1956 Rochester 4GC idle by-pass screw

[Lexi]

I am not much of a carb guy, but have reason to believe my idle by-pass screw on my Rochester 4GC is leaking air. Is some of that normal and if so, is the spray of brake clean a proper way to test this? If it is determined that it is faulty how is that corrected? Do I look for another cast iron base plate and/or screw, or is that repairable? Thanks, Clay/Lexi

[Dave Shepherd]

What makes you think is happening, what are the symptoms?

[Lexi]

Hello Dave. I can hear air hissing out of it at times, as if the threads of the screw are not as tight as they should be. Was wondering if a slight wrap of teflon tape, (as I do not believe fuel comes into contact with that area), might tighten it up a bit-but not sure if some air is supposed to enter normally by that route also. Not 100% certain how that circuit of the carb even works. I had just completed a lot of work on the car and took it to a shop to investigate why I could not get a good idle after a major tune up, among other things I had completed. They to commented on the idle by-pass screw leaking air as well as the need for a throttle bushing service, (also leaking air). That accounts for my somewhat erratic and "wandering" idle which recently manifested. Just not sure what to do with this by-pass screw business. Will have to send carb out for service, but can this idle by-pass screw issue be addressed by a rebuilder? Clay/Lexi

[klinebau]

How many turns out is that screw now and what does the factory service manual recommend as a starting point?

[Dave Shepherd]

Your erratic idle is caused mainly by the worn throttle shaft, also make sure the secondary throttles are closed. It sounds like to try and compensate for the poor idle quality,  the air bypass is too far out.

[Lexi]

Dave I agree. That said, I don't think the idle by-pass screw is too far out. Guessing a turn or so. Worn throttle shaft issues are well known in the carburetor community, but I have not read much on the idle by-pass screw or on how to test it for proper operation. The Rochester products 1956 4GC training booklet indicates how the idle air is introduced to the system, with the idle by-pass screw acting as a mixing valve of sorts. The booklet says this screw regulates air which comes from an "adjustable passage". I infer that the screw itself should not provide much if any air into the system. If what I heard is air hissing from it as did the shop mechanic, it seems that this would be an unconventional source of air. Looks like the air screw is fed air internally, rather than allowing much (if any) to enter from where it is mounted. I gather that it is not a point of air entry, just a regulator for what it is diverted to it. Hope I understood this correctly. If that is the case then a spray of brake clean for example, on the idle by-pass screw should not effect idle if it is working properly. Would that be a correct assumption? If so, that would be at least one test. Clay/Lexi

[Cadman-iac]

Hey Clay,

Your assumption is spot on. It's just the regulator for the internal air passage, and should not have any air going past the threads from the outside of the carburetor.
I guess it's possible to hear the air from the internal passage,  if the engine were quiet enough. Even more so if the air cleaner was removed.
I'm not sure how air tight the threads are supposed to be, but it should be able to control all but the smallest amount, so a bit of carb spray on it shouldn't affect it very much if at all.
As was stated earlier, the throttle shaft wear is usually what causes most of the problem. The spray on this area will verify that.

Rick

[fishnjim]

I hope this helps(below).
I doubt many today's mechanics ever saw a carb with the vacuum idle circuit.   They're "terrible" designs in my opinion.   Much prefer the throttle plate idle screw.   Some can be adapted.   One of the reasons I changed from a 4 to 3x2s - simpler.
It's just a needle valve that controls the air for the idle venturi.  So you should hear air sucking from the carb.   If adjustment is not right and the vacuum not stable, smooth idling is a fantasy.   There's a procedure in most shop manuals on how to set, but if one never has done it, may not be so easy.   

[Lexi]

Fishnjim thank you so much for that PDF file. That is one that I didn't have. Gonna study it this eve & I am sure it will help. You are correct, that idle by-pass idle adjustment was not a good system. The Rochester 4GCs I believe are among the first, (or the first), 4 barrel carburetors so we cannot expect perfection with something from this era. Just a headache 6 decades later. Rick, thanks for your input. Next good weather day I am going to have another look and do some spray testing Clay/Lexi

[walt chomosh #23510]

Clay,
  I run a 4GC on my 55CDV and struggled to get it to perform properly. (I have a spare also and ditto on it) When I finally got it to adjust the idle up and down, the RPMs didn't vary much it seems. Both primary and secondary throttle plates must be closed completely and mine are. Fact is, it's idling so I guess I'm happy. I believe Art Gardner helped me when I was struggling to find common ground on the system.....walt...tulsa,ok

[Lexi]

Walt thanks. Nice to know I am not alone with this problem. I started the car today and took it out for a run. Seemed a lot better since the shop opened up the adjusters to make the idle more rich to compensate for the lean condition caused by the air leaks. Prior to the shop looking at it I had set the carb up with a vacuum gauge, but it was running lean due to the air leaks. Ran well at idle at about 20 inches mercury. I think the "popping back" sound I was beginning hear when pulling away, was due to that lean condition. It is now gone. My 4,000 mile spark plugs also had a slight, white frosty look to them which may also have been from a lean running condition.

Today I did spray brake clean on the 2 throttle shafts and the idle by-pass screw. The secondary bore must be leaking as there was some increased rpms noted when hit with spray. Primary seemed unaffected. When I sprayed the idle by-pass screw the rpms noticeably increased. As stated earlier, not sure how much of a vacuum leak is permitted at this location, but think what I heard today was excessive. The shop said all 3 areas leaked air, (primary, secondary and by-pass screw). As the secondary bore is leaking, it also may not always permit the linkage to return properly, which might have given me a false pass reading on the primary throttle bore today. Both here and on the old Mid-Century Cadillac site, the woes of the idle by-pass screw would often be discussed. Perhaps in some of those cases the failure of the screw to effect idle speed was that it was leaking air. Something for all with this system to consider. Gonna study this some more and then consider my options. Clay/Lexi

[Cadman-iac]

  I don't want to hijack your thread Clay,  but I thought since this is on the same subject, I would post it here.

I looked at my other 56,  the parts car, and it has the Carter WCFB carburetor on it. I have been thinking all this time that it was the Rochester 4GC carburetor.
But the Carter has the air bypass s rew on the right side.  I was looking at the service manual for the Rochester and it looks like it's got the screw on the left side.
Now I have a Rochester 4GC carburetor that is supposed to be for a 56 that I got from my friend years ago. However it does not have the air bleed screw on it anywhere. It is the small bolt pattern base plate,  like it's supposed to be.

Now I have 9 Rochester 4GC carburetors total,  only one with the small bolt pattern.  All the rest have the bigger pattern.  And out of those other 8, only one has the air bleed screw,  and it's actually in the back of the carburetor at an angle.
  So my question is, what years did they use the air bleed system on the Rochester, and does anyone have any idea what year this carb might be? Unfortunately it doesn't have a tag on it, like most of them anymore. 
And the tag on the one that is supposed to be for a 56 is stamped with the number 6220. I don't have a way to check it out,  does anyone happen to know if it is for a 56?

  Something else that I have noticed,  is that the 56 Rochester 4GC doesn't use the secondary air valve in it, nor does the Carter for 56, unless it's the dual 4 bbl setup. That's straight out of the service manual.
I kept looking at my 56 Rochester 4GC carburetor and wondering why the air valve was missing, all the others have one.
Just a little trivia.


Rick

[cadman56]

My experience is this:
I always rebuilt all my carbs.  when the base plate is apart  I would always place it on a flate surface and  with oil on a fince sandpaper  'fiqure 8' it  to remove any  warping, etc.
When reassembling the throttle plates I would hold the assembly  up to light.  If I could see even the  slightest bit of light around the plates thery were not properly installed.  There must be no light visible.
Also, I have the bushing kit for the later  spread bore carbs with aluminum base plates.  The bushings will not fit the 4GC of the 56, they are too big.
The threads on the idle bypass screw are very fine and the retaining spring is pretty stron.  I would think the spring tension may be a bit weak if a leak is experienced at that location.
There could also be internal leaking of the carb.  I seem to remember that is an issue with the 4GC?
I have also seen these where the linkage to the secondary binds and the throttle cill not fully close..
Fortunately, my experience with these carbs  has not been what most seem to decry.  I was always able to get a consistent idle in all 4 cars through the thousands of miles I drove them.
Good luck.

[walt chomosh #23510]

The throttle shaft wear in Clay's throttle body secondary shaft MUST be corrected before going any further.My 1955CDV had wear on it's secondary and caused intermittent motor issues that had me running around in circles. My experience with bushing the shaft bores was zero so I sent my carb to Daytona and was not only pleased with the quick turnaround but the price also! Problem solved! This also raised the question of carb return springs. Too much pressure will cause premature wear whereas not enough will prevent proper consistent idle......walt...tulsa,ok

[fishnjim]

Maniac - Without the triangle number tag, it's not very easy to ID a Rochester.  Especially after a number of years, parts maybe altered/replaced.   Some have different stampings.  I didn't look at your pix.   Look for 4-jet or other lettering.   I don't think the 4 GC has any, but going from memory.   They changed every year and sometimes more than once, and variations for different makes.   Just look at all the part numbers. 
I know the vacuum idle goes past '58, and any manifold that has the "exhaust gas crossover" under the carb probably had a factory vacuum idle carb.   This goes back to the early aircraft era, and the concept of carb "icing", ie, ice forming from the water in the air plugging off the fuel flow.   Expansion causes a drop in temperature, so once the temp crosses the freeze point, icing is favorable.   But I don't think that's much of a problem at atmospheric pressures and not nearly as fatal, but the auto carb people were slow to realize.   
I have both "Correct" Rochester and Carter 4 bbls for '58 and they both have the 3 screw adjustment.   You'll see that disappear and the idle screw on the idle cam appear later.   Some of the tweeners have the screw hole but no screw.   
ps: Clay/lexi: If your throttle base is that leaky, better install new bushings or get a different base plate.   The throttle shafts wear and reusing them may not solve the leak with fresh bushings, but I don't think they were ever meant to be air tight.   Thermal expansion would cause binding, etc. issues.   Seals would be more cost.   That's why I recommend using a rebuild vendor that can "bench" a carb after it's rebuilt.   That'll show problems and is tested/adjusted on a live engine.   
Tuning is a bit of an art.   It's more iterative than most think.  I grew up in carb/points era.   So you have to keep the instruments hooked up and recheck after every adjustment.   For carbs, access to exhaust gas analyzer is superior to "by ear".   But not a common DIY tool.   Plus these engines were designed for completely different gas than is now available, so that also affects them.   No magic here.
PPS:There's a ton of literature available on line about carbs from the various major carb parts vendors, Mike's etc. so searching helps.   That's half the fun, figuring it out for yourself.

[Lexi]

Thanks to all who contributed. See chit chat between Jose and I down below for more information on these carbs and Rick'e enquiry.

Cad-maniac (Rick), without the tag it will be tough to identify your carburetor. If memory serves me correct, rightly or wrongly, I have not seen a 1956 Cadillac Rochester 4GC without an idle by-pass screw. Perhaps they were made without them and I just have not seen one. I looked at a couple of MPLs and I nearly passed out from carburetor information over-load. That said, I am going to stick my neck out & say that the 1959 MPL appears to list 2 part numbers for the 1956 Rochester throttle base, (the cast iron piece which is where the idle by-pass screw is located). A third listing provides the mechanic with the option to order the base complete with all attachments including the "adj. screw" (quote). It did not differentiate between any base plates that did not have the adjustment screw. I presume they all had them, and that the reason for the 2 different part numbers (based on engine unit #), probably indicate some other difference, which was not noted. Not 100% on this though. As the MPL is not a "how to" source, just a parts list, it often fails us at critical points. In this case it does not note what the differences in the 2 base plates are. Perhaps if all of those sections were studied it would become clear. Not sure. Attached is an image of the base of a 1956 Cadillac Rochester 4GC. Another image shows the same base "top side", (the part that the bowl would attach to). Hope this helps you with your enquiry...which yes it is related to this thread of mine and I am glad you made that post. Clay/Lexi

[J. Gomez]

Rick, Clay,

As far as know Cadillac was the only model with the idle-bypass screw on the throttle plate, other Rochester GM or other brands did not have it.

Clay,

The throttle base you show with that hole is for the vapor vent on the ’55 and ’56. Since the throttle plates are closed at idle there is no air going through it the air goes on the idle-by pass passages going up to the carb. 

Larry,

I recall reading a post or article on installing brass bushing on old Rochester carbs, I do not remember where or who posted it and which carb it was done, maybe a 2GC but I’m not 100% sure. The throttle base is cast iron so not sure how easy/hard will be to rimmed the holes to fit the brass bushing and if any rebuilder will tackle the job.   

[Lexi]

Jose thanks again. I was studying my base plate and later noticed that the "vapor holes" are just above the throttle plates which as you and Walt noted are closed at idle, so there should not be any significant vacuum there at idle. So back to checking the threads on my by-pass screw. What you said about Cadillac's 4GC and the idle by-pass screw makes sense. Will have to modify my earlier post so as not to mislead others in the future who may read this. Clay/Lexi

[Cadman-iac]

Hey Clay,

Thanks for looking anyway, I appreciate your efforts.  This carburetor, I'm not sure it's for a 56, my old friend, who is now long passed away,  gave it to me,  and he had it tagged as being for a 56, and that's what I was going by.
It does have a factory triangle tag on it, but who knows if it's the right one. It's got 6220 stamped in it, but I don't see anything in the books that I have that would tell me if in fact it is a 56 carb.

Does anyone know how many years Cadillac used the small mounting hole pattern on their engines? That would at least narrow down the search parameters for the correct fitment.
It does have the "4 jet" cast into the top piece. It does not have the bowl vent mechanism on it, or provisions for it. The choke assembly is mounted on the top piece instead of the side of the bowl.
It's not critical that I figure it out,  as I'm not planning on using it for anything yet,  but eventually I would like to know what it's for.
Thanks for the help and information.

Rick

[Lexi]

What Jose said makes sense. I also have never seen a 1956 Cadillac 4GC without that screw. Attached is a youtube video link that offers some tips that may help in identifying a 4GC without the tag. Clay/Lexi

https://www.youtube.com/watch?v=yChXh-r_dF8