Wooly15

Rochester Carb Woes

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Ever since I bought my ‘56 Super, my Rochester carb has presented the notorious stumble off idle/low RPM bog.  I’ve tried everything that I have found online from the MANY different forum threads I’ve found on the topic. I’ve even taken it to two separate retired mechanics very familiar with these, and they both agreed that the carb is just worn out and needs replacing.  I cleaned it, replaced all gaskets and the accelerator pump with no success.  I even suspected a vacuum leak at the intake so I pulled, cleaned and put new gaskets there.  Here’s my question to you guys:  Is it worth sending it to one of the guys that advertises restoration of carburetors or would I be better off going new, aftermarket and losing my factory pedal start?  When old carbs become “worn out”, are they salvageable? That being said, does anyone have any experience with or know of any reputable carb rebuilders?

 

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My feeling is that carb just does not like our modern gas formulation...even ethanol free.  A 4gc that ran flawlessly on the old leaded premium never ran right on unleaded and this is is on a very low mileage car.  I fought mine like you did and finally installed a Carter WCFB which solved all the driveability issues.  You will need to modify the heat pipe for the choke and block off the exhaust in the manifold that goes under the carb; but you retain the pedal start feature.

A stock rebuild will not fix it and I doubt that anyone knows how to modify the 4gc to function on the available fuel.

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I had a 4GC on a '56 Olds that seemed incurable of the same systems. I never did get it right. Over the years that carb stuck in my mind and I have had a lot of windshield time to think about it. The carb always worked fine on the bench and had a good shot from the accelerator pump nozzles until it was on a running engine. Then there was very little when you watched down the carb.

 

Today I think there may have been some porosity in the pot metal between the accelerator pump well and a hidden vacuum passage in the body. If I was in your position or had one myself, today, I would mix up some gasoline resistant epoxy and put a very thin coat on the walls up the pump well. Be careful not to cover the check ball. I think it is worth a try. If the hesitation goes away you are good as long as "resistant" works. A light coat of solder may be the end cure but a little harder to get thin and smooth. If it doesn't work you aren't out a lot, but it eliminates a possibility.

There is also a GM tool to seat the check ball. It is just a cup tipped punch that taps the steel ball into the soft diecast to make a seat. That prevents part of the squirt from going back into the float bowl. You can probably make one from the shell of a ball point pen.

 

Watch out for those retired mechanics. They repeat stories they hear as if they lived them and like the feel of wind on the back of their teeth. I learned that a long time ago. And, nope, I never collected a paycheck as a mechanic.

Bernie

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I had my original 4GC rebuilt three times be people who "knew what they are doing". The first guy put the choke rod on backwards and mismatched the two check balls. The second guy did an awesome job refinishing it, I think that's where he focused on the carb. The third guy dipped it in acid and handed it back to me, removing the nice finish the second guy put on. I ordered a kit from Carb King and admittedly the first week the carb was weak. I must have drove it really hard because it started to clean up. I bought a second one from MrEarl and went through it the same. No modifications. There was always a slight bog, but an 8th turn rich made it really unnoticeable unless you were in low trying to mash it at a stop light. My 4GC collection is a love hate relationship. 

 

The carb I got from MrEarl ran better initially than my original, I think in part to my car being a high mileage car and the throttle shaft was worn. When I did my dual carbs, I greased up all the shafts with the same grease I use to seal my wiper motor actuator and it seemed to make a difference. Now with just my original carb, about twice a week on payday I go out with a sewing needle and apply some silicone grease around the throttle shafts the same as my wipers. 

 

I think there's a reason why the AFB carb was picked for aftermarket reproduction and not the 4Jet. Others on the forums have had success with a 600cfm Eddy, but then you're looking at rewiring your car. 

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I've had both, 4GCs and AFBs. No noticeable difference in performance but I prefer the AFB for two reasons:

1) The AFB has no gaskets below the float level (no propensity to leak) and

2) the AFB is much easier to rebuild (for me the amatuer.)

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During the rebuild, get the original specifications for the idle tubes in the carburetor. Drill them 0.002 oversize.

 

Make certain the "factory fix" is done in the choke vacuum supply circuit!

 

Make certain that the primary and secondary floats are NOT interchanged. Set the floats to specifications.

 

DO NOT set the idle mixture using a vacuum gauge for the highest vacuum!

 

Should run like a sewing machine.

 

Jon.

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3 minutes ago, carbking said:

During the rebuild, get the original specifications for the idle tubes in the carburetor. Drill them 0.002 oversize.

 

Make certain the "factory fix" is done in the choke vacuum supply circuit!

 

Make certain that the primary and secondary floats are NOT interchanged. Set the floats to specifications.

 

DO NOT set the idle mixture using a vacuum gauge for the highest vacuum!

 

Should run like a sewing machine.

 

Jon.

 

With my level of expertise, I'm afraid to drill anything. Lots of great info here, guys, I appreciate it. I think my plan is to deal with it until I can dig up a carter from somewhere. Maybe someone wants for trade for a Rochester! ;) Just thought I'd mention this, when the engine is cold, it runs just fine...

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OK - reread the thread, and a few more comments:

 

(1) A 4GC wear out??? maybe, after half a million miles. It MAY be worn beyond the abilities of some.

(2) The throttle shaft to throttle body clearance was/should be 0.004~0.006 inch new. Wear of an additional 0.003 is acceptable. Total wear of more than 0.009 should be addressed, either with bushing the throttle body, or machining a new throttle shaft, or both, depending on what is worn.

(3) The accelerator pump in MANY 4GC carbs (haven't checked the print on this one) have an internal check valve to relieve percolation air bubbles. If a pump without the ball replaces a pump that should have the ball, percolation in the pump cylinder will cause the volume of fuel in the pump cylinder to slowly be diminished, by pushing the fuel out of the pump jet (while driving) to relieve the pressure normally relieved by the ball. This can cause a very weak or non-existant FIRST pump shot, as the pump cylinder is empty or close to empty. The second (or maybe the third) pump squirt should be normal as pump activity would refill the pump cylinder.

(4) The AFB was chosen for aftermarket rather than the 4GC because Carter went after the aftermarket and performance aftermarket, and Rochester did not. Rochester did offer some service replacement carbs, but never aftermarket carbs. Marketing decision. The aftermarket Carter AFB's and especially the clone AFB's are not really suitable for non-racing Buicks. A really good carb person MIGHT get one close, but NEVER as good as an original. If you want an AFB on your Buick, use a genuine made for Buick AFB, not one of the aftermarket, either genuine Carter or clone.

(5) And there are NO man-made materials which survive in ethanol as well as the underbelly of a cow! Use leather accelerator pumps!

(6) Carters DO have a better reputation, primarily because Carter did offer a plethora of aftermarket tuning parts, and were much easier for a non-carburetor professional to modify when an engine was modified.

(7) There are other tricks, but I have to hide something! ;)

 

Jon.

Edited by carbking (see edit history)
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Drilling  to 0.002 took me back to the South China Sea in 1969. While in the Navy I ordered an ICS, International Correspondence Schools, course on automotive tune up and carburation. I still have the books on a shelf in the garage. But I remember the detail the course went into on curing a light throttle stumble for some variation of a 1949 Plymouth. I still smile when I see that book and think about reading such intimate detail on 20 year old clunkers at the time. I guess I was a critical leaner at that time as well.

 

As I was reading I was thinking, just drop it off at my house. If I'm not home leave it in the driveway and put a couple of hundred dollar bills under the wiper. :) I have sorted out lean situations with an oscilloscope. There are a lot of components to the system from the flex hose in the fuel line to the valve guides and a whole ton of stuff in between. Keep it in your hands. You'll get it. Remember "The level of perfection YOU can achieve is directly proportional to the number of times YOU are willing to do it over."

 

I bought a carb kit from Jon a year or so ago for my Packard Carter WDO. I am very happy with the results. If Jon had been watching my casual approach to the job I bet he would have just shook his head and left. Maybe the ICS course made me this way.

Bernie

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Bernie - glad you were happy with the kit.

 

For many years, I have advocated that the most essential "tuning" tool for the hobbyist is an oscilloscope.

 

And the most essential non-tuning tool, other than hand tools, is a good used metal lathe.

 

Jon.

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Looks like we both might be waiting for our wives to cook supper.

Sometimes a lazy guy will skip the scope and just use one of those old Sears meters with the clamp on induction KVA pickups. Secrets, I like them.

B

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geez, too bad you guys aren't local. I'd have this thing tuned and running great!

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Every once in a while, I'll pull out my Sun Machine, but my favorite carb tuning tool is my wideband O2 sensor.  I don't use it for tuning to a specific AFR, but it does tell me what the engine's doing and if I need to make any carb modifications.  For example, my Firebird's 2GC was SUPER lean on the transition circuit, as was the Holley 570 Street Avenger on my Mustang.  A little work with the pin drills on the idle feed restrictors got everything running well.  I am still living life without a metal lathe, but I do have a MIG welder, which helps ease the pain.

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(Will the scope indicate when a plug misfires in "lean misfire"?  So you then go slightly richer for the final adjustment?  Detecting lean misfire without and AFR meter?)

 

Back when the BCA BOD quarterly meetings were hosted by BCA Chapters, we were in the hospitality room and one of our members had a recently restored '55 Roadmaster Riviera.  He had been having an off-idle stumble that would not leave.  He's bought an original carb from another BCA member and then had a professional rebuild it.  Even after the second "rebuild", same issue.

 

There were two members from Arizona (Phoenix, I believe) whose minds were being picked on this subject.  One noted that he'd had a 4GC on a Pontiac with a similar issue.  He discovered that, after at least one "kit" job, he finally put the original accel pump (leather) back into the carb and the stumble either went away or diminished greatly.  I'd always felt the "rubber cup" pumps to be more responsive, but his firm orientation was that ONLY the leather pumps were any good (even if they showed a little wear).  Our guy and his brother-in-law put the leather pump back in and it did much better.  This was well BEFORE ethanol was increased to what we now have.

 

As far as the "lead work", there might be suitable tools and supplies in Eastwood for doing body work with lead (although there are some hazards in using those things!), like before "Bondo" arrived.

 

NTX5467

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On mine the off idle stumble that could not be fixed, was when you were just easing away from a stop.  If you punched it (and got a shot from the accelerator pump) it ran fine...hard to drive that way.

That 4gc is really a pretty carb...sitting on a shelf with some dust on it!

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11 minutes ago, old-tank said:

On mine the off idle stumble that could not be fixed, was when you were just easing away from a stop.  If you punched it (and got a shot from the accelerator pump) it ran fine...hard to drive that way.

That 4gc is really a pretty carb...sitting on a shelf with some dust on it!

That's exactly my symptom. It also helps if I pump the gas pedal before I take off. It's very frustrating. 

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Jon is almost certainly right.  Opening those idle tubes up a couple thousandths with some pin drills should do it.

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In an earlier post in this thread, I suggested to NOT set the idle mixture by setting for the highest vacuum.

 

In most modern (post mid-1930's) carburetors, there is both a lower idle port, and the idle transition slot. The fuel to prevent hesitation when slowly accelerating from an idle is supplied by the idle transition slot, NOT from the accelerator pump.

 

Vacuum is measured below the throttle plate, and the highest vacuum reading will be obtained when the primary throttle plate(s) are closed. This condition forces 100 percent of the idle fuel through the lower idle port, as the transition slot is completely covered by the throttle plate(s). Thus there is no fuel in the transition circuit.

 

OK to use the vacuum gauge, and find the highest vacuum, but then the idle mixture screws should be turned in maybe 1/2 turn (each engine will be different), and the throttle positioner screw also turned in slightly, to crack the throttle plate(s) at idle. As there is now fuel flowing through the idle transition circuit, there will be a smoother transition from idle to the main metering circuit.

 

It is important to know the design idle mixture setting range for the specific carburetor. If this figure is NOT known, and cannot be found, a good rule of thumb is from zero to 1 1/2 turns on pre-smog carburetors. Once the screws are out past the maximum design figure, no additional fuel is added.

 

The caviat about pre-smog carburetors is because more precise idle mixture control was necessary once smog emissions became the law of the land. The taper and angle on the metering section of the idle mixture screws was changed. The length of taper is much longer. This changes the rule of thumb on smog carbs to maybe 1 to 3 1/2 turns.

 

Why should you increase the diameter of the idle tubes?

 

Contrary to popular opinion, the idle mixture screws to not change the idle mixture; rather the idle mixture screws meter the amount of a pre-determined mixture into the engine. The mixture is pre-determined internally to the carburetor by the diameters of the idle tubes (jets) and the idle bleed(s). Think of old-fashioned shower controls where there is a hot control, a cold control, and a pressure (volume) control. The individual enjoying the shower would set the hot and cold controls to their individual comfort, and then generally turn the volume on full. So think of the idle tube as the hot, the idle air bleed as the cold, and the mixture screws as the volume. The design engineers pre-set the hot and the cold (idle tubes and idle air bleeds) for non-ethanol fuel. Generally the idle tubes are going to be 0.032 inch plus or minus 0.005 inch depending on the exact application. The 0.002 addition is a good rule of thumb (sometimes not enough) to enrichen the pre-set mixture to compensate for the reduced energy of ethanol fuel. For those that add more aggressive camshafts to the engine, generally necessary to go more than 0.002, but that is always a good starting point. 

 

Jon.

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10 hours ago, old-tank said:

On mine the off idle stumble that could not be fixed, was when you were just easing away from a stop.  If you punched it (and got a shot from the accelerator pump) it ran fine...hard to drive that way.

That 4gc is really a pretty carb...sitting on a shelf with some dust on it!

 

Willie - the last "pretty" carburetor was one of the pre-1930 updraft carburetors that was of brass construction! ;)

 

Jon.

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2 hours ago, carbking said:

Why should you increase the diameter of the idle tubes?

 

Jon, I always enjoy your posts!  The reason for things is so important to determining the root problem.  Please pardon my ignorance, but when you say to increase the size of the idle tubes, can you post a picture of where this tube is in the Rochester 4 GC?  Are you saying to increase the size of the idle ports in the throat of the carb?  Or is this a port inside the body of the unit?

 

As far as drilling these out, is this something that needs to be done on a drill press? 

 

Sorry if these questions show my ignorance.  I am just having a hard time trying to figure out where the enlargement needs to occur.

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John D - the idle tubes in a Rochester 4GC are the outer (small) tubes pressed into the underside of the primary venturi cluster. The Doug Roe book on Rochester carburetors is helpful in nomenclature.

 

As to drilling, as the hole is very small; best to use a "pin vise" and a drill from a 61~80 numbered drill set. Or, if you do this for a living, a set of furnace orifice drills.

 

ALSO - those drilling carburetor jets, either main or idle, will find there are "skips" in the "thousandths" in the number drill set. To partially fill in those skips, think fractional metric drills. These may be ordered (they are not available in the Baja boondocks of central Missouri) from the online supply places such as McMaster-Carr, MSC, etc.

 

Jon.

Edited by carbking (see edit history)

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Imagine, you can't get some of the small fractional drill bits in central Missouri, but they have the internet. I can just here my Mother saying "Can you fraction that?"

 

Interesting comment- works fine cold but hesitates when warm. That could translate into "Works fine when it is supposed to be running richer".

 

Oh, edit. When you pull a vacuum on the distributor advance does the point plate move freely? And when the vacuum is released does the plate return all the way nice and smooth?

Bernie

Edited by 60FlatTop (see edit history)

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8 hours ago, 60FlatTop said:

Interesting comment- works fine cold but hesitates when warm. That could translate into "Works fine when it is supposed to be running richer".

 

Pretty sure that's why Jon recommends enlarging the idle tubes.  But I'm more interested in the idle transition slot. 

It seems to me that so many of us report the same situation.  This hesitation after a carb rebuild.  I'll go back to something mentioned a long while ago; could a gasket be made without a required port, such that so many folks seem to be having the same problem? 

 

The driveability issues reported here match my 56 exactly!  After a rebuild, no hesitation when cold and up to operating temp. At operating temp, light acceleration = hesitation.  Slightly more accelerator and it seems to be okay.  Further, I may be imagining this but if the car is facing downhill when started, I have no hesitation. I know it did not have a hesitation prior to the rebuild.  I had to rebuild mine due to a hole in one float causing severe flooding.  But I did not have a hesitation before this rebuild.

 

Could an internal gasket be blocking the transition slot? 

Edited by JohnD1956 (see edit history)
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I got my kit from Jon and when compared to a virgin gasket, they were the same. The shop manual explains how to set the idle speed screw in such a way that it blocks the idle transition circuits until the butterflies are open. Something about checking the gap with a feeler gauge or drill bit, and then adjusting idle bleed screws. These cars were made to idle at 450, but that's not really possible on ethanol, and I've read a lot of people open the idle speed up to 600. Maybe the butterflies are not sealing the gap at this higher RPM and should be adjusted to do so? I know the butterflies have an enlarged hole and the screws only seat in the shaft. 

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1 hour ago, JohnD1956 said:

 

Pretty sure that's why Jon recommends enlarging the idle tubes.  But I'm more interested in the idle transition slot. 

 

 

John, the idle tubes feed the transition slot as well.

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