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Ford carburetor stalling and hesitation

Have a 1966 Ford Mustang with stock 289 -2v. The carburetor is rebuilt (2x) by Pxxx Carburetors with the same off-idle / hesitation / flat spot. The carb has been sent back to the same rebuilders twice for the same issue - no change. The engine timing is set at the factory 6 degs. BTDC and advanced incrementally - per the rebuilder recommendations - no change - with each base timing setting.
The engine is rebuilt; strong vacuum at idle (20-21") steady guage needle, strong compression all eight cylinders, fresh spark plugs, plug wires, new Pertronix Ignitor (no points or condenser), new distributor vacuum advance, new distributor vauccm line (stainless steel), new accelerator pump diphragm and spring.


How about your fuel pump?

How about your fuel pump?

Fuel pump is OK -no fuel

Fuel pump is OK -no fuel delivery issues to the carburetor. The carb seems adjusted OK too - the fuel float level is within specification.

Jimm, Pretty soon it’s gonna

Jimm,

Pretty soon it’s gonna be a year that you asked the same question on a different forum. For your convenience the link for that forum appears below. Are you sayin that this problem still has not been resolved? If the answer to that question is yes, then I'm findin that hard to believe, because the guys at the other forum NAILED the problem.

http://forums.mustangmonthly.com/70/8629211/mustang-drivetrains/66-musta...

Nope -the problem has not

Nope -the problem has not been resolved yet - been dealing with this since 1994- since we purchased the car (so - yes - well over a year!). There is no leak in the carb at the fuel idle tubes. Several suggestions but these did not turn out to be a problem.

Jimm, "MechanicalGuy48" is a

Jimm,

"MechanicalGuy48" is a colleague. He and I "Walk the Walk and Talk the Talk" if you can "Dig what I'm sayin"? :-) Are you sayin that you DID ALL THIS (below) without results? If the answer is yes then replace the distributor:

AFTER YOU RULE OUT SLACK IN THE TINMING CHAIN!!!!!

Well it sounds like you have done your homework on advance. The high vacuum seems to put away any concerns about a vacuum leak, it almost seems too high. I don't know what the background is with this carb if you just got this one or whatever, but occasionally at the factory we ran across calibrations that had the throttle plates closed too much at idle(for various reasons) and this would affect transition flow in the idle slot. When you take your carb off and turn it over, you will notice a slot(two of them) in each bore above the idle adjustment needle. This slot flows fuel that allows you to transition to the main jets. While the accelerator pump should shoot fuel in quick throttle openings, the fuel in the transition slot also helps in slower throttle openings. The transition slot fuel comes from the same source as the fuel and air mixture that flows through the idle screw hole which is well below the throttle plates. When the throttle is closed the transition slot is only partially exposed below the throttle plates and it flows no fuel because it backbleeds above the throttle plates. As you open the throttle plates you expose more of the transition slot and get more fuel. It is entirely possible that this slot needs to be lower for your car. You can try two things. One is to put more fuel in the idle system by enlarging your idle jets. The other is to take a small flat file and file down the slots a small amount. Both of these things are destructive of carb parts if they don't work. I guess when your ready to try that,,,, I can show you how to do it. Exhaust all your other Ideas first,,,make sure the power valve is not leaking and all the carb passages are clear and all your spark plugs are good and wires etc.

Thanks for the response; I've

Thanks for the response; I've made sure the power valve is not leaking. All the carb passages are clear. All the spark plugs are good and wires are OK -this was one of the fundamental checks done before sending the carb out for rebuild to the experts the first time.
I'd be inclined to experiment with the transion slots only after all other possibilities are fully exhausted. I do have another (spare) 2100 model carb that I can experiment with - though this second one is not rebuilt or been placed on the engine in years.

Jimm, What's the mileage? Any

Jimm,

What's the mileage?
Any slack in the timing chain?
Has the chain ever been replaced?
Is it the chain due (or long overdue) for a change?

Drive with a fuel pressure gauge attached ("T" in if necessary) Post the fuel PSI values under different speeds (engine loads). You can also do the same thing with an "Engine Vacuum Pressure Gauge".

Hello Jimm, During the 60's

Hello Jimm,

During the 60's the 289 Ford had problems with a soft cam. It is possible that the problem you are describing is due to one or more slightly worn lobes on the cam. Note that on a 60's 289 slight cam lobe wear is possible without popping through the carburetor.

Thank You
Helene

Jimm, I find myself in (FULL)

Jimm,

I find myself in (FULL) agreement with (HELENE). I’ll (CLARIFY). The term (SOFT CAM) is an (AUTOMOTIVE TECHNOLOGY) term for the (PREMATURE) wearing down of camshaft lobes.

The engine is a rebuilt Ford

The engine is a rebuilt Ford long block; new rotating assembly - including the camshaft and lifters.
If there were a worn cam lobe (soft) then it may show in poor compression in one or more cylinders. The engine has strong comression in all cylinders and strong / steady vacuum at idle. This means the basic sealing and pumping integrity of the engine is in good shape; no excess wear or slack in the timing chain, gears or crank or camshafts / pistons or valves.

Jimm, When faced with a

Jimm,

When faced with a problem like yours, that’s been goin on for a long time, it’s easy for a person to miss or overlook something. That’s what I think is goin on here buddy. You’re a skilled guy but you missin something. Here’s a helpful link. You can’t just click on the link as it appears. You gotta copy and paste it into your address box then click on go. When the page opens you’re gonna notice the subject titles in blue. Start your troubleshooting with “Hesitation during acceleration”. For me, I think what you’re missin is something in the fuel circuit or a vacuum leak. Best way to find a vacuum leak is with a smoke machine, and the cigar trick can also work. Let us know if you’re not familiar with the cigar trick. Good luck man, and keep us posted.

http://www.recarbco.com/technical/newtrouble.html#Hesitation

Big Block 409- Thanks for the

Big Block 409-
Thanks for the suggestions. There are no vacuum leaks - the engine has only two (2) vacuum lines; one hose from the intake to the transmission - this is new and intact. The other line is the solid tube (stainless steel line) from the carburetor to the distributor vacuum advance - this is also new. In addition, the distributor vacuum advance is also new.
Given the history, and the off-idle stumble that so far - even the Ford carburetor experts canno resolve - I'll take all the suggestions and information. That's part of reason why the same question is posted on more than one site - have not been able to solve problem yet.

Jimm, Thanks buddy, but I'm a

Jimm,

Thanks buddy, but I'm a MARINE SNIPER and you gotta understand that we NEVER quit even when the mission is over :-). The problem you're havin is compounded because we're tryin to resolve it online, but that doesn't mean shit to me. I'm not gonna give up man, and I'm sticken with this till it's fixed. I got a question for you, and answer honestly because it's to your advantage.

Did you try EVERYTHING the guys on the other forum recommended?

Next thing is to hook up a fuel pressure gauge, and drive with the gauge attached. You're doin this to find out if there is any change in fuel pressure when the off idle hesitation occurs. If there is then you found your problem. It means that the diaphragm in the fuel pump is defective, and the pump has gotta be replaced.

The fuel pressure and fuel

The fuel pressure and fuel pump is OK. The fuel line is OK - no leaks or kinks.
The suggestions were useful - that is, some of the ideas are already tried and tested. Some, such as the idle tubes leaking inside the carburetor - does not apply, as there are no leaks in the idle tubes / no reason to seal them up.

Jimm, (You said) The fuel

Jimm,

(You said) The fuel pressure and fuel pump is OK.

Sorry buddy but your above answer is useless. Remember we’re doin online auto repair here and, we don’t have your Mustang in front of us to run any tests. If we did have the Mustang in front of us to test then WE could say that fuel pressure and fuel pump are ok, but since you gotta be our hands and eyes here you gotta tell us in DETAIL what’s goin on. You gotta post the ACTUAL fuel pressure PSI. Simply sayin the fuel pump and fuel pressure is ok, is just not a sufficient answer Do you understand what I’m sayin here man?

(You said) there are no leaks in the idle tubes / no reason to seal them up.

Same as what I’m sayin above guy. Describe in DETAIL how you confirmed the tubes are not leakin?

You also gotta tell us in DETAIL the conditions of the hesitation. In other words does it happen:

All the time?
Sometimes?
Under what conditions does it get worse?
Under what conditions does it get better?
Does ambient temperature impact it?
Do engine temperatures impact it?
Does the weather impact it?

The hesitation only happens

The hesitation only happens off-idle, just as the throttle is cracked to accelerate the engine speed. It happens mostly when the engine is cold - only at the off-idle throttle position. It will also happen to stumble, though slightly less severe - when the engine is warmed-up.
The outside or ambient temperature will no have any effect; happens regardless of the outside or underhood temperature.
There are no fuel line leaks on the basis on visual observation - no fluid lekas anywhere in the fuel system; from the fuel lines to the carburetor.

JIMM, Look man we’re tryin to

JIMM,

Look man we’re tryin to help you here, but at this point (and on your part) there are still far too many unanswered questions. Answer all the questions that you have yet to answer, and remember you gotta answer ALL of them with details and cold hard facts and details because, you've spent years on different forums lookin for answers to your hesitation problem. Furthermore by this time I’m sure you’re ready to put this problem to rest.

IT’S ALL UP TO YOU BUDDY.
YOU DECIDE WHAT YOU WANNA DO.

After you have answered all the unanswered questions, if the hesitation continues, then replace the parts in your 2100 carburetor with the (NEW UP-GRADED) parts shown below, and post the results.

C4AZ-9B559-A Pump Diaphragm (Motorcraft CM-463)

C4AZ-9576-A Check Valve (Motorcraft CM-825)

Then if the hesitation continues (and you feel like it) you can post the fuel pressure PSI via a fuel pressure gauge.

BUT:

Remember and understand that you don't have to do a damn thing man. You're livin in AMERICA (The Greatest Free Democracy In The World). That means you are free to struggle and just live with you hesitation problem (as you have for years), because of MARINES like me that fought for your right of freedom and for your right to be WRONG. Best regards and good luck Jimm.

Thanks for the suggestions. I

Thanks for the suggestions. I thought all the questions were answered up to this point. The conditions of when the stumble / off-idle hesitation is given above.

The carburetor accelerator pump; diaphragm and spring are new and have already been replaced.

Jimm, (You said) I thought

Jimm,

(You said) I thought all the questions were answered up to this point.

Not really buddy you just think you answered all the questions. For your convenience the unanswered questions appear below. You’re allover the web with this problem, and on countless other forums, so understand that some of these questions are comin from those forums.

Tell us in detail how you confirmed the transion slots (emulsion tube) are not leaking?

Tell us detail how you ruled out issues with the primary and secondary ignition circuits?

Tell us in detail what the fuel pressure PSI is (via a fuel pressure gauge) at idle and under different engine loads?

(You said) The carburetor accelerator pump; diaphragm and spring are new and have already been replaced.

I know that, but were they replaced with the (NEW UP-GRADED) partsbelow?

C4AZ-9B559-A Pump Diaphragm (Motorcraft CM-463)

C4AZ-9576-A Check Valve (Motorcraft CM-825)

The carb transision slots are

The carb transision slots are not leaking - as I can tell visibly and with the Mity-Vac handheld vacuum pump.
The primary ignition components are already replaced; new ignition coil, new wiring harness, new coil wire to the distributor. The secondary components are all replaced and are new; new spark plugs, new Ignitor electronic module (no points or condensor), new spark plug wires.
The carburetor accelerator pump; diaphragm and spring were replaced twice (2x); once at the carb rebuilder and once again-later-by myself.
As far as I can tell, these wrere OEM-quality or equivalent

Jimm, Still waitin to see the

Jimm,

Still waitin to see the fuel pressure PSI stats via a fuel pressure gauge. If those stats check out, the primary jets are at 47 thousandths. Pull the jets and drill em out to 51 thousandths and post the results.

Need an explanation of how

Need an explanation of how drilling out the primary jets would help an off-idle stumble? More specifically, why to the 0.051" size? Why not 0.050" or 0.052" size? Is this based some guesswork or experiment, maybe an actual carburetor?

Hello Jimm, An off idle

Hello Jimm,

An off idle hesitation (lean condition) that is secondary to a carburetor jet being too small can be resolved by using a slightly larger jet. The larger jet changes volume of fuel to a slightly (richer condition). By the same token you don’t want to go too large on the jet size because that could cause a flooding issue. Actually your questions started me thinking that since the carburetor was rebuilt twice maybe it was rebuilt with the wrong size jets? Do you know what size jets are in the carburetor now? If necessary contact the rebuilder and let us know the size jets that were installed. You can also look inside the float bowl on the left side and let tell us if you see numbers like 1.01, 1.21, and 1.23. From those numbers we can get an idea of what the jet size should be.

Thank You
Helene

Thanks for the suggestion.

Thanks for the suggestion. The rebuilder tested the carb and made all adjustments on a test engine at their facility, before sending it back to me. At any rate, the stock jet size for this carb (C6DF-B)model and vehicle application is; 1.145 - and according to the 1966 Ford factory service manual.

Hi Jimm, You've stated that

Hi Jimm,

You've stated that the carburetor was rebuilt and tested twice on the re-builders test engine. Very well then but that raises some new critical questions.

Is the re-builders engine the same engine as yours?

Is the re-builders engine on an engine stand or installed in a car?

If installed in a car is the car driven after the carburetor is installed and adjusted?

So let’s say the engine is installed in a vehicle and the vehicle is driven after the carburetor is installed and adjusted. If there is no off idle hesitation on that vehicle but there is an off idle hesitation on your vehicle then that raises the question of what is wrong with your vehicle. Maybe a defective carburetor base gasket? Replace the gasket tighten it down using a criss cross bolt pattern, torque to spec and post the results. 1.145 is the wrong size jet. The stock size is 1.147. Try a 47F and if that doesn't resolve the hesitation then go to a 51F and post the results.

Helene

The rebuilders have a 302

The rebuilders have a 302 engine on a test stand that they (reportedly) test each carburetor on before returning the unit to the customer.
The 1.145 size jet is correct for the C6DF-B (289 - 2v with automatic transmission Mustang) for the 1966 year vehicle. The carburetor base gaskets (two) - one for each side of the carb spacer - are new ones - no leak at this point.
Besides the engine test performed at the rebuilders, I've also adjusted the carb (idle mixture, choke pull-off, idle speed, ect.) to match the engine in my car - several times over the years - to try and resolve the off-idle stumble / hesitation issue.

Jimm, (You said) The

Jimm,

(You said) The carburetor is rebuilt (2x) by Pxxx Carburetors with the same off-idle / hesitation / flat spot.

(You said) The carb has been sent back to the same rebuilders twice for the same issue - no change.

(You said) The re-builders have a 302 engine on a test stand.

So how many years have you had this problem and how many forums have you been on tryin to resolve it? With that being said, let’s take a close look at your above statements and the ASS HOLE re-builders lack of common sense. Furthermore buddy, you’re partly to blame for the hesitation goin on this long as well. Read on and you’ll see why.

First thing to understand is that, if you put a C6DF-B 2100 with 45F jets on a 302 and drive it WON’T stumble off idle, but put it on a 289:

Mustang
Falcon
Comet
Or Fairlane and it WILL stumble off idle.

Now when I say you’re partly to blame here’s what I mean. At ANY TIME did you ask the re-builders:

Why are you testing my carburetor on a 302 when the engine in my car is a 289? (And don’t believe the BULLSHIT answer that a 302 is simply a bored out 289) because there’s much more to it than that.

Now here’s why the re-builders are ASS HOLES. At some point they should have told you that:

Our test engine is a 302 and we don’t have a 289. Also our test engine is mounted on a stand and not in a car, and we DO NOT do any driving to test a carburetor that we have rebuilt. ALSO, the fact that our test engine is mounted on a stand makes it IMPOSSIBLE TO ACCURATELY DUPLICATE the off idle stumble that you have, so what we’re going to do for you is RE-JET your carburetor with larger jets and send it back to you to try. Let us know if the RE-JETTING resolves the stumble.

From what I understand and

From what I understand and can gather - the rebuilder is out-of-business. I'm not sure of this as I'm getting conflicting stories - even on-line (imagine that!).
But they have sent the installation DVD which is explained the hot engine test process. They recognize the limitations of the engine test and usually provide detailed written, and now the DVD - to guide the installtion and timing adjustments.
I can't fault the engine test - given the limitations - this type of test is mre than most carb rebuilders provide.
At any rate, my repeated e-mails to the organization have gone unanswered and un-responded to - now I may have learned why.
Looks like I'm on my own for the most part - which also explains why the repeated and multiple postings on different sites.

Jimm, (You said) Looks like

Jimm,

(You said) Looks like I'm on my own for the most part - which also explains why the repeated and multiple postings on different sites.

Look man, you're on your own because you choose to be and because you're not listening. Go to a size 51 jet and be finished with this problem once and for all.

So you are convinced the

So you are convinced the stock jet size in the carburetor is incorrect? And your advice is go with a slightly larger size jet? What am I left with as options if this does not resolve the stumbling issue? I have no problem trying this fix - though the logic to go to a larger size does not quite make sense. I'm not sure a slightly richer condition is the answer here - but I'm willing to give this a try.
Back to my original question - what are my alternatives if the different jet size does not work?

Jimm, (You asked) So you are

Jimm,

(You asked) So you are convinced the stock jet size in the carburetor is incorrect?

Yes

(You asked) And your advice is go with a slightly larger size jet?

Yes

(You asked) What am I left with as options if this does not resolve the stumbling issue?

Going to a 52 jet but not higher than a 52 because flooding will occur.

(You said) I have no problem trying this fix - though the logic to go to a larger size does not quite make sense. I'm not sure a slightly richer condition is the answer here - but I'm willing to give this a try.

You're like me JIMM.... A smart man, and I would be interested in hearing your logic and why you feel a larger jet makes no sense IN TERMS OF RESOLVING YOUR PROBLEM?

(You asked) Back to my original question - what are my alternatives if the different jet size does not work?

Puttin on a HOLLY. Here's a link :-)

http://www.holley.com/

Well - first off - a larger

Well - first off - a larger jet will not necessarily solve the problem. There is little evidence the stumble or hesitation is due to lack of fuel.
It may be related, however - I've swapped carburetors to another stock 2100 Autolite (with stock jet size) and the problem vanished. Of course, this was from another Mustang and not my vehicle - but belong to a friend whom had recently restored the car.
So far, all the evidence points to an issue strictly with the carburetor and not necessarily with an incorrect jet size; as in - too small of a jet size.
But, like I stated previously - at this point I'm willing to try the swap. Going to a different carburetor - not as stock 2100 Autolite such as a Holley 2 barrel - is pretty much out of the question.

Jimm, (You said) Well - first

Jimm,

(You said) Well - first off - a larger jet will not necessarily solve the problem. There is little evidence the stumble or hesitation is due to lack of fuel.

OK, but between this forum and the other forums you’ve been on (for years) you’ve stated the above quite a few times. For the sake of understanding and proving yourself do you think you could backup your above statement, with some cold hard facts and the tests you did to confirm the above?

(You said) I've swapped carburetors to another stock 2100 Autolite (with stock jet size) and the problem vanished.

Well, if this is true then why have you been allover the web with this problem (for years) looking for a resolution?

(You said) So far, all the evidence points to an issue strictly with the carburetor and not necessarily with an incorrect jet size; as in - too small of a jet size.

Again, for the sake of understanding and proving yourself do you think you could backup your above statement with some cold hard facts and the tests you did to confirm the above?

(You said) Going to a different carburetor - not as stock 2100 Autolite such as a Holley 2 barrel - is pretty much out of the question.

Again, for the sake of understanding and proving yourself do you think you could backup your above statement with some cold hard facts, and the tests you did to confirm why going to a Holley is a mistake?

(I said) You're like me JIMM.... A smart man,

Even a Grand Master like me can make a mistake. With that being said I ask that you humbly forgive me for my mistake in saying that you’re a smart man who is like me. You're not a smart man, and you're like me only in your dreams man. Have I pissed you off yet buddy? Well I certainly hope so, because you’re talkin like a man who has a PAPER ASSHOLE. If not a jet problem then WHAT? Make sure you back up your answer with COLD HARD FACTS. REMEMBER WHO YOU’RE TALKIN TO? Just for the record here guy the stumble IS NOT due to a:

Float problem
Needle and seat problem
Accelerator pump problem
Or metering rod problem

MORE IMPORTANTLY DO YOU KNOW WHY?

Also you stated that larger jets will create a richer mixture. Again this is a WRONG statement, and again do you WHY? On the other hand larger jets will_________________ fill in the blank buddy?

Still waitin to see those fuel pressure PSI values that you got via a FUEL PRESSURE GAUGE.

Since the different stock

Since the different stock 2100 model carburetor - with the stock jet size - seemed to resolve the problem, as in there was no noticeable stalling or hesitation. I'd say the problem is not with the jet size being incorrect at all. The problem appears to be with the carburetor itself - exactly what, I'm not sure.
At least I can largely rule out all other possibilities at this point. Instead of changing jet size, I'll target the carburetor and the rebuild steps first.


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