Old Blue - 1984 XL Flareside

It sounds to me like your accelerator pump needs adjustment/tuning.

Even with my limited knowledge, I’d agree that makes sense.

It has been replaced twice now in the last 18 months. I have also tried changing the throw of the pump and no difference.

Are you sure the check ball is properly in place.

Do you see a squirt of gas if you cycle the throttle lever by hand?

certainly, if there is not transition from idle circuit to venturi flow there is no stumble. at 1200 rpm you are already on the venturi flow (or you have a vacuum leak and a rich idle) and just adding throttle should be very smooth. being that it seems to come and go, how thoroughly have you checked for vacuum leaks? one point that gets dismissed a good bit is the brake booster. notice that it has the largest vacuum line of all going to it. this is because it is the largest vacuum reservoir on the truck/car. and if that diaphragm leaks it can do exactly this. the most common sign is noise or noise changes by merely pressing on the brake pedal with the engine running. usually, a hissing sound that comes from the area around where the actuating rod connects to the brake pedal. the only true test is to cap the intake port disconnecting the booster and run the engine. possibly even tune the engine that way before reconnecting the booster. if it runs differently than it is a problem even if it's not the whole problem.

Hi Jim,

I will confirm, but last time I checked it was working, yes.  The carb shop guy installed it, but never hurts to check again, I shouldn't take nothing for granted!

Thanks Mat, I'll plug the brake booster and see if it helps.  Instead of driving it around, I can duplicate it in the garage now.

I just have to put it in gear, (set the parking brake) and also have one foot on the main brake.  I then give just a little gas, and the RPM stays flat when it should increase.  Eventually it does catch up after 3 or 4 seconds...

I'm not too well versed in Cali spec feedback carbs.
I certainly don't want to doubt "the carb shop guy"
It is his trade after all.

There should be a spec for gap if you have a set of feeler gauges.

Consider yourself lucky

I am probably missing something, but gap of what?

Actuator arm to diaphragm plunger.

Let me see if I can find a diagram outlining this (though it may not be specific to your California emissions carb.

Oh nevermind, I can visualize it now.  I am working and posting here in the background - a bad combo!

Well Gary has some information here on the site.

Perhaps Bill Vose will bless us with some of his infinite carb wisdom? (I'll ping him)

I know the Holley on my truck calls for 0.015" gap.

Thank you.

I now have a few things to check and I will report back. Thanks for the ideas, keep ‘em coming!

First item, there is no gap adjustment on a Ford/Autolite/Motorcraft 2100/2150/4100. There is (on most) a pump stroke adjustment which is done on the throttle shaft over travel lever. It is done, on one with multiple holes, by moving the accelerator pump rod into different holes. The further up you move it the larger the volume of gas delivered.

The first thing I would check would be, are you getting a "shot" of gas when the throttle is opened? Choke open, engine not running (choke can be held open for this) air cleaner obviously off. Open the throttle and observe the two "shooters" a decent stream on gas should come from both. If this is good, then other things need checking, if not, then you have your cause. The question then would be, "is the carburetor still under warranty from the shop that rebuilt it?" I used to give a 1 year warranty provided a new filter was installed.

One of the items that could cause issues is the timing chain and cam gear. Cam gear is originally a plastic covered aluminum gear, eventually the plastic breaks down and the chain can jump one or more teeth resulting in both the valve timing being off and the ignition timing being off. Another is a over active EGR system, if the valve opens too far, it will make the engine stall or at best stumble badly. You can temporarily disconnect the EGR vacuum hose and see if the problem goes away or at least changes.

There is a way to check some of the functions of the EEC-IV system, but, the first versions do not have a check engine light, so there is no visible indication of any problems. A parts store code reader will not work on these, it is only good for OBD-II (1996 up) systems. Any codes on your truck can be read using either a test light or an analog volt meter, once the Self Test Output mode is activated, the EEC will pulse the output with the codes.

I went out to the garage and verified that the accelerator pump works well.

I am not sure about the warranty, I suppose I can call and ask!  When I brought up on the last visit that the hesitation was still present, I was told it was because "they have to run so lean" and that the "EGR doesn't help".  I have enriched the idle mixture to no avail, and verified that the EGR isn't open at idle (just to double check).  I also disconnected the EGR and capped the line, no difference.  I will try to call back and make more of a stink about it - he's a good guy which is probably why I have a problem pushing it (that's on me)

I did check the timing with the spout out, it was 10 degrees right on the money.  I suppose that wouldn't catch a timing mismatch between the valve train and the ignition though!

I have a Ford OBD-I reader, the truck is currently clean in KOEO, KOER, and Continuous modes...

I did a smoke test and found a few small vacuum leaks and fixed those a few months back.  No change. I didn't try to cap off the brake booster though, I'll try that next.

I would try giving the accelerator pump more stroke by moving holes, as Bill described.

One thing to check, it will require a 15/16" socket, a deep well is perfect, but a regular one and a short extension will do. Remove the distributor cap and use the socket and a long enough breaker bar if you have one, but a ratchet will do. Turn the crank clockwise until the rotor moves, then turn it counterclockwise until it moves the other way. If you can do this where the timing scale is, it will give you a positive indication of how much slack there is in your timing chain. A lot of slack will cause the cam to be retarded, you can make the ignition timing up, but it can still make the engine run poorly.

Another item to consider, catalytic converter(s), they can become clogged or the strata inside can crack and crumble, obstructing the exhaust, you are dealing with 39 year old parts.

I had already tried moving the arm to the top hole of the arm, it didn't make a difference.  But some variables have changed since then, so it wouldn't hurt to try moving it there again.

(It was/is set to the middle hole)

Thanks Bill, I'll give this a shot as well.

I hadn't considered the cats, my Googling (for what it's worth/not worth) of the symptoms of a clogged cat didn't see like a total match, as the hesitation only happens right off idle.  But, maybe it's only partial obstruction that isn't readily apparent at higher RPMs?

I was hoping the video would come out clearer, but here is a video of the symptom:

https://youtu.be/dZrQu0H9oPI

Sorry for the weird angle, but I wanted to show what I was doing with the gas and the tach/sound all in the frame.

You'll see giving a harder push on the gas skips over the problematic part.  When you give just a bit of gas, the RPMs increase, stay steady (when I think they should go up a little higher) and you'll notice it dips.  The dip is when things feel the roughest.  It will stay at that dip until I give it more gas.  I had it hesitating more earlier when the truck was more cold (is this a hint) but of course that isn't in the video.

Hopefully this can give a better mental picture of what I'm seeing.  Thanks everyone.

I just took another video - the hesitation and especially the rough sound at this RPM is more evident.  (See previous video for how it's smoother when giving more throttle instead of feathering it)

https://youtube.com/shorts/vQWa26tu1qE

I also capped the brake booster vacuum line, no difference. (Thanks for the suggestion, Mat)

Honestly, to me that seems like it's just the convertor starting to load the engine.

While the engine may become a little 'rough' it's ultimately a carbureted engine hobbled by 39 year old emissions standards.
Do you actually drive it that way?

I don't want to sound dismissive, I'm just trying to understand.  🙂