
    1.23 356
    1.33 424


    Since I was coming from a 1 barrel Carter YFA and liked the low end response/power and the fact that this is a 1976 engine nestled in a rattle trap jeep I figured high RPM was not in my future so I opted for a 1.01 bore 2100 carb off of a 1964 Ford Falcon with a 260ci Windsor engine sourced from Ebay.

    Once installed I had the new to me carb starting and idling perfect but needed to tune the other circuits. On a 2100 carb you have essentially 4 adjustments on the early models,

    1) Idle mixture screws for each of the barrels to dial in the fuel air mixture when on the idle/transition circuit. (Transition circuit is a fuel "short cut" ported just above the closed throttle plates that pulls extra fuel as you barely tip in off idle to resist a lean bog before the main circuit catches up to the air quantity.)

    2) Accelerator Pump, on some years/models of 2100s with the accelerator pump you can change the pump delivery amount via changing the hole the accelerator pump rod is attached to on the pump arm (0-2 options) attached to the front of the carb and/or the delivery speed where the accelerator rod is attached to the arm on the throttle shaft (3-4 options). The accelerator pump delivers fuel when the throttle is opened to enrich the fuel air mix as the increased air is entering the carb to resist a lean bog before the main circuit catches up to the air volume.

    3) Main Jets, 2 jets in the bottom of the fuel bowl to adjust the high speed fuel delivery between idle transition to the power valve opening. They are referenced by number on these carbs relating to numbered drill size so a jet stamped 40F translates to Ford 0.040". The Ford portion of that is important because the 2100 can use 5/16" Holley jets which would be stamped 40H and be about 7 jet steps away from 0.040 but I cannot remember which direction. (will update if I come across it later)

    4) Power Valve activation point. You can install power valves with different vacuum settings from 0.5inHg to 10.5inHg to accommodate different vacuum situations on different motors. General rule of thumb is 50% of your idle vacuum. The power valve will enrich the air fuel mix as a load is placed on the engine while on the main circuit.The amount of fuel delivered by the power valve is set by the Power Valve Circuit Resistors located in the runners that deliver the power valve fuel to the main circuit. The PVCR's are generally not adjustable on OEM carb setups but there are some work arounds I'll cover later.

    On to tuning, for the idle mixture experts that are smarter than me have gave a general start for first fire on these as 2 turns from closed to get the engine to run. Once the engine is running you will set the engine speed to about 700rpm and either tune by ear via the lean drop method, vacuum gauge for most vacuum at idle 18-22inHg, or by O2 gauge for 12.5-15.0 A/F ratio and “optimal” has generally been accepted at 14.7 for emissions, at least that’s what the electronically controlled carbs aimed for at idle. Anecdotally the main jets can affect the idle mixture to the tune of 1 full size change on mains can have 1 full turn of change on the idle mixture.

    Tuning the accelerator pump methodically may start with the smallest/slowest (furthest out on the pump arm and closest hole to the throttle shaft) setting and work up from there until the bog goes away when the throttle is pressed quickly

    Main jet tuning the old school way went by feel and pulling spark plugs to judge the running condition by the color left on the plug by combustion events. The newer and easier (cheating) way is to install a wide band O2 meter with a corresponding gauge to get real time Air/Fuel ratios. They are most popularly used for DIY EFI systems but should make tuning a carb super easy. I’ve found reading plugs to be difficult because if tuning wide open throttle events you have to jam the clutch and shut off the engine and pull over all at the same time to “save” the plug color. Jeep setups on the 1.08 venturi models usually end up in the 45-48 jet range.

    Changes in temperature and altitude will affect the main jets due to air density since the molecules of air change in both of those. I won’t get into X amount of feet times the air density divided by earth’s rotation equals the 4th root of the weight of a peanut blah blah blah… but these are things to consider if you tune your carb right on the edge of lean on a 70 degree day at sea level and then drive to the snowy mountains 11,000 feet up. I’ve been told 2500 feet of elevation change can have 1 full change on the main jet size.

    Power valve “tuning” is more of sizing to the vacuum condition of your engine/altitude, old engine lower vacuum, high altitude lower vacuum. I’ll just use half of the idle vacuum reading so the power valve opens below “normal” cruising throttle input and only comes in when extra load is on the engine like a hill or acceleration change. If you were to set the power valve to open too soon you would be driving at full enrichment all the time and using a lot of gas and running pretty rich.

    I say running on the power valve will be rich partly due to the theory that its more fuel but also the fact that the 2100 carbs run notoriously rich on the power valve since they were originally designed in the 50’s/60’s so fuel usage and excessively rich engines were standard. Others who have gone down this road see 12-13 to 1 air fuel ratios at WOT so they are considered safely rich. Older engines like a 258/232 should make optimal power in the 12ish to 1 range.

    Further the power valve uses 2 brass restrictors (PVCR’s) in the passage below the power valve and the main circuit to meter (limit) the fuel delivery when the power valve opens. Here is where we start getting into the weeds a bit since these are not an adjustable feature. In the hot rod world they can be opened up with precise drill bits to deliver more fuel on upgraded engines to stay in the safe fuel ratio. Unfortunately you cannot un-drill these so requiring less fuel on the power valve gets complicated. There are ways to accomplish this by drilling out the PVCR’s and installing some 6-32 threaded brass set screws that were center drilled with precise bits to the desired size. On a 2100 this is difficult because of where the PVCR’s are located partially obscured by the carburetor body. Depending on the year and application of the carb the PVCR size can range from 25-50% of the size of the tuned main jets. There are other ways to affect this circuit as well but is a bit further than required as well. For drag race setups the power valve can be replaced with a solid plug and all the fuel metering ran off the main circuit BUT a 6-10 increment step in main jets is needed AND fuel metering at partial throttle is way rich which would cause issues on a street driven vehicle.

    Also some years of 2100s/2150s can have a 2 stage power valve installed that will partially open with restriction to flow at a 1st stage of vacuum (9-11inHg) and fully open at the 2nd stage (3-5inHg) to further limit fuel delivery at mid to heavy throttle input. Reportedly this setup doesn’t have as good of throttle response but is interesting as we want to further control fuel through the full vacuum range.
    In a nutshell with these carbs you can manually do what an EFI system does a multitude of times a second and a lot better but be able to fix things with a rock and some chewing gum and strap a fuel container to your roof and run if the pump goes out…

    I'll try to cover what I find while tuning, I have a wideband O2 gauge on the way and Jets from size 40 to 49 to play with. It just so happens the later style 2 barrel intake/exhaust setup I have has a 18mmx1 threaded hole where the single wire O2 sensor was used on the OEM setup that I can use for my new wideband setup.
    Last edited by mtnbiker4evr13; 04-02-2019 at 04:18 PM.
    94 4runner SAS Elocked 35's "Fiona"
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    The running issue I've been chasing is the engine falling flat and missing at high load heavy throttle input. Using the vacuum gauge while driving I found it drives great right up till vacuum drops to 6-7in which is where the power valve was set to open. To confirm my thoughts I ordered an O2 sensor/gauge and an array of jets to dial things in.

    So O2 Gauge arrived yesterday and confirmed my initial thoughts on the problem. My ear and vacuum tuning had me at 10-12 A/F at idle, cruising was around 8 and power valve was out of range under 7.4 that the gauge bottoms out on.

    Stepping from the 46 jets to 45s did not make more than .5 to 1 point of change but should have 41-44 jets arriving today.

    I am a bit surprised that I'm having to step so far back on the jetting. Either the smaller venturi is pulling higher speed air and pushing more fuel or my 232 isn't pulling enough air for the fuel delivered. Definitely interesting tinkering around with these carbs.

    Before installation I would have put money on needing bigger than average jets for the 1.01 carb since to run the 1.21 carbs guys have to jet way back.
    94 4runner SAS Elocked 35's "Fiona"
    97 F350 Powerstroke CCLB "Marge"
    79 CJ7 Rustomod

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    High Altitude Adjustments:

    So base elevation in Colorado where I took the Jeep was 7700ft. This is about 5400ft above where my home elevation is and essentially made the Jeep undriveable down the road. O2 gauge confirmed 7-8 A/F ratio and a 5-8in drop in vacuum in the motor. So I decided to go ahead and do the PVCR modification or at least plug both the restrictors to see where I ended up.

    I had previously purchased some brass 6-32 set screws to use as jets once drilled with small gauge drill bits and a pin vice, the correct size bit and a soft material 6-32 bottoming tap from McMaster-Carr since these were not areas to skimp. I did get an Amazon special ping vice and small gauge drill bits on the cheap side and have regretted it. Similar kits are available to modify air bleeds on hot rod carbs. I don't know that I ever saw a 6-32 kit, most are 8-32 and may be too big under the power valve.

    Anyway, to do the modification you need to remove the carb, remove the power valve cover and finally the power valve. If you are OCD or make good decisions you should plan on doing a full clean on the carb to catch any stray drill/tapping chips. The Power Valve Circuit Restrictors are the 2 brass inserts under the power valve that look impossible to get to.


    I was able to drill them out without them spinning with a standard battery drill. Probably best to do this in the shop with a drill press since the soft insert will pull the drill bit down in without warning.

    After drilling I tapped them about 1/8in since that is how long my set screws were and I wanted to tighten the new restrictors down to something. I first ran the setup without holes which shuts off the power valve circuit completely which at elevation may not be terrible. It ended up being in the 17-18 A/F ratio running down the road at WOT. This is uncomfortably lean and risk burning up a valve or 6. Next I tried 2 restrictors drilled to 78 or 76 which netted an A/F ratio of 11.5-12. Ideally it would have been a little leaner since from 7,700ft we would be going up to 13,200ft for some passes. I left it rich since that was at least safe for the motor. At elevation I would have prefered to step back my main jets a few sizes but I was already down to a pair of 41 main jets.

    A neat trick my dad thought up was drilling and tapping the handle of a screw driver to hold the set screws while drilling them... Worked awesome and would be a great way to store/separate them since they are tiny.

    The super small main jets has confused me since day one, similar setups guys are running 46-48 jets without issue. To try something new and fun after the first day of wheeling I added a 2nd air cleaner on top of my original one since I was only running a 6.5" round filter open under the hood(picture a double stuffed Oreo). It seemed to help a little and was at least piece of mind. If I was getting an air restriction it would act as a choke and pull additional fuel on all circuits requiring me to step back on jets. Also pulling open hot air from under the hood affects the whole system as well. I think the next tinkering will be getting an enclosed filter of some sort in there in a larger diameter.

    As for the off road prowess of the 2100, with the simple mods it ran over 13,00ft passes nearly without issue and the steepest things Moab could throw at it without a hiccup. Definitely poor man's EFI.

    Now that I'm back to 2,300ft I need to look at all my numbers again and make sure I'm not burning anything up...
    Last edited by mtnbiker4evr13; 08-13-2019 at 03:06 PM.
    94 4runner SAS Elocked 35's "Fiona"
    97 F350 Powerstroke CCLB "Marge"
    79 CJ7 Rustomod

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    Stumbled on these and needed to get them in here for future reference trying to clean up my transition circuit.

    Thank you to 69612 at Hotrodders.com for these!



    94 4runner SAS Elocked 35's "Fiona"
    97 F350 Powerstroke CCLB "Marge"
    79 CJ7 Rustomod

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