Jump to content

Rochester Carb Woes


Wooly15

Recommended Posts

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?

 

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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. 

  • Like 1
Link to comment
Share on other sites

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.

  • Like 2
Link to comment
Share on other sites

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...

Link to comment
Share on other sites

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)
  • Like 2
Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

002a.JPG

001.JPG

Link to comment
Share on other sites

(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

Link to comment
Share on other sites

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!

  • Like 1
Link to comment
Share on other sites

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. 

Link to comment
Share on other sites

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.

  • Like 4
Link to comment
Share on other sites

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.

  • Like 1
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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)
Link to comment
Share on other sites

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)
Link to comment
Share on other sites

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)
  • Like 1
Link to comment
Share on other sites

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. 

Link to comment
Share on other sites

First, the Doug Roe book referenced (on Rochester carburetors . . . .1bbl, 2bbl, 4bbl AND scads of specs relating to spreadbore QJets) explains very well the purpose and operation of the transition slots.  PLUS the relationship of the throttle plates to the slots at hot idle.

 

At this stage of the carb's life, there probably is some additional wear on the throttle shaft bores.  There is a "production clearance", but now there probably is "wear clearance".  As a Holley engineer once mentioned, there IS a vacuum leak through those shaft bores, but the metering specs compensate for that.  IF it gets to be too much from wear, that's another situation which would mean the bores needs to be bushed to restore that clearance (kits exist for QJets to do that, but probably not many others).

 

NOW . . . in the old service manuals (some of which have been mentioned in here) and MOTOR Manuals, it mentions removing the "stake" on the bottom of the throttle plate screws to remove them for the rebuild/soak procedures of the rebuild.  Then when it's time for reassembly, you reinstall the plates on the throttle shaft(s) and re-stake them so they don't fall out.  In later years, this was not recommended!  But it was in the 1950s-60s.  Therefore, what happened was that any accumulation in the throttle shaft bores was cleaned out, making the wear more apparent AND a slightly larger (uncompensated) vacuum leak.  ADDITIONALLY . . . the original production relationship of the throttle plates and the transition slots was disturbed.

 

The Rochester book and the similar Holley book have diagrams of the transition slot/throttle plate design relationship  .. . plus how and WHY the throttle plate not being at the specified relationship to the transition slots will cause hesitations, bogs, and flat spots off of hot base idle and accelerations.

 

The additional size Carb King mentions seems very minor, but in the Rochester book, it details how being more concerned with the AREA of the hole rather than the basic hole diameter itself.  In the tuning of QJets, it details this and it makes sense.  This DOES generate the need for certain sizes of twist drills!

 

In many cases, cars of the 1960s would end up idling at about 500rpm in gear with the AC going, yet the spec "N" or "P" idle speed was 550-600rpm.  On our '69 Chevy pickup 350 QJet in order to get it to act "right" (to me), it needed a "N" idle speed of 750rpm!  But when in gear, it would sit there and idle all day long in the summer with the AC going (as Dad talked to somebody he knew) and not overheat or cause any performance issues.  So, EACH carb has it's own "sweet spot" of hot idle speed and mixture, it seems.  Finding it can be a little illusive sometimes AND not specifically at "factory spec" in all cases!

 

The OTHER side of the idle speed combination can be ignition timing.  A few crankshaft degrees, either way, can sometimes change the idle speed enough to need to be reset, which can affect the throttle plate/transition slot relationship in some cases.

 

So, after setting the points, reset the base timing, THEN do the carb adjustments.  When you do an engine rebuild, take the extra time to verify TDC and mark the appropriate marker to that effect.  This takes some production tolerances out of the mix.  Flexplates/flywheels might not be marked exactly correctly or the keyway slot in the crankshaft might not be machined exactly on target.  You might even make your own indicator and mark, rather than use the factory markings.

 

ONE thing a carb soak will not remove is internal corrosion in the fuel passages.  It'll clean the varnish and other soft accumulations, but not the harder ones (which would need to be removed mechanically, if possible).  Sometimes, the very metering orifices which are critical to proper performance can become smaller (as I found out once), but still appeared to be open as they'd flow spray B-12 when I kitted the carb.  So looks can be deceiving!  Only when I found the "Low Speed Jet" spec for the particular Carter carb (and location in the bottom of the idle tube!!), went large enough on the twist drills I bought until I "got brass", did the carb's idle system work so the car would idle when hot and not die.

 

These things are from my own research and experiences.  In some cases, it might be splitting hairs, in others, not so much.  REMEMBER, too, that we're dealing with half-century-old mechanisms in these carburetors . . . or older!  Not allowing for the "new fuels", "factory specs" will not be as optimum as they were when the cars were newer and gasoline still had lead in it.  Therefore, use those "factory specs" as a starting place, not the final destination, unless things work well with them.  Maybe the idle speed needs to be 25 rpm higher, at optimum idle mixture?  Perhaps 2 degrees more ignition timing might help?  Or both!

 

It's been long-noted that fuel issues and ignition issues can act similarly.  Might be that the plug wires or other ignition components need some attention?  Probably not, usually, but "could be"?  What about the vacuum advance ported vacuum port in the carb?  Something ELSE affected by the throttle plate position!  Otherwise, doe it work as mentioned in another post?

 

I've mentioned many things which can fit into "the dance" of getting several engine systems to work in concert with each other for the great performance which our vehicles CAN provide.  Just might take a few extra steps to get it now, considering the fuels we now have (plus the beloved mechanism age issues!).  Variable situations!

 

HAPPY HOLIDAYS!

NTX5467 

 

 

 

 

  • Like 1
Link to comment
Share on other sites

A comment in regards to Willis' last post:

 

On the subject of throttle shaft bushings kits; I don't think these are a good idea for the hobbyist. Why? If the hobbyist has the milling machine which I consider necessary to do a proper job of throttle body bushing, then he/she does not need a bushing kit. And if the hobbyist does NOT have the milling machine, then (opinion) he/she most certainly does not need a kit! Yes, it is possible that a hobbyist could use an expensive floor drill press with end mills, but the tolerance on spindle drift, even on the better drill presses, is much greater than on a milling machine. Do it wrong, and the results are worse than not doing it! For those that do have, or have access to a milling machine; OR choose to ignore the content of this paragraph, I have bushings available. But (opinion), this procedure is best done by a professional.

 

And why (comments) by JohnD and others, does the hesitation issue arise after a rebuild? If the carburetor NEEDED a rebuild because of a bad float, or a groove in a fuel valve causing the fuel level in the bowl to be too high; the higher fuel level WOULD act on the idle circuit similar to increasing the diameter of the idle jet, but do so in an incorrect manner, thus masking the issue which arises after the rebuild.

 

Bottom line.....these carbs were calibrated for a specific fuel. But understanding the circuitry of the carburetor will allow the enthusiast to recalibrate for a different fuel, just as recalibration is necessary for those who move to higher altitudes, it CAN be done.

 

Jon.

  • Like 4
Link to comment
Share on other sites

I'm still curious why some, like John, did not experience the off-idle hesitation issue until after they rebuilt their carb.  In John's case it sounds like there was a pre-existing float issue which may have caused an over-rich condition that may explain things.  Seems like there must have been similar pre-existing conditions in other cases.  The rebuild fixes the over-rich condition, leaving the mixture too lean for today's gasoline formulation?

  • Like 1
Link to comment
Share on other sites

1 hour ago, EmTee said:

I'm still curious why some, like John, did not experience the off-idle hesitation issue until after they rebuilt their carb.  In John's case it sounds like there was a pre-existing float issue which may have caused an over-rich condition that may explain things.  Seems like there must have been similar pre-existing conditions in other cases.  The rebuild fixes the over-rich condition, leaving the mixture too lean for today's gasoline formulation?

In my case, the hesitation was present before and after. It was the main reason I rebuilt it, well, "refreshed" it. 

Edited by Wooly15 (see edit history)
  • Like 1
Link to comment
Share on other sites

Maybe Jon wouldn't have walked away shaking his head if he had seen me just replace a few parts that were obviously bad, left a bunch of shiny stuff in the bag, and just put it back in. I didn't want to rebuild it. I just wanted it to work right.

Bernie

  • Like 2
Link to comment
Share on other sites

In the case of a "pre-existing condition", we've felt that we might as well "kit" it anyway in hopes that procedure would "make everything right again".  "Making it better" is a good orientation, but doesn't always work out that way.  Yet the minimalist approach (sometimes the BEST way) might always leave us wondering "IF . . . " when the issue we desired to minimize/fix wasn't.

 

As flaky as the minimalist approach might be (very sub-optimal to many, understandably) it can apply to many vehicular systems, especially engines.  Back to the "if a little bit will fix it, don't go farther unless you have to" orientation.  At least this way, you still know what you have and you're not at the mercy of an allegedly-competent repair person/facility and their magic wand.  BUT if you might desire the perceived peace of mind that something's been fixed, your money and judgment call.

 

Back when an auto supply chain started advertising that they'd pull codes on your car and tell you what it needed to fix it, a customer would let them pull the codes for free, then bring the list to us to get the OEM parts.  Computers are supposed to always be right?  What I'd advise them is that unless it was known how all of those codes worked together AND that one problem could result in many different codes, UNTIL they got a good diagnosis, they were just throwing parts at "a problem" when only ONE part might fix it . . . unless they desired to know that they wouldn't necessarily need to replace the "unnecessary" parts later on.

 

Carburetors have, to me, two basic types of parts . . . wear parts and deterioration parts.  Wear parts would be where a harder metal moves against a softer "sacrificial" metal, as throttle shafts or power pistons for metering rods (in this case, the main jets themselves).  Accelerator pumps, too, but with softer pump cups against the metallic carb body.  Deterioration parts would be gaskets and seals, typically.  Some, like needles and seats, could be both, possibly. In the case of metering-rod carburetors, the springs can age and lose some of their earlier tension, which makes it easier for engine vacuum to pull the power piston "down" and keep the metering rods in the learner-orientation in some particular engine operational situations (probably mainly part-throttle, lower vacuum, but not "power level" vacuum, accelerations or "pulls" up hills).  In some cases, though, it might be better for earlier engines to run a tad leaner in some situations than their earlier calibrations, plus the Rochester book mentions that best power is made with a slightly lean mixture . . . whereas some like to use a richer calibration for "cooling" to help decrease detonation at WOT. 

 

In prior times, Holley carbs were bad about gasket deterioration between metering blocks and the main housing, which is what made some Carters and Rochesters better choices for OEM and probably one reason Ford (a larger Holley user from 1957+) design their carbs more in the Carter orientation for non-performance engines.  It seems that modern gasolines (even pre-E10) had lost many of the varnish-producing components so carbs don't usually "pattern" from those fuels on the outside of the housings.  Even back then, I would get a kit and two spray cans of B12 to do a carb "kit" activity.  Usually with good results, IF the issue could be fixed by a kit.

 

In more recent times, the solder holding ball bearing "seals" in carb main bodies has become a definite deterioration item with ethanol-blended fuels!!!

 

I STRONGLY CONCUR with Jon's orientation on the throttle body being bushed for throttle shaft-related excess wear!  Everything HAS to be dead on in the resulting machine work.  LIke when you chuck a cylinder head in a fixture, various relationships are already set when the drill press bit refinishes the valve guides.  Once the guides are machined, then you do the valve seats.

 

NTX5467

Link to comment
Share on other sites

10 hours ago, 60FlatTop said:

Maybe Jon wouldn't have walked away shaking his head if he had seen me just replace a few parts that were obviously bad, left a bunch of shiny stuff in the bag, and just put it back in. I didn't want to rebuild it. I just wanted it to work right.

Bernie

 

Concur!  I have done likewise, most recently with the carbs on my 115 Merc Tower of Power.  One carb had a needle valve issue and was flooding.  I replaced only the needles and seats (on all 3) and put it back together.  On the other side of the coin, I have fallen victim to the "while I'm in there I may as well..." with mixed results.  Sometimes there's no problem; other times I wind-up replacing a functioning OEM part with an aftermarket piece (because I was in the neighborhood) only to have it fail a few months later.  Leaves me wondering whether I should have simply left well-enough alone...  :unsure:

  • Like 3
Link to comment
Share on other sites

  • 1 month later...

I still haven't corrected this issue, but I wanted to share another observation I made today. While trying to quiet my ever returning throttle squeak, I played with the throttle at the carb while it was running. I noticed that if I open the throttle at the carb, with the car in neutral, there is no hesitation. It's only while in drive.

Link to comment
Share on other sites

In "D", the engine has the load of the torque converter/trans it's running against.  I'd make sure the base idle speed and mixture, combined with the ignition timing AND accelerator pump adjustment are all "in spec" first.  Then, increase the base idle speed about 25rpm and see how it acts in gear.  Might try tweaking the ignition timing for about 2 degrees BTDC more, too.

 

Might also be some wear in the accelerator pedal pivot bushing on the inside of the firewall, too!  You push on the accel pedal, but until all of the wear/slack is moved through, nothing gets to the carburetor throttle lever.

 

NTX5467

  • Like 1
Link to comment
Share on other sites

10 hours ago, NTX5467 said:

In "D", the engine has the load of the torque converter/trans it's running against.  I'd make sure the base idle speed and mixture, combined with the ignition timing AND accelerator pump adjustment are all "in spec" first.  Then, increase the base idle speed about 25rpm and see how it acts in gear.  Might try tweaking the ignition timing for about 2 degrees BTDC more, too.

 

Might also be some wear in the accelerator pedal pivot bushing on the inside of the firewall, too!  You push on the accel pedal, but until all of the wear/slack is moved through, nothing gets to the carburetor throttle lever.

 

NTX5467

Probably due to lower starting rpm in "D" and slower rpm change due to load.  With your foot on the brake and in "D" bring up the rpm to produce the stumble; repeat in the the hood (with a helper's foot on the brake!).

Link to comment
Share on other sites

The comment about the hesitation in gear versus neutral simply reinforces that the engine WANTS more idle fuel. When in gear, there is additional load of the fluid coupling losses in the torque converter. This requires more fuel.

 

The comment someone made about these issues often turning up AFTER a rebuild can probably be explained by the fact that fuel valves tend to wear on the sealing edge, allowing the float to move slightly upward (do the math, a change of only 0.01 at the fuel valve may mean a change in the float at the far end of almost a tenth of an inch!). This raises the fuel level in the bowl artificially high, and increases both idle fuel and main circuit fuel. The main circuit addition causes the engine to run rich and burn too much fuel, but the idle circuit performs normally.

 

There have also been some comments about leather versus synthetic accelerator pump cups. The manufacturers did not change to the synthetic material because it was better, rather because it was CHEAPER. At manufacturer quantities, even the modern synthetic pumps cost less than a dollar each. Contrast that to a minimum of about $9.00 for a easy to make leather pump, and as much as $20.00 each for some of the more complex ones. It has been proved over and over that no synthetic pump material is as good as the underside of a cow!

 

The comments about replacing only those parts which need replacing. I won't argue the merits of a hobbyist doing this, but consider this philosophy. When I was still rebuilding carbs; if I rebuilt a carb and the carb gave trouble, and the customer opened it up to find used (even good used) parts, what kind of comments do you think the customer would post on the internet? As far as the quality of parts is concerned, all of the parts I use are equivilent to (or better than) original (NO offshore parts here, and there won't be as long as I wake up in the morning on the top side of the ground!).

 

Jon.

  • Like 1
Link to comment
Share on other sites

It was an easy test; but you don't need more pump squirt, what you need is more idle fuel (not idle fuel and idle air). The fuel that prevents the hesitation BEFORE the pump squirt is that coming through the idle transition slot, which is why you increase the size of the idle tubes (jets).

 

Jon.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...