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Discussion Starter · #1 ·
I consider myself a fairly decent troubleshooter and can punch my way out of most paper bags, but I have gotten a bit frustrated by trying to dial in my idle in when I put the car into Drive on my 66 LTD, 352, 4bbl Autolite 4100, stock distributor, Pertronix 2 w/Per 2 recommended Flamethrower coil for Per 2. I also I running the 7mm carbon core Pertronix wires which I cut to the proper size and crimped and tested with an ohm meter to make sure they were done right. I last rebuilt the carb in June 2021.

So, with those details out of the way, the car was running OK with the points(the usual points issues of lower rpm unsteady but still driveable) until I upgraded to the Pertronix 2 last month. I thought I had the carb dialled in good air fuel mixture wise, the idle was set about 650 in Park, but when I put it in Drive, I did notice that the engine was a bit rough at the approx 500 rpm, to the point that I wouild have to make the idle while in Park higher so it would not die out when in Drive.

My first thought was "vacuum leak". So, I sprayed down the carb base, the PVC hose and anything else pulling vac from the engine. I did find the PVC hose attached at the base of the spacer behind and under the carb had a small split, and replaced that hose, and that made zero difference. I even took the effort to spray down the rubber part of the trans hose at the engine and even went under the car and replaced the small rubber section of the trans modulator valve, in case that was an issue. I sprayed down the trans modulator rubber hose with carb cleaner while the engine was running after replacing that and the PVC hose and there was no noticeable RPM difference. BTW, my trans shifts normally and has no issues, so I am not considering the trans modulator as the issue.

The car is pulling good vac at idle, 18-20 lbs steady, so that does not seem to be an issue.

Here is where the issue gets a little strange and out of my level knowledge. When I am adjusting the carb, with the air cleaner off, the idle speed is significantly lower than it is with the Air Cleaner cover on. This is something new to me and this car. I never remember having to adjust the carb to compensate for when the air cleaner cover was on or off. Again, the troubleshooter in me said, "ah, maybe the air cleaner filter is really clogged and that is causing this weirdness of the idle speed increasing when the air cleaner is on". No dice, I put the air cleaner cover back on without the air filter in and the idle speed still immed jumps up about 100-150 RPM. I really dont remember the car ever reacting to the air cleaner cover being on or off when adjusting the carb , but I cant pinpoint when exactly it got like that, other than maybe when I put the Pertronix in, but a Pertronix should have nothing to do with an RPM jump when I put an air cleaner cover on.

What essentially is happening is that I have to now sort of compensate for this disparity with the air cleaner cover off or on. Here is a chart to show what happens:

Air Cleaner Cover off, in Park, idle 625, vacuum 17 psi
Air Cleaner cover off, in Drive, idle - none- Stall
Air Cleaner cover on, in Park, idle 750, vacuum 20 psi
Air Cleaner cover on, in Drive, idle 475-500, vacuum 16, but idle feels like it may stall and has stalled on occasion, doesnt feel right as it has in the past.


So, a pretty significant difference in RPM with the air cleaner cover off and on. The engine seems to run ok at higher speeds, it is just that rough idle either when I am in the driveway after it is warmed up, or at a stop sign. I cannot imagine putting a Pertronix has caused this, or my spark plug wire building is that deficient, as the car does seem to run smoothly otherwise. I am fully baffled and am at the end of my current ability to understand what would case that condition. Worse case I pull the Pertronix again, put the points back in again and see where I am at, but I get the feeling that is the wrong direction. Maybe another carb rebuild? Doesn't seem to add up to me.

What can cause such an RPM change with an air cleaner cover on or off, other than a vac leak or some carb malady? I currently can use the car, but the idle is set to about 750-800 in park and goes down almost a full 250 RPM when I put it in drive, and even then I still worry about a stall from the feel of it, that is not normal to my knowledge.

I am open to any thoughts, and thanks for reading my tale of woe as I hope to get this all under control so I can take the car to the All Ford Nationals in Carlisle in a few weeks.
 

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Is the air cleaner hitting the throttle linkage or somehow the choke set up? What spacer do you have under it? Stock or aluminum? I had an issue for years with the low rough idle in gear on mine and it wasn't til a friend with OCD studied it that the issue was found. I sprayed stuff everywhere trying to find the leak and never did. That leak was as big as the grand canyon! The front of the 4100 underneath where the accelerator pump is there is a cover with 4 screws. The back 2 screws were sitting up on the ledge of the phenolic spacer and creating a huge gap. We filed off the corner of the spacer or the ledge so that the screw heads could clear it. Now i've changed carbs and trying to catch up with the guy to adjust it for me as he had that thing so well tuned and idling so smooth we could hear the little RV cam as the only hiccup. It was like that for like 10 yrs before it was found but made no dif in fuel consumption. lol
 

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If there is no leak at the base of the carb, then I'm with Extech. After the engine is warm, adjust the mixture screws and speed screw for highest vacuum at a comfortable RPM in drive. When you put it in park, if the idle is below 900-950, I would be happy. Do all this after you have the timing set where you want it of course. Changing timing will change the idle. And if possible, I would adjust the mixture screws with the air cleaner on. That it idles better with the cover off indicates to me that it's currently set a little rich so turning in the screws a bit might help, assuming no mechanical interference like Shotrod mentioned.

Pat
 

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Discussion Starter · #5 ·
Thanks for the replies so far. I know trying to diagnose on a web forum without the tangible smell and sounds is pretty impossible, but nice gentleman thus far are chiming in, and that gives me some ideas.

To each generous commenter so far a reply:

Extech- I agree, the idle in Park is not crucial to driving the car, but there is a certain point where the curb idle and the in drive idle is so out of whack that it is in not normal operational standards for the carb. Too high an idle in Park, then going into Drive or reverse is not optimum for engagement of the transmission, so that is what I am trying to fix. That I now have to dial in the carb with the car in Drive and then compensate for the increased RPM that putting the air cleaner cover creates when trying to dial in the carb is the issue.

ShotRod- I know the 4 screws you are talking about on the 4100. I have the stock spacer and those screws and spacing there is A OK. Also, I have not had this issue with this very same carb and spacer before- used to run pretty well, but something has changed. (I have the timing pretty well advanced- I use a vac gauge for highest vac when timing, instead of a timing light, that has worked well for me....usually). I also had the thought about the air cleaner hitting some linkage, but that is not at all going on. Just this weird increase in RPM when putting the air cleaner on that I never had to deal with before is screwing up my curb/park/drive idle settings. I basically now am setting the idle with the Air Cleaner off about 150 rpm lower to achieve the ballpark 800 RPM at idle when I put the cover back on- to compensate for the increased RPM when putting that cover back on. I have not had to do this in the past, mystery to me, other than installing a Pertronix what the heck I may have done.

1964Fastback- I am going to try to adjust with the car in Drive, which I never had to do previous to this problem, but I have to try something different since I am going in circles and will try that. And, the idle is actually higher and doesn't stall with the Air Cleaner cover ON, which adds to my bewilderment. To truly dial in the car properly with idle now, I pretty much would have to keep the air cleaner cover on and try to get to the idle adjust screw with the cover on is pretty tough. Never had to do that before now. And, yes, I agree, I have been making Air Fuel screw changes with the Air Cleaner cover on, those screws are very reachable with the cover on.

Keep the opinions coming- we all know there is some weird answer that will ultimately solve this one.
 

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Yes, I misunderstood and thought it idled faster with the cover OFF. So yes, lean and theoretically need to back the mixture screws out. I've always been able to adjust the idle with the air cleaner off as well. You are right; that's odd. In fact, that's one thing I usually do if I'm unsure if it's rich or lean - cup my hands over the air horn. If it's warmed up, ideally the engine should start to stall. If it speeds up, it's lean.

It started doing this after installing the Pertronix? Doing that affects the timing of course. Hopefully you checked/adjusted that already.

I know you did the spray test, but I would consider running the car in park and clamping off the rubber hoses one at a time going to the transmission, PCV valve, and power brakes (if equipped) and see if the idle changes. Pretty sure there should be no change for the brakes or transmission, maybe a small change on the PCV valve.

Pat
 

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Thanks for the replies so far. I know trying to diagnose on a web forum without the tangible smell and sounds is pretty impossible, but nice gentleman thus far are chiming in, and that gives me some ideas.

To each generous commenter so far a reply:

Extech- I agree, the idle in Park is not crucial to driving the car, but there is a certain point where the curb idle and the in drive idle is so out of whack that it is in not normal operational standards for the carb. Too high an idle in Park, then going into Drive or reverse is not optimum for engagement of the transmission, so that is what I am trying to fix. That I now have to dial in the carb with the car in Drive and then compensate for the increased RPM that putting the air cleaner cover creates when trying to dial in the carb is the issue.

ShotRod- I know the 4 screws you are talking about on the 4100. I have the stock spacer and those screws and spacing there is A OK. Also, I have not had this issue with this very same carb and spacer before- used to run pretty well, but something has changed. (I have the timing pretty well advanced- I use a vac gauge for highest vac when timing, instead of a timing light, that has worked well for me....usually). I also had the thought about the air cleaner hitting some linkage, but that is not at all going on. Just this weird increase in RPM when putting the air cleaner on that I never had to deal with before is screwing up my curb/park/drive idle settings. I basically now am setting the idle with the Air Cleaner off about 150 rpm lower to achieve the ballpark 800 RPM at idle when I put the cover back on- to compensate for the increased RPM when putting that cover back on. I have not had to do this in the past, mystery to me, other than installing a Pertronix what the heck I may have done.

1964Fastback- I am going to try to adjust with the car in Drive, which I never had to do previous to this problem, but I have to try something different since I am going in circles and will try that. And, the idle is actually higher and doesn't stall with the Air Cleaner cover ON, which adds to my bewilderment. To truly dial in the car properly with idle now, I pretty much would have to keep the air cleaner cover on and try to get to the idle adjust screw with the cover on is pretty tough. Never had to do that before now. And, yes, I agree, I have been making Air Fuel screw changes with the Air Cleaner cover on, those screws are very reachable with the cover on.

Keep the opinions coming- we all know there is some weird answer that will ultimately solve this one.
Hello jdstefan66,

I have some ideas for you to check. I use this exact same carburetor (from a '66 with a 352) on my '68 XL. It should be the 1.12 venturi. There's a couple of things that caught my attention whilst reading your original post, withcidentally you did a really go job of detailing the symptoms and modifications you made along the way.

My first suggestion is to rule out any contributing factors from ignition timing problems. I will 'assume' you are using ported vacuum? Are you making the mixture and RPM adjustments with the vacuum advance hose connected? Have you made sure there is absolutely no ported vacuum reaching the vacuum advance at your chosen idle speed setting by pulling off the vacuum advance hose and listening for an idle change?

One last thing to check concerning ignition wires is did you double check the firing order? It's really easy to switch two around.

Once you've ruled out ignition and it's not complicating the matter and you've double checked for other vacuum leaks as others have made great points to check then one can focus on mixture problems. The fact the air cleaner makes a noticeable difference in idle quality draws my attention to idle circuit problems. Even if running lean it shouldn't make a difference assuming the air filter isn't plugged up.

Have you checked the float levels? This is easy enough, after the car has cranked sufficiently or ran for a moment or two, turn it off and remove the air horn on the carb, the 4100 is so easy is this regard to check float levels. Also look for debris at the bottom of the bowls. If you see anything even the slightest I would suggest taking it off and apart and cleaning everything. There are air bleeds for the idle and transfer ports on primary venturi assembly. These are tiny holes on top and the sides. These have to be clear, otherwise you can end up with funky problems like your experiencing. Whilst your in there you can check the emulsifier tubes for any debris. Also if you remove the carb for cleaning you can cover all your bases and check the power valve and accelerator pump diapgrams for signs of cracking/hardening.

One last thing with the carburetor, I ended up having problems with the hot idle air compensator after a short time. It wouldn't stay sealed, so I just removed it and made a block off plate for it. It's job is to sense if too much hot air is entering the air intake and it will cause a vacuum leak to increase the idle RPM's to allow more cooling, well in theory anyway. My cooling system is up to par so I never have to worry about it overheating even sitting with the air con on at idle on a hot day so off it went as the bimetal was showing signs of fatigue and I didn't have a spare.

I do not know how familiar you are with the carburetor, but should you hone the problem ultimately to the carburetor I did go through mine and provided quite a few pictures for reference should you need them here 1968 XL - Keeping it Roadworthy Whilst Reparing it.

Just my thoughts. I wish you luck on your diagnosis!

Cheers
 

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… (I have the timing pretty well advanced- I use a vac gauge for highest vac when timing, instead of a timing light, that has worked well for me....usually).
Replacing components (like the Pertronix) can shift the timing curves. This can be a problem, as the timing curves are "locked" into the distributor, only easily adjustable for where they either start or end. Adjusting curb idle timing other than with a timing light and vac plugged will throw the rest of the curves off, causing weird operation. Use a light to see where warm idle timing is at now, and then try at spec advance setting.

Follow other suggestions for hoses, vac advance, etc, as-usual for base timing. At least you'll get a proven baseline for where to go from here, and to begin diagnostic checking of other issues knowing that part is "correct" by-the-book. You can do all kinds of unofficial stuff for improvements to ignition, but you have to start at a known point and move away from it to see all of the other effects.

TL;DR - yes, always a timing light, and always adjust mixture when fully warm with a vacuum gauge for the center or slightly rich side of lowest attainable vacuum (the range can be surprisingly wide near peak torque/ efficiency).
 

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I was also going to mention the hot-idle compensator, as it is designed to create a vacuum leak, and may be somehow displaced when the air cleaner is on. The other things that I can think of are that the choke linkage is getting bumped by the air cleaner base or gasket when you put it on, or is possibly the upward pull on the air cleaner stud when the wing-nut is tightened lifting the carb up off the base or even the air horn off the body? It would be worth it to pull the air horn off and check that it is true. The design of the casting is rather thin in the middle, especially where the vacuum passage that leads to the secondary diaphragm is. This is also where the hot-idle passages are, and might be where you are getting a vacuum leak. You could always give those a good tightening (within reason), and check the gasket. You can also run them over a piece of sandpaper on a slab of glass or stone that you know to be true. Sure sounds like a small vacuum leak to me with the way you're describing the symptoms.
 

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Discussion Starter · #11 ·
Again, all you guys are great taking the time to throw stuff out there, and trying to help a fellow hobbyist, I applaud you.

I did try some of the ideas mentioned earlier today. I did check the plug wire routing, totally routing to the correct cylinders, No wires are arcing or crossed. The dashpot is not adjust in any weird way to affect performance. I did take off the vac advance from the distributor. My advance line is steel from the vac cannister on the distributor and screws into the base of the 4100. I plugged the hole where the vac advance line was screwed into and then set timing and adjusted like that, and no changes were noticed that helped my situation.

All good ideas. However, I did notice something today that did strike me as odd, but not enough that I mentioned it in my write up last night. The Air Fuel screw on the drivers side, seemingly is doing nothing regardless of how many turns it is in our out, or for that matter even fully seated. . I had given both A/F screws the customary default settings- 1.5 turns out from fully seated. After the car warmed up to temp, and I was at idle, around 750 rpm, in park, I was able to get the proper RPM response from the passenger side A/F screw- it did start to starve the engine as I turned it in a half turn, and the RPM sped up as I backed off that 1/2 turn. In other words, the passenger side was working normally from the base setting of 1.5 turns out. The passenger side A/F screw seems to like being about 2.5 turns out, that is what my ear and vac gauge is telling me.

BUT- the drivers side A/F screw, well, that is a completely different story. I backed that out from being seated the 1.5 turns out. And then turned it in a half turn, with the engine warm, etc. as I had done with the passenger side A/F screw. No response was noted. Fully seated it and it made no difference. I then backed it out, like 5 turns, and that far out made no difference in RPM or engine idle.

Maybe I am stumbling onto my fix. Does each A/F screw on each side of the Autolite 4100 meter the fuel into each side of the bowl or jets? I am guessing that I am pretty much running the carb at a half of idle circut and then when I put that air cleaner on, it is really exacerbating the issue and probably causing all this havoc.

Simply, would the passenger side A/F screw passageways being plugged or clogged cause me all these issues with rough idle and even cause the RPM to go up with the air cleaner back on. (and, BTW, I have tried the air cleaner on without the Air Filter in place, to eliminate a clogged Air Filter and the issue still was present.)

Is it likely another through rebuild would flush my troubles away, or am I off on another tangent with the A/F passenger side adjustment doing nothing?
 

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Discussion Starter · #13 ·
Extech, I agree, that could save me the time on the rebuild just pulling that, but if there is some sort of blockage there, I would feel better just doing the full rebuild.

I am hoping that this gets me closer to the answer. One A/F mixture screw that is not functioning must have some detrimental effect to performance.
 

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While you have the carb off, check your mounting studs where they go into the intake. I'm not looking at an intake manifold right now, but if memory serves, at least one, maybe both of the driver-side mounting stud holes on the stock cast intake are open to the runner for the intake. If the threads are not sealed, or tightly held down, you could have a sneaky vacuum leak in that area which may be worsened, again, by the upward pull of tightening the air cleaner to the assembly.

I had a small-block that once gave the exact set of symptoms. One idle-mixture screw did nothing, the other did, but less than normal. It idled, poorly, low vacuum, worse in drive. After a ton of troubleshooting, I (with the help of the smartest engine pros I know) discovered that the intake had an internal casting flaw that was allowing communication between two banks of the manifold, so vacuum could not properly build. That car only had 13K original miles on it, and we speculated that the reason for that is that it probably ran like $hi* from the beginning, and remained parked for most of its life. A new intake was the fix. On that car, you could pull every other spark plug wire and make no difference on the idle. Very weird situation. Basically, I had a 4-cylinder at idle. Hopefully this is not quite as elusive as that one was. That engine got torn down and re-assembled three times before getting to the bottom of it. Still couldn't see or detect anything externally, so it was a very tough thing to find, but proved that not all vacuum leaks are external.
 

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quick dirty fix is to take screw out and blast carb cleaner thru the hole w the straw. (not while its running of course) if that fixes it, (temp fix), then you have it solved. reason say temp is more than likely whatever clogged it will be floating around in the fuel bowl and needs to be removed. .02
 

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Discussion Starter · #16 ·
I agree with you m in sc- I could try yours or Extech fix, but either way I will end up worried that whatever gunk I may flush out is already in the system and ready to wreck some more havoc. I poured some Berrymans B12 down the carb yesterday, to see if a jolt of that would help, after that I actually had a surging noise from the carb that was not there before, but would go away at the higher idle speed. I surmise the Berrymans probably flushed the gunk to a worse area and created more issues.

Hopefully a rebuild will clear up the confusion. And, to address Jazzmeister comments about the the higher RPM being a vac issue when I tighten the air cleaner, just me putting the air cleaner on, with or without the filter without the wing nut causes that RPM to go up. Hopefully that non functional A/F screw and a good cleaning will solve this mystery, to be determined.
 

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If you can see the idle air bleed holes in top you can blast them with the carb cleaner tube. If you can't see them...
Make sure your choke spring housing is not loose. And the air cleaner snorkel does not touch the hood.
 

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Discussion Starter · #20 ·
I always try to come back after I ask for help and let those kind enough to respond to my original question, and those others who are wondering now or in the future, "how did this work out?"

Here is how it worked out.

I painstakingly rebuilt the Autolite 4100. I mean, not my average 6 hour rebuild that I have done a couple times on this, no, this time I probably spent close to 20 hours between removing from the car, to the process of disassembly, cleaning, and reassembly. I was wanted perfection and even if it is unobtainable, I wanted it as close as I could, within my abilities.

I had rebuilt this same carb last year, but it was a too quick rebuild and it never really ran right after wards. My car developed some stalling issues the night before I was to take it to Carlisle last year for the All Ford Nationals in June, so I did a quickie rebuild the night before I left, and I can see I made at least one mistake with a gasket and probably didn't clean up the passages as well as I could have, and that eventually led to my issues.

So, once I tore the carb down and gave it a good cleaning and a passage by passage laborious check, I finally got it reinstalled on the car and what do you know, the car idles better than it has in a couple of years! Even though I had referenced Mikes Carb YouTube videos on the 4100 rebuild in the past, this time I found the nice gentleman with a 66 Galaxie convertible, which he just sold, on YouTube on the channel called Old Car Alley. I watched his 4100 tear down and rebuild videos and that really pushed my project over the top. If you have the time, look him up on YouTube. His Galaxie videos are really inspirational and he is a true craftsman.

These were my two biggest findings that I could visually tell that were not right, you cant see the dirt in the tiny holes, so I am sure I got some of that out of them also:

1.My PCV hose was leaking at the clamp that clamps it onto the carb to manifold spacer. I bought a new hose to fix that.

2.The gasket I used on the last rebuild between the carb and the carb spacer was the wrong one. I used the gasket that I should have used between the spacer and manifold instead, and I am sure that caused some restricted flow. See the pic for the gasket I used vs the gasket I should have used and you can see the difference. (the clean un used upper gasket is the one I should have used)

I also put my original Ford aluminum gasket back under the spacer to manifold, as it was in fine shape and I should have never removed it a few years ago.

My idle, air cleaner on or off now is consistent. The air fuel screws I actually dialed in very well at the bench and after road testing determined that they were exactly where they needed to be, 2 turns out.

Now, of course, once I fixed this and was in mini celebration mode, I noticed that I had smoke coming from the exhaust pipes under the steering gear box. Never had that before. And, wouldn't you know it, my power steering pressure hose, original to the car I believe, had sprung a leak. After replacing that and the return hose, and letting all the smoke burn off finally, I am happy to report that I have a good running car again.

For all those that offered help, I again thank you. Rebuilding the carb, very very carefully was the solution in this case.
 

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