1950s Rochester rough idle and idle by-pass screw

[Cadman-iac]

Here is a shot of my carburetor with the 2 fuel sight plugs clearly visible. So they are there for float examination only and not to facilitate any float adjustment? Clay/Lexi

 Unfortunately adjusting the float through the sight plugs would be like doing arthroscopic surgery. The pivot point/ adjustment point is in the front center of the carburetor directly below the fuel inlet. The only way to access it is to remove the top of the carburetor.
 The only thing that the sight plugs do for you is let you know exactly where the fuel level is, which is helpful. If you get it set and check it through the plug, you'll know right away if you have to do it again or put the plug back in and drive away.

 Rick

[PHIL WHYTE CLC 14192]

Clay, before you set the float level remove the floats and put them in a bowl of water and see if they float. If they don't float they have a leak ie.full of fuel. Don't ask me how I know this, also don't ask me how I know that if you try to resolder a fuel float it makes a very good flame thrower.🤣

[Cadman-iac]

  One problem with the floats on the 4GC is that they are a dual float per bowl. You can have one half of one that is slightly heavier than the other side and it compromises the setting.
When you're checking them in water, look closely at it and make sure that both sides ride at the same level. Let it sit in the water and let the water calm, just quickly looking at it won't tell you anything other than it floats.
Also check to see that the arms going to each half are equal, that one isn't higher or lower than the other, that will mess up your setting as well.

Rick

don't ask me how I know that if you try to resolder a fuel float it makes a very good flame thrower.🤣

Tried that one myself. It didn't explode, but it got a lot heavier with the extra solder making it pretty much useless.

[Lexi]

Clay, before you set the float level remove the floats and put them in a bowl of water and see if they float. If they don't float they have a leak ie.full of fuel. Don't ask me how I know this, also don't ask me how I know that if you try to resolder a fuel float it makes a very good flame thrower.🤣

I have heard that repairing them is very "ify" at best. I have lots of parts here, so would probably dig out a replacement set. Is it feasbile to drain any fuel that may be in there then patch any leak with JB Weld? Not sure I like the idea of that product sitting permanently in a fuel filled enviroment in my carburetor. Clay/Lexi

[Lexi]

Unfortunately adjusting the float through the sight plugs would be like doing arthroscopic surgery. The pivot point/ adjustment point is in the front center of the carburetor directly below the fuel inlet. The only way to access it is to remove the top of the carburetor.
 The only thing that the sight plugs do for you is let you know exactly where the fuel level is, which is helpful. If you get it set and check it through the plug, you'll know right away if you have to do it again or put the plug back in and drive away.

 Rick

Well, thoroughly ticked off now. A rebuilt carb that is not performing safely. So I dug out a spare that I have had in storage for 4 years and installed it this afternoon. Works very well and car seems to be back where it was before all this nonsense began in early July. Ruined my driving season though. I was thinking about your comments Rick about basically controlled flooding (too high fuel level in bowl), and gasoline weeping through the gasket. Makes sense that it should not do that prior to driving the car on the road. I tried rapping the nozzle, hoping to perhaps dislodge any foregin matter that might be there. No luck. Car so far is now running great with my replacement carb from today's install. One thing I noticed is that although these are both 1956 Cadillac 4GCs, the one I just removed ran like a scared rabbit and I could not slow it down to idle properly. The one I installed this afternoon ran way to slow and would stall after fast idle off. Long story short, I got it running now but I have a theory that all of these carbs probably have more or less, some level of vacuum leak that may not have been there when new. Whether or not such are repairable or whether a service tech would even want to tackle some of these issuess, is debatable. My dislike for the idle by-pass screw system has also been re-enforced. As usual I had to do everything twice to get the car running, but Lexi can now hit the road and chow down on some Reynolds wrap imports prior to hibernation, LOL. Thank you for your sound advice (and humour) Cad-maniac. Clay/Lexi

[Jon S]

Here is a shot of my carburetor with the 2 fuel sight plugs clearly visible. So they are there for float examination only and not to facilitate any float adjustment? Clay/Lexi

Correct. Been following this thread

[Cadman-iac]

I have heard that repairing them is very "ify" at best. I have lots of parts here, so would probably dig out a replacement set. Is it feasbile to drain any fuel that may be in there then patch any leak with JB Weld? Not sure I like the idea of that product sitting permanently in a fuel filled enviroment in my carburetor. Clay/Lexi

The issue with using JB Weld is that you're adding weight to your float, and I'm not sure if it would stay in place being submerged in fuel constantly.
Getting any fuel out of a float is difficult too. You either have to open up a hole in it so it'll drain, or maybe heat it and hope that the fuel can escape as a vapor without blowing it up in the process.
Then you'd still have to locate the leak, which isn't the same as looking for something escaping, but rather something entering.
You could try covering the entire seam between the two halves of the float, but like I said above, you're adding weight.

Rick

[Cadman-iac]

Well, thoroughly ticked off now. A rebuilt carb that is not performing safely. So I dug out a spare that I have had in storage for 4 years and installed it this afternoon. Works very well and car seems to be back where it was before all this nonsense began in early July. Ruined my driving season though. I was thinking about your comments Rick about basically controlled flooding (too high fuel level in bowl), and gasoline weeping through the gasket. Makes sense that it should not do that prior to driving the car on the road. I tried rapping the nozzle, hoping to perhaps dislodge any foregin matter that might be there. No luck. Car so far is now running great with my replacement carb from today's install. One thing I noticed is that although these are both 1956 Cadillac 4GCs, the one I just removed ran like a scared rabbit and I could not slow it down to idle properly. The one I installed this afternoon ran way to slow and would stall after fast idle off. Long story short, I got it running now but I have a theory that all of these carbs probably have more or less, some level of vacuum leak that may not have been there when new. Whether or not such are repairable or whether a service tech would even want to tackle some of these issuess, is debatable. My dislike for the idle by-pass screw system has also been re-enforced. As usual I had to do everything twice to get the car running, but Lexi can now hit the road and chow down on some Reynolds wrap imports prior to hibernation, LOL. Thank you for your sound advice (and humour) Cad-maniac. Clay/Lexi

I would suspect the float setting is too high. If you had something lodged in the needle/seat you would have uncontrolled flooding until whatever was dislodged, then you would expect to not have any more issues.

As for how each carburetor performed, the first one was dealing with a high fuel level, allowing it to easily draw fuel from the bowl, and may have contributed to your high idle issue.

The second carburetor I'm assuming has the float level set correctly, sounds like you have all vacuum leaks fixed, and it just needs to be adjusted. Without being there to witness it running I'm just speculating.

But get out and let Lexi stretch her legs, feed her a turdbox or two before she goes into hibernation. Enjoy the ride and have fun.
No charge for the humour!

Rick

[Lexi]

The Gods are not smiling upon me. Went to take the car out for a short test drive and was rough idling & tough to start. Smelled gas. Shut down immediately. Under the hood notice lots of uncontrolled flooding. Back to the drawing board. Yet with this carb which I only installed 3 days ago, (which ran fine when parked), it now also needs servicing. Looks like Rick's comment about blown needle/seat valve has now infected this carburetor.   Clay/Lexi

[Cadman-iac]

The Gods are not smiling upon me. Went to take the car out for a short test drive and was rough idling & tough to start. Smelled gas. Shut down immediately. Under the hood notice lots of uncontrolled flooding. Back to the drawing board. Yet with this carb which I only installed 3 days ago, (which ran fine when parked), it now also needs servicing. Looks like Rick's comment about blown needle/seat valve has now infected this carburetor.   Clay/Lexi

I'm really sorry to hear that Clay, it sounds like you're taking a page out of my play-book, or you're just having a Hee Haw kind of week/month or whatever. You know, "If it weren't for bad luck I'd have no luck at all, etc, etc, etc...."

Question, are you looking into these carburetors and making adjustments yourself, or just relying on your rebuilders expertise to  get it right?
If it's the latter, maybe you need to consider taking up a new hobby, carburetor 101.
My issues this last couple of months have been with the quality of the parts I'm getting. F'ing Chinese crap not holding up like it's supposed to, but it's the only thing out there anymore.
Just today I put the second upper plenum assembly on my wife's car in 2 days. The one I installed yesterday began to split around the bolts after being torqued to the required 89 INCH POUNDS! Piece of crap plastic made in China.
This is the 5th one I've done over the years, so I know about the torque specs, and the last one went 7 years before giving up.
So today I installed large flat washers under the heads of the bolts to spread the load and hopefully the bolts won't just pull through the plastic like the one yesterday did.
Plastic has its uses, but engine parts ain't one of them!!

How many of these 4GC carbs do you have Clay?  You may want to try and take the best parts from all of them and assemble yourself 1 good carburetor.

I was thinking about those air bleed screws,  they are accessible from outside the carburetor, but they control an air passage inside the carburetor.
It seems that the issue with these is that the threads on the screws themselves are not airtight.
There must be a way to correct this, either by use of an O-ring on the screw if it has an area that would allow for that, or to use something like a thread sealant that doesn't solidify and lock up the screw.

I'll have to look again at my collection of carburetors and see if any of them use the air bleed setup. If I've got one, I'll see what if anything can be done to seal the screw from the external air.

But that's just one of the issues that you seem to be having.
The flooding issue would require a close examination of your floats, and the needles and seats.


It's funny, I'm fighting a similar issue, but with my Suburban and the throttle body injection. It seems that the computer and the camshaft are not getting along very well. As a friend of mine told me about the early computers they only have to monitor the air/fuel ratio and adjust the timing.
It's got two jobs and it failed one of those!

One day I'd like to wake up and everything goes right. Maybe next year that'll happen.

Rick

[Lexi]

Been relying on the rebuilder. This latest carb was also rebuilt by him 4 years ago. It ran the car fine when removed, then I sent it out for servicing as I had another rebuilt unit to install on a newly rebuilt engine. Wanted everything new or rebuilt, a fresh start. So when I got that carb back I packed it up, where it has sat in a box for 4 years. Installed it this past Monday and it ran the car great. Best it has run in some time. This morning I wanted to ensure that the fast idle was not going to run the engine like crazy and perhaps spin my cam shaft, so I pushed the car out of storage to put eyes on the fast idle cam and also to be able to watch for any fuel anomolies (which cannot be done where it is normally parked). Cranked it a bit (without touching the gas pedal), to get some oil up into the upper block. Then put on the electric fuel pump to fill the carb bowl for an easy start. Right off I noticed an issue when listening to the E pump which after filling the carb bowl, it's sound usually modifies. Not this time. WTF I thought. Just sounded like it was still filling the carb. So I knew then that it was flooding the carburetor and engine. And it was. Gas all over the engine bay. So between parking it 3 days ago and this morning, something died inside it. Ruined the pavement. Can't win but for losing. But at least I got back under cover just prior to the light rain that later fell. Yes, it has been a horrible year.

As I have to be able to move the car, I put the other carb back on it which although it weeps a bit a fuel around the upper bowl gasket, it is not (yet) flooding it. So I got the car started with that one and parked her.

As for carbs, I have 3 useable ones, (all now requiring some work), plus 2 cores, as well as parts to make 2 or 3 more, all Rochesters. I also have a rebuilt Carter which has been stored for sometime, but might run as is. It came off of a Cobble Beach show car. I as well thought of putting an O ring on that idle by-pass screw to see if that will modify the air leak. Fed up right now. Seems I got to do everything 3 times or more. Was thinking the same thing; put one together myself with my best spare parts. This is a huge PIA. Hope my bad Karma did not pass on to you Cadmaniac! Clay/Lexi

[Cadman-iac]

  Clay,

The idle by-pass screw is 50's technology that has been reinvented in the late 80's on fuel injected engines. It's called an idle air control valve, or IAC,  but unlike the 50's version, this one is constantly adjusting to the engine load by the computer.
Unfortunately you can't adapt the electronic version to your 50's carburetor.

I looked at my carbs, the one with the sight plugs has an idle air bleed screw on the back driver's side. Where is the screw located on your carburetor? I keep thinking yours is between the primary and secondary throttle shafts on the driver's side, but I'm not sure.
 
Edit: I looked back through this thread  and verified the location of the screw on yours, it is in a different spot.


Another question, when you had the carb rebuilt, did he re-bush the shafts, or were they ok?

I took the screw out of my carb and it measured 3 inches long. There's enough threads and they're fine enough that I don't believe that if any air were to get past the threads it couldn't be enough to really cause an issue.
As was stated earlier by someone else, the spring does two things, it keeps the screw from turning on its own and it puts pressure on the threads that would help in sealing.
I didn't get a chance to look closely at mine, so I'm not sure if there is anything that can be done to ensure a better seal. Your idea of using an O-ring under a washer and the spring pushing on both may be the best/only thing you can do.
On mine, there is no recess around the screw where it enters the base plate, which would have been an ideal place for an O-ring.





Another thing I noticed about mine is it has slightly larger rear bores than the front. Like an early spread-bore design. Is yours like that?

  Rick

Edit:
Just an update on my Suburban that has a bearing on this issue in a way.
Yesterday it was running really well and all of a sudden it was missing and stumbling on acceleration and the vacuum was really low according to my gage.
After checking the plugs again and the timing, I was just getting ready to adjust the distributor when I noticed that one of the vacuum lines had pulled apart. I put it back together and checked the rest of the vacuum lines and found the suction line to the vapor canister had also come off.
So after reconnecting both lines it ran just fine. Both lines are very small, but apparently they are still enough to mess up the computer.
You may want to trace all of your vacuum lines to check for leaks, disconnects, bad accessories or whatever.
I think someone else suggested unplugging all the things plugged into your carburetor and try running the engine again and see if it makes any difference. If so, then one thing at a time, begin to reconnect them and see what happens. When it starts to run badly again you have found the source of your problem.
If it still runs like crap after disconnecting everything, then you'll know the issue is internal.
I got lucky that I found those lines disconnected. I looked yesterday but I kept looking for something more serious, like a bad plug or wire, map sensor, IAC malfunction, ESC issues etc....not something as simple as a loose hose.
What's that saying or principle, keep it simple stupid, K.I.S.S.!!

[Cadman-iac]

  Hey Clay,

I was just looking at my carburetor again thinking about how best to stop a possible air leak around that bleed screw. Just placing an O-ring on the threads with a washer to push it against the carb base will eventually crush the O-ring.
However, if you were to use a drill bit to put a small taper in the entrance for the screw for the O-ring to sit in, just a shallow depression, nothing to deep, then put the washer over it so it keeps the O-ring from coming out if you back the screw out, this would at least eliminate one possible vacuum leak for you. You could use a thicker O-ring so it would last longer as well. You will have to experiment with how deep a taper to make in the base, start small and increase if necessary.

I checked, the threads on mine at least, are a 3/8-18, a standard size, so if necessary to clean up the threads in the base after putting a taper in it, you can easily run a tap through it.

Just food for thought.

Rick

Here's a picture of a regular fine thread nut on the screw I used just to verify the thread size.

[Cadman-iac]

  Here's what I've come up with for the bleeder screw.


I would recommend using a fatter O-ring like one from a fuel injector as it's going to be stronger and resist being crushed by the spring and washer, plus it's made to handle the fuel, unlike most regular O-rings.
Here's a regular O-ring beside an injector O-ring.


So you can see the sizes.


As you can see it's much more substantial and less likely to break and leak. And if you put a taper in the hole for the screw it'll help it seal even better. This was the base before I added a taper to the hole.


And this is after I tapered the hole.


With the taper there and the spring pushing it it'll form to the taper and that will also keep it from breaking and falling off. Just sandwiched between two pieces of metal, (the base and washer),it won't last long.

And here's what it looks like installed.


After being exposed to the heat of the engine the O-ring will eventually be pushed into the taper and form an even better seal.

I looked again at all of the 4GC carbs I have and this one is the only one that has the bleeder screw in it, all of the others are normal.

Hope this helps some Clay. Let us know how it goes.

Rick

[Lexi]

Rick my thoughts as well, except I would not have thought about that fuel injector O ring. Good tip. Thanks! Earlier in this thread, (post #21), I uploaded 2 images of my idle by-pass screws. One looks near pristine, while the other has some obvious wear which suggests that it may be very slightly out of round. Looks similar to the one you posted an image of, with the same sort of wear marks. That could also add to the vacuum leak problems especially if the screw threads are not sealing. Do you think when these screws were new, even then they would have sucked in some brake clean during a test? Or would there have been virtually no perceptible leak rate? Clay/Lexi

[Cadman-iac]

Clay,

I doubt that any of those screws would become out of round unless they were put in a vise and beat with a hammer. They're just too thick, and the tip isn't tapered like a mixture screw would be, They're blunt ended, as they only meter air flow, and much more air flows through this than the amount of fuel that flows past the mixture screws.
As for the threads leaking air, it's possible, but I also believe that's why Rochester designed them with such a long threaded portion, to cut down the chances of too much air getting through from the threaded end.
Even a good throttle shaft and base will still have a minute amount of air seepage past the shafts, but in most cases it's negligible.
If you just put an O-ring on the screw with a washer over it as a test, (before you go to the hassle of tapering the screw entrance), then you can check for leaks around the throttle shafts without the bleeder being a possible positive indication because of its location on your carburetor, centered right between the two shafts. It would be really hard to tell exactly which one is leaking with them that close together as you spray, if that makes sense, at least on the driver's side, it won't make a difference on the passenger side.

Rick

[Cadman-iac]

  Something I'm not sure about on your carburetor is how the air bleed air is distributed evenly with the outlet port being on the left side of the carburetor. Does your gasket have a passage cut out that directs this air into the center of the manifold/carburetor?

 On mine, even though it's located in the back of the carb, the outlet port is dead center of the carburetor just behind the secondary throttle shaft.


 Even on mine, being in the middle of the secondary throttle shaft, it's still not in the middle of the carburetor, (meaning dead center of the carburetor), so the distribution of air I would think would be more toward the rear half of the engine, which would cause a lean condition there, and a rich(er) condition in the front.
 I've studied carburetors and throttle bodies and most will have a passage that dumps evenly to both sides on 2bbls, and towards the front or primary side on a 4bbl for anything that affects the air/fuel ratio, such as where the PCV connects to it. Too much air in one side leans that side out.
 Unfortunately I don't have the base gasket that goes with this particular carburetor, so I honestly don't know if there's a channel to distribute this air evenly across the manifold so as not to cause any part to have a lean condition.
 What does your base gasket look like for that carburetor?
 I'm wondering if maybe you might have the wrong gasket that's causing you problems.

 Just more food for thought.

 Rick

[Cadman-iac]

Here's more on this. I took the base off of my carburetor to see where it gets the bleed air from and to see if maybe I missed an internal passage.
No hidden internal passage below the screw, it comes out right between the left and right secondary throttle plates.
But it does pull the air equally from both just above the plates in a channel cut between them.

So it is drawing filtered air here, HOWEVER, just above this point, in the gasket between the base and the body, it has those cutouts that are supposed to vent any fumes presumably to prevent hard starting, or a rich condition, and this just goes directly to the engine bay and comes out below the bowl in the rear.
(As you can see the gasket is in sad shape after sitting for who knows how long).


There are similar cutouts in the primary side of the gasket too.


 Here you can see the evidence of the cutouts from the darker color of the base.
 These are hidden between the bowl and base, nothing but dust could get through here.



There is a recess in the bottom of the bowl that may help keep dirt from getting sucked into the gaps in the rear as the engine is running.



Rick

[Cadman-iac]

  It's been a long, long while since I messed with a 4GC carburetor, like 40 years now. I've forgotten a lot about them.
I was looking at a bowl assembly I had lying loose and I noticed that it has what I'd call an "equalizer passage" on the right side. It goes from bowl to bowl to keep the fuel levels the same between them, I'm guessing for times when there's a lot of bouncing or tilting up or down of the vehicle, and maybe even if one of the needles gets stuck.


It's not large enough to compensate for a stuck closed needle, but it might help keep a little fuel flowing from one to the other. The holes the fuel passes through are small and there's a restriction in the passage from one of the screws that hold the cover in place. They did drill through this and capped/plugged the access point as you can see in this picture.

There is a pressurized fuel passage that runs around the top of the cover that supplies both front and rear bowls.


Here's the float, both front and rear use the same type, so you can swap them without any problems.


I'm thinking about filling up one of the bowls on this just to see how much it holds.

Look at how big they are. Don't want a backfire if this leaks!


Clay,
What this has me thinking about what would happen if one of the floats was set higher than the other, and your carburetor has this equalizer passage in it?
You need to check both floats for the correct setting because of this equalizer passage.
 I just looked at several of these carbs and they all have the passage.

Yes, a very over complicated carburetor for sure.

Rick

[Lexi]

Here's more on this. I took the base off of my carburetor to see where it gets the bleed air from and to see if maybe I missed an internal passage.
No hidden internal passage below the screw, it comes out right between the left and right secondary throttle plates.
But it does pull the air equally from both just above the plates in a channel cut between them.

So it is drawing filtered air here, HOWEVER, just above this point, in the gasket between the base and the body, it has those cutouts that are supposed to vent any fumes presumably to prevent hard starting, or a rich condition, and this just goes directly to the engine bay and comes out below the bowl in the rear.
(As you can see the gasket is in sad shape after sitting for who knows how long).


There are similar cutouts in the primary side of the gasket too.


 Here you can see the evidence of the cutouts from the darker color of the base.
 These are hidden between the bowl and base, nothing but dust could get through here.



There is a recess in the bottom of the bowl that may help keep dirt from getting sucked into the gaps in the rear as the engine is running.



Rick

\

Very interesting observations Rick, on a topic that as far as I know, has not been explored to this depth here. Guess we are breaking new ground. I won't fiddle with one of the carbs as the rebuilder who worked on it recently should make it right. In my spare time I am contemplating cleaning and rebuilding one of my spares on my own. In answer to one of your (?) earlier questions, putting a caliper into the disassembled carb, revealed that the bores for both primary and secondaries were the same measurement. The idle by-pass screw design seems intended to fit snuggly in it's passage much the same as the throttle shafts fit into theirs. One carb is back on the car, limping around. Other is packaged for return to the builder. One in pieces. Clay/Lexi