1950s Rochester rough idle and idle by-pass screw

[Lexi]

Chasing down a poor idle problem. Tests indicate a vacuum leak at a couple of spots on the carb. One of them is at the idle by-pass screw. They provide air at idle that is "variable" as that screw is adjustable. I believe it adjusts air that passes through a passage inside the carburetor. I am not sure if that passage air is obtained externally, past the screw, OR fed internally from inside the carb.

Does anyone know of a "fix" for these idle by-pass screws once they are leaking air, and a source of a vacuum leak? If the air feed is NOT external, so unwanted air flows past the idle by-pass screw, would a rubber O-Ring work? If the feed is internal what can be done? I imagine a clogged passage might force air in past the screw as a path of least resistance, thus grabbing air externally? Any thoughts? Thanks. Clay/Lexi

[J. Gomez]

Chasing down a poor idle problem. Tests indicate a vacuum leak at a couple of spots on the carb. One of them is at the idle by-pass screw. They provide air at idle that is "variable" as that screw is adjustable. I believe it adjusts air that passes through a passage inside the carburetor. I am not sure if that passage air is obtained externally, past the screw, OR fed internally from inside the carb.

Does anyone know of a "fix" for these idle by-pass screws once they are leaking air, and a source of a vacuum leak? If the air feed is NOT external, so unwanted air flows past the idle by-pass screw, would a rubber O-Ring work? If the feed is internal what can be done? I imagine a clogged passage might force air in past the screw as a path of least resistance, thus grabbing air externally? Any thoughts? Thanks. Clay/Lexi

@Clay,

Not sure if you have the full Rochester 1956 Cadillac Service Training manual, here is the section discussing the idle-bypass section, this may be of help on your topic.

Let me know if you want the full copy (large file) so I'll be glad to send it over via email.

[Lexi]

@Clay,

Not sure if you have the full Rochester 1956 Cadillac Service Training manual, here is the section discussing the idle-bypass section, this may be of help on your topic.

Let me know if you want the full copy (large file) so I'll be glad to send it over via email.

Jose, much thanks! I do have that Rochester training booklet. It helps a bit, but fails us at critical points as it shows adjustment set up when all was new. Does not delve into trouble shooting or how to fix 70 year old worn parts. Update on our phone call; I only got to look at the car a day or so ago and carb vacuum leaks appear to be the problem. Just not sure what rebuilders do with one of those idle by-pass screws if it is at fault. As we discussed they are notorious for not working properly or at all. As Fishnjim once said a way back in this Forum, that was a "terrible" system. Well its come back to haunt me and not sure what to do. Digging through my parts and hope that I can cobble something together. Might try an O-Ring and/or if there is a spring there, look for a stronger one or increase it's tension with a couple of washers. Clay/Lexi

[tcom2027]

Without knowing what tests you made, what follows may be repetitive. Ag=after closing both of the idle jets and then opening them to the base setting of 1 1/2 turns +/-  does the engine run and if so how. If you turn the crews all the way in does the engine stall. Does opening them past the base affect the idle rpm or the smoothness of the engine?

Did you spray starting fluid, Brakleen, hair spray etc. around the base of the carb and especially around the throttle shaft to see if the idle smoothes or the rpm increases. Best to use the  aerosol cans with a snorkel tube on and the air cleaner installed. That will help avoid a false positive.

My thought is the leakage may be around the throttle shaft followed by the base gasket or a slightly deformed carb base, the latter easily cured with a flat plate or glass and emery cloth. Obviously if the gasket is cracked , deformed, etc. replace it.

I don't remember if the 56s have an insulation block under the carb, but if so. remove the carb and check the block for cracks.


tony

[Lexi]

Without knowing what tests you made, what follows may be repetitive. Ag=after closing both of the idle jets and then opening them to the base setting of 1 1/2 turns +/-  does the engine run and if so how. If you turn the crews all the way in does the engine stall. Does opening them past the base affect the idle rpm or the smoothness of the engine?

Did you spray starting fluid, Brakleen, hair spray etc. around the base of the carb and especially around the throttle shaft to see if the idle smoothes or the rpm increases. Best to use the  aerosol cans with a snorkel tube on and the air cleaner installed. That will help avoid a false positive.

My thought is the leakage may be around the throttle shaft followed by the base gasket or a slightly deformed carb base, the latter easily cured with a flat plate or glass and emery cloth. Obviously if the gasket is cracked , deformed, etc. replace it.

I don't remember if the 56s have an insulation block under the carb, but if so. remove the carb and check the block for cracks.


tony

Hey Tony, thanks for chiming in. My test time was very limited, but I did spray brake clean around the base of the carburetor and at a few other areas of interest. The air cleaner was in place during testing. When brake clean hit the idle by-pass screw, engine RPMs jumped quite noticeably. Engine RPMs jumped even more when in contact with the secondary throttle shaft, (but not reactive with the primary throttle shaft). Covering the carburetor with a gloved hand also noticeably increased the engine RPMs, rather than stalling the engine. The '56 has no insulator base gasket, just a regular gasket. When sprayed, that area seemed fine. My testing will continue. Wish I knew more about that idle by-pass screw. It would make sense for perhaps some vacuum leakage, but not what I experienced, in my opinion. On a side note, I got a nasty shock from #1 spark plug wire, so must be a break down in the insulation there. So I installed another wire, but that had no noticeable impact on the idle. Clay/Lexi

[J. Gomez]

Jose, much thanks! I do have that Rochester training booklet. It helps a bit, but fails us at critical points as it shows adjustment set up when all was new. Does not delve into trouble shooting or how to fix 70 year old worn parts. Update on our phone call; I only got to look at the car a day or so ago and carb vacuum leaks appear to be the problem. Just not sure what rebuilders do with one of those idle by-pass screws if it is at fault. As we discussed they are notorious for not working properly or at all. As Fishnjim once said a way back in this Forum, that was a "terrible" system. Well its come back to haunt me and not sure what to do. Digging through my parts and hope that I can cobble something together. Might try an O-Ring and/or if there is a spring there, look for a stronger one or increase it's tension with a couple of washers. Clay/Lexi

Clay,

Agree that document is more of an operation theory than a troubleshooting guide, and even the Service Manual does not provides enough details for troubleshooting.

There should be a very limited vacuum leak at the adjustment screw as the spring would provide tension against the threads to seal depending on how far the screw is out. If the screw has to much play it may not provide the proper sealing.

Agree, you can try adding an "O" ring and a flat washer at the end of the screw and sandwich them against the spring and see if that helps.

[tcom2027]

Good morning,

Strange that the threads are worn or the seats in the carb are damaged to the point you have a significant leak. I assume you took a close look at the tips of the needles. They or the seats may be damaged from over tightening.

How does the car run with the needles turned all the way in to the stops?

The throttle shaft and the idle screws are going to leak a little even when new, more as they wear but that is compensated for in the design of the carb.

This may be a bit afield but have you checked all the vacuum hoses, particularly the ends where they attach to a vacuum operated  component? Nip off the ends to get a tight fit. The brake booster can have sneaky leaks. Same for a vacuum hose that got pinched or laid against the exhaust manifold a long time ago and finally died.

THere are guys here that know more about the Rochesters than I do. Remember it ran well .....until it didn't, so the problem can be corrected. To loosely (very) quote Sherlock Holmes, "When all the other possibilities, however impossible or improbable have been eliminated only the truth remains."

tony

[Lexi]

Clay,

Agree that document is more of an operation theory than a troubleshooting guide, and even the Service Manual does not provides enough details for troubleshooting.

There should be a very limited vacuum leak at the adjustment screw as the spring would provide tension against the threads to seal depending on how far the screw is out. If the screw has to much play it may not provide the proper sealing.

Agree, you can try adding an "O" ring and a flat washer at the end of the screw and sandwich them against the spring and see if that helps.

I agree. A process I will have to investigate Jose. I assume that the air which the by-pass screw is regulating is internally fed, and is first filtered through the air cleaner. Why pull dirty air into the engine and by-pass the air cleaner? I agree that there would probably be some leakage there, but in my case it seems extreme. Clay/Lexi

[Lexi]

Good morning,

Strange that the threads are worn or the seats in the carb are damaged to the point you have a significant leak. I assume you took a close look at the tips of the needles. They or the seats may be damaged from over tightening.

How does the car run with the needles turned all the way in to the stops?

The throttle shaft and the idle screws are going to leak a little even when new, more as they wear but that is compensated for in the design of the carb.

This may be a bit afield but have you checked all the vacuum hoses, particularly the ends where they attach to a vacuum operated  component? Nip off the ends to get a tight fit. The brake booster can have sneaky leaks. Same for a vacuum hose that got pinched or laid against the exhaust manifold a long time ago and finally died.

THere are guys here that know more about the Rochesters than I do. Remember it ran well .....until it didn't, so the problem can be corrected. To loosely (very) quote Sherlock Holmes, "When all the other possibilities, however impossible or improbable have been eliminated only the truth remains."

tony

I have not had the time to conduct all the tests that I would like to. In some instances I am not sure that my physical examination would even be revealing. I am not a "carburetor" guy, and am not qualified to evaluate the condition of some parts. The idle by-pass screw is also a long abandoned technology that even niche specialty rebuilders might be unfamiliar or uncomfortable with. Most present day mechanics at least up here, have never even worked on a carbureted car. Quick tests seemed to reveal the other vacuum connections were good. Good point, the car ran well the day before so I am hoping it can be corrected in a reasonable manner. All I know now is that when sprayed with Brake clean at those 2 carb spots, the RPMs jumped significantly, which I understand is indicative of a problem. There could be other gremlins lurking about as well. Hope to get back at it this week. Clay/Lexi

[Jon S]

Jose, much thanks! I do have that Rochester training booklet. It helps a bit, but fails us at critical points as it shows adjustment set up when all was new. Does not delve into trouble shooting or how to fix 70 year old worn parts. Update on our phone call; I only got to look at the car a day or so ago and carb vacuum leaks appear to be the problem. Just not sure what rebuilders do with one of those idle by-pass screws if it is at fault. As we discussed they are notorious for not working properly or at all. As Fishnjim once said a way back in this Forum, that was a "terrible" system. Well its come back to haunt me and not sure what to do. Digging through my parts and hope that I can cobble something together. Might try an O-Ring and/or if there is a spring there, look for a stronger one or increase it's tension with a couple of washers. Clay/Lexi

Clay/Lexi - I believe the spring is there to prevent the adjustment from turning. The screw should have a tapered end. I've never heard of them "leaking."  I would suspect a failing mounting gasket.

[David King (kz78hy)]

For the idle screws to be effective, the throttle plates must be sealed to the carb bores, otherwise, air can be drawn in there and they will never be adjusted correctly.  Also, you found the throttle shafts leaking, same issue, too much air being allowed in.  The internal orifice in the carb is tiny for the idle circuit, so a good soaking, cleaning, thin wire to feed through to make sure nothing is blocking the passage way, compressed air to blow out will be needed.  I have a Rochester carb book and can take a photo of the info there. 

[James Landi]

"For the idle screws to be effective, the throttle plates must be sealed to the carb bores, otherwise, air can be drawn in there and they will never be adjusted correctly.  Also, you found the throttle shafts leaking, same issue, too much air being allowed in.  The internal orifice in the carb is tiny for the idle circuit, so a good soaking, cleaning, thin wire to feed through to make sure nothing is blocking the passage way, compressed air to blow out will be needed.  I have a Rochester carb book and can take a photo of the info there. "

COMPRESSED AIR!!!!   Gum Out usually can not blow out fuel separation, aka brown gunk that settles, gets heated up and bakes into these tiny passages at the super heated base of the carburetor. 

[Lexi]

About 4 years ago this carb was reportedly leaking air at both throttle shafts as well as the idle by-pass screw, by my local mechanic. I had it rebuilt and saw it running on the engine test stand prior to taking it home for installation. It ran fine and I seem to recall that the rebuilder sprayed it while running and RPMs were not significantly affected, if at all. He did not have to re-bush the throttle shafts, as it was determined that there was "gunk" inside presumed key passages. So a good cleaning as James suggested is what fixed it before. The rebuilder also advised that many classic car owners come in wanting a throttle bushing job, which usually isn't required, he said.

This time, 4 years later, just the idle by-pass screw and secondary throttle shaft is leaking, (and not the primary). Noteworthy, is that after running 7 minutes the car will idle without me having to nurse the accelerator pedal, (but still not 100%). Seems that enough heat build up after 7 minutes may be partially swelling a key air leaky area, thereby causing a partial seal at the source of the leak.

As my car makes many local short trips, it seems reasonable to perhaps blame the carbon/gunk build up on these driving habits. Even though I often try and incorporate a (very) short highway run, (being mindful of this possibility), perhaps my driving habits are still insufficient, and causing this issue?

Happened with the carburetor prior to this one. Again, it needed to be tanked after about 4 years. Same issues with vacuum leaks at these same areas. Seems something to be mindful of for those who drive these older carbureted cars, and that a much longer periodic highway drive is required as preventative maintenance-to keep the "gunk" build up at a miniumum.

Now that these carbs are pushing 70 years of age, and that we are filling our gas tanks with inferior fuel, rebuilt carburetors may not "last" as long. Any thoughts? Clay/Lexi

[James Landi]

CLay,

May I respectfully suggest that Lexi needs a long trip during cool weather on a flat highway that is not congested with stop and go traffic with a full tank of high(est) octane gas?  Having owned two SDV's during the 60's and 70's, my two engines remarkably perked up after long highway runs. If your vacuum advance is working, and a vacuum gauge indicates a relatively steady 18 inches, then a long ride will likely enhance Lexi's operation... here's what I discovered--- acceleration became more responsive, engine idle smoothed out, exhaust "stacks" turned brown instead of black. The improvement, as I recall, was noticeable.    I have absolutely no solid evidence regarding what was going on to improve performance, but I am certain these engines do not run well if they're doing short,  around town trips.  Benefits of electronic technology introduced forty years ago include electronic engine management systems that our older  models engines don't have.  I also recall that the exhaust heat riser tended to get stuck, and the exhaust passages under the carburetor tended to plug up. Hope this helps, James     

[Lexi]

Makes sense to me James. Lexi did make a long highway trip last August, but none this year. She has made a lot of short highway trips like 10 or 20 minutes each. When doing these short runs I usually try to take her out on the highway even if for a bit, to avoid what you describe. As I noted in my last post, those may not be enough. Living around the Greater Toronto area is brutal as traffic is terrible, some 50 miles in either direction. Its an odyssey just to get far enough away to avoid the congestion. But even after those short runs she did seem to run better. Oddly enough, the day before her problems surfaced, I had her out on the highway for about a half hour. She has no heat riser valve. Too much life getting in the way lately, so hoping to get back to look into this problem soon before the driving season is done. One question though James, why a long drive in "cool" weather? Clay/Lexi

[Jeff Rosansky CLC #28373]

Ok, you are going to think this is absolutely crazy, but here goes--
What about covering every external hole that doesn't need to move (such as the throttle shaft) with something to see if it helps at all.
I don't know if some sort of tape would stick, but how about--I can't believe I am even going to say this-- sticking something like a wad of chewing gum over things like idle screws, etc. You are going to rebuild it anyway so it will get clean. Doing something like this would tell you if your air leak is internal or external.
I'm sure there's something better than sugar free Trident, but you need something that will form to the openings and be removeable.

I can't believe I suggested chewing gum-Ugh.
How about some plumber's putty?

[tcom2027]

Maybe screw the idle bypass screws in to the stops and see what happens. If the engine keeps running you can assume the problem is internal or there  is a vacuum leak elsewhere.

[tcom2027]

@Clay,

Not sure if you have the full Rochester 1956 Cadillac Service Training manual, here is the section discussing the idle-bypass section, this may be of help on your topic.

Let me know if you want the full copy (large file) so I'll be glad to send it over via email.

Excellent explanation with very clear drawings.

[Lexi]

The Idle By-Pass screw is basically a controlled vacuum leak. Not sure what it looks like inside where it is supposed to regulate filtered air I believe already passing into the carb, for curb idle. I am told that for later vehicles that still used them, they got much bigger and tapered, presumeably to better control air flow. On a couple of my carb cores I removed these screws and they were both about 1.5 turns out. Probably the last adjustment when they were last on a running vehicle. At the business end, inside the carburetor where the air idle flow regulation takes place, these ends are cut off flat. Not sure if they mate 180 degrees up against an air passage port or whatever. Upon close examination one of them appears to be more at 90 degrees, (the "flat" part), than the screw from the other core. One appears more "worn" than the other, eyeballing it and also confirmed when I put a watch makers minature brass square on it. Perhaps over torqued at one point by some ham fisted individual? Just like the idle screws up front, not good to over tighten and bottom this idle by-pass screw out? Thinking out loud, I wonder if one repair we could investigate is whether squaring that end to 90 degrees, if off, would better address such problems as I have heard so often that the idle by-pass screw does not even work? Could that be because the end is slightly worn so not as efficient at blocking unwanted air? Will try to get a clear photo, but a tough task as everything is so small and I probably don't have the magnification to show this. Mention this FWIW. Clay/Lexi

[Cadman-iac]

  Hey Clay,

From what I can recall, those air bleed screws on the carburetor are not drawing air from a filtered source, it pulls directly from the engine bay. That's one reason why they did away with those in later years.
Some of the early 2 barrel Rochesters used to have a slot cut into the gasket between the base plate and the bowl assembly that did basically the same thing but without adjustment.
Shutting down a hot engine would hot soak the fuel in the carb, and the fumes would escape through the vents and wherever there was not a good seal, like throttle shafts and those air bleeds.

  Rick