The Gray Lady

Rochester 4GC Carburetor Rebuild 

This page presents information on:  My rebuild of a Rochester 4 bbl.  (4GC - 4 Jet) carburetor, with photos 

I have undertaken the rebuild of my 4 bbl. Rochester carburetor.  This project has been on the "to do" list since the fall of 2009.  I finally made the determination I would cross it of my list!  The first task was to order the rebuild kit.  Mine was acquired online from Daytona Parts Co.  I purchased it only after confirming via email that the kit materials were ethanol compatible.  The cost for the kit was around $50.

In preparation for this project I bought a bunch of used Kent-Moore tools/gauges, and a Moroso carburetor stand.  I also collected as much Rochester Carburetor info/data as I could.

TOOLS REQUIRED:  Camera, box wrenches, flare wrenches, screwdrivers, hammer, needle-nose pliers, a good metal ruler graded in 32nd's of an inch and with  a set capable T-bar, parts cleaning fluid, parts cleaning brushes, toothbrush, q-tips, a source of compressed air and of, course, shop towels.  Depending on your circumstances, you may also need a tubing cutter, tubing bender and a tubing flaring tool.

Let's get a little nomenclature down at the start, to assure we are all on the same page when describing both the carburetor and the rebuild process.  My use of the term carburetor refers to the entire assembled unit.  That unit is made up of 3 main parts.  I shall refer to them as the airhorn, fuel bowl and throttle body.  Each constituent part has a number of smaller, but nevertheless critical parts [See exploded view diagram].  References in this article reading (#00) are to the exploded view diagram found on page 1 of the Rebuild Instruction Sheet. 

Step #1, to mention the obvious, is to remove the air cleaner housing.  In doing the actual rebuild, the very first tool I reached for was my digital camera.  I took pictures of the carb, in place, from all sides and angles.  This was so I would have a visual reference of how a properly installed carb looked when I got around to putting her back on the intake manifold.  I took shots of the linkage connections, automatic choke setting, etc.  I also made a handwritten note of the choke setting.  This is what the carb looked like at the start of the project   

Carb at start of rebuild      Venturi, Throttle Bore, Throttle Plates  

Next, I disconnected all the linkage, so the carburetor would be free to come off the manifold once it was unbolted.  Then I removed all the lines going to the carb:  choke tube, fuel line and vacuum line (?).  The question mark is there because there is a small diameter, flared metal line with a fitting attached to the base of the carburetor/throttle body.  This line runs down between the underside of the intake manifold and the top of the engine valley cover, toward the back of the engine.  I have yet to ascertain what this line is for, or does?  [NOTE:  ==>] I have since ascertained that it is, in fact, the distributor vacuum advance line.

Once I removed the four nuts from the manifold mounting studs, the carburetor came off easily.  At this point I covered the intake manifold ports, to prevent dirt or parts from falling into the manifold, and ultimately the engine.

The airhorn is what one sees when looking down at the top of the carburetor.  In vintage cars, the air cleaner sits directly on top of it.  The airhorn top is home to the  accelerator pump and choke butterfly plate.  On the right (passenger) side of the airhorn is the automatic choke.  The underside of the airhorn is where one finds the floats, power piston, float needles and seats.

With the carburetor on the workbench, I drew a diagram of the top of the airhorn, giving each airhorn screw a number.  Each screw was removed and then labeled with the appropriate number.  This permits putting the same screw back into the same hole on reassembly.  One thing I did notice when removing the airhorn screws was that none of them seemed to be terribly tight; nor were the carburetor mounting bolts for that matter.  In fact, they all seemed to be rather loose for my taste.  I wonder if they had ever been touched since being originally installed over a half-century ago,  =  w   a   y  =>   back in 1954?  For inexplicable reasons, one of the 13 airhorn screws did not have a lockwasher?  Nor was there an in-line fuel filter in the system?

One of the airhorn screws also holds the carburetor ID tag.  While not critical to the operation of the carburetor, it is important for both identification and historical purposes.  It provides the carburetor model # and the manufacture date code.  The numbers on my tag are 700790 and M4.  For the date code, the letter "M" is the month.  The number "4" is the year in the decade.  M4 equates to December of 1954.  This is because M is the twelfh letter of the alphabet, in sequence.  "I" is not used to avoid possible confusion with the number '1'.  Do not lose, or destroy this tag and be sure to reinstall it!

BE ADVISED:  The folks at Daytona Parts must assume if you are buying one of their rebuild kits that you know what you are doing.  This is neither a good, nor a necessarily valid, assumption.  (Even though I have turned a lot of wrenches on a lot of cars, and even replaced carburetors in the past, this was my first ever carb rebuild.)  I say this as, the accompanying kit instructions are more than sparse [See Rebuild Instruction Sheet].  In order to determine what went where, I found myself comparing the replacement parts with parts I could spot and remove.  Not the best way of doing things, in my humble opinion!  It is also a bit frustrating, as not all of the supplied parts are actually used in the rebuild.

Fuel BowlRemoval of the airhorn exposes the innards of the fuel bowl.  Here is where one finds the venturi, nozzles, valves, jets, springs, etc.  In my case, I also found a  rather heavy layer of gunk and varnish!  So much so, that I was amazed the carburetor functioned at all.  Everything seemed to be coated.  (As this is an "after picture," the layered gunk is not depicted.)  Flushing the fuel bowl with parts cleaner removed some of it, but not all.  Here, the use of brushes, q-tips, shop towels and elbow grease is most definitely required.

The venturi clusters are easily removed.  Three screws hold each in place.  New gaskets for the clusters come in the kit.  The clusters, as well as the fuel bowl body, contain orifices, jets, nozzles, etc.  Any where there were such, they were flushed with parts cleaner and blown out with high pressure air.  One kit part I could not locate on my carb was a small check ball (ball bearing like item).  I knew the carburetor had one, but did not know where it was.  A review of the exploded view of the carburetor provided in the kit showed it (#61) to be in the fuel bowl body, but where?  I also noticed a small metal bar inside a bore in the fuel bowl, under the primary venturi cluster (#59).  As there was a new one in the kit, actually a very small T-plate, I knew it would come out.  I was able to remove it with a pair of needle-nose pliers.  Under the T-bar (properly named as a pump discharge spring guide) was a long, thin spring called what else, but the pump discharge spring.  When the spring was removed the mysterious check ball was revealed!  Both it and the spring were replaced.

Here is where the hammer comes into play.  Yes, I know it sounds a little strange to be using a hammer on a carburetor.  The new T-plate lock/pump discharge spring guide has to be set in the bore.  The way to assure it is in fact properly set is to lightly tap it in place, using a screwdriver, dowel, or somesuch as the driving object.

I must confess, while the kit comes with two (2) check balls of different sizes, I found only the one; the larger of the two.  I do not know if the second is used in my carb or not?  I have emailed Daytona Parts for clarification, and will report on the answer received.

I saw no replacement parts for the throttle body portion of the carburetor, other than the idle adjusting needles/air mixture screws.  Consequently, I initially determined that in my case, there was no need to remove the throttle body from the fuel bowl during the rebuild process.  However, the throttle plates and bores needed some serious cleaning.  The gunk buildup on them was such as to prevent the throttle plates from fully seating.  Both sides of the throttle plates are not easy to access, absent dismantling the throttle body.  I subsequently decided to remove the throttle body from the fuel bowl.  I am glad I did.  On thoroughly cleaning it, I found many orifices and passages.  They only became visible after separately flushing the throttle body with cleaning fluid and using the blow gun.

Float AssemblyOk, back to the airhorn, specifically the underside.  Here one finds the float assemblies.  The float assemblies are held in place with (drift) pins (#42).  These pins come right out by hand.  Each float arm pivots on a needle which travels inside a needle seat.  BE CAREFUL HERE!  It is critical that one observe which way the needle is positioned inside the seat.  It can be installed two different ways.  NOTE:  Make a written notation which is the seat end, and which is the pivot point end.  With the needle removed, the needle seat comes out easily with a wide, flat blade screwdriver.  Be sure to put the supplied gaskets on the new needle seats before installing them in the airhorn.

My floats had a slight coating of gunk on them.  I presume this coating could affect float movement/travel.  Consequently, it was gently removed.  Once cleaned, the floats have to be inspected for leaks.  If a float has a pinhole leak, the float can fill with fuel, and not function.  A simple test is to place the float in a pan of water.  Does it float?  If it does, all is ok!  If not, you have to either repair, or replace it.

A critical step in reinstalling the floats is to be sure to FIRST INSTALL THE AIRHORN GASKET.  Once the float arms are in place the gasket will not go on!  The airhorn gasket must be in place, as its absence throws all your measurements off.  Measurements, what measurements?  The float levels must be precisely set for the carburetor to function properly.  The float levels determine when the needle comes off the seat, allowing gas to enter the carburetor fuel bowl.  The float arm travel parameters determine the low and high levels of gas in the fuel bowl.  This is where use of a good ruler graduated in 1/32nd's of an inch is critical.  Technically, one could use the flimsy, paper ruler that comes with the rebuild kit.  Truthfully, I did not find it to be very handy, practicable, or useful.  All required measurements are taken from the underside of the airhorn, with the ruler held tight against the gasket.  Without it being in place, all measurements are off by the thickness of the gasket.  The rebuild kit does provide a measurement specification sheet.  This sheet also tells you where to bend the float arm to adjust for the needed measurements.  A WORD OF CAUTION:  The float assembly is delicate.  Handle it with care!  Be prepared to have to make repeated measurements and adjustments.  When setting the float for the closed position, don't be surprised to find that doing so may throw off the open position setting.  One must also make sure the floats are parallel, and are able to travel freely within the confines of the fuel bowl.  The best way to assure this is, with the float arms in the closed position, does the airhorn gasket clear the floats?

Now, it is time to start buttoning up the carb.  First, install the throttle body back on the bottom of the fuel bowl, using the new gasket that comes in the rebuild kit.  Next, insert the new accelerator pump spring in the accelerator pump bore/well of the fuel bowl.  Put a small amount of gas in the fuel bowl to ease the initial start.  DO NOT TRY AND FILL THE FUEL BOWL!  I recommend filling it 1/3 full.  Install the new accelerator pump in the airhorn.  Batten down the airhorn to the fuel bowl.  When tightening the airhorn screws, use an alternating, criss-cross pattern.

The next step is to install the new idle adjusting needles/air mixture screws in the throttle body.  THE CURRENT SETTINGS OF THESE SCREWS MUST BE DETERMINED BEFORE THEY ARE REMOVED.  This is accomplished by counting and recording the number of turns to lightly seat each screw.  Once that is accomplished, the screws may be removed.  The new air mixture screws are installed and lightly seated.  Then, they are backed off the precise number of turns you previously recorded.  This is your starting point when adjusting the settings on your installed and running carburetor.

It is now time to clean the intake manifold gasket seat.  Use a scraper to remove all traces of the old gasket.  TAKE CARE THAT NO GASKET DEBRIS ENTERS THE INTAKE MANIFOLD.  Install the new carburetor gasket on the intake manifold.  No gasket adhesive is required to be used here, or anywhere else in the carburetor.

HINT:  Now is the best time to do any work you want to perform on the distributor.  Your access to it will never be better.  At the very least, check and set the point gap before reinstalling the carburetor.  Oil the distributor bearing while you are in the vicinity.  In my case, at this point I also installed an in-line NORS glass bowl type fuel filter.  I cannot believe there wasn't one in place!  Hopefully, the fuel filter will capture all the previously observed gunk before it can enter the carburetor?

Installation of the carburetor is the reverse of the removal.  Put it in place and attach the stud bolt mounting nuts.  Now, attach the linkage.  Check that the linkage moves freely through its normal cycle.  Then attach your fuel and vacuum lines, and the choke tube.  At this point it is necessary to adjust your choke setting to assure it closes properly, and opens correctly during the engine warming process.  The choke butterfly should not be tightly closed on a cold engine.  In fact, the specs say there should be .040 clearance between the closed butterfly and the carburetor bore.

If the fuel bowl of the carburetor is bone dry at this point, it will take some pumping of the accelerator pedal and a considerable amount of cranking of the engine to fill it.  Remember, without gas in the fuel bowl, the engine will never fire.

Once she fires, adjust the idle adjusting needles/air mixture screws to fine tune the carb to run at its most efficient.  Set the fast and slow idle speeds and enjoy your newly rebuilt carb! 

Here are some select pics of the rebuild in progress

Accelerator Pumb Bore in Fuel Bowl     Accelerator Pump     Float Needle Seat  

Accelerator Pump Bore (Highlighted)           Accelerator Pump                                             Float Needle Seat on Airhorn Underside
Venturi Cluster/Spray Nozzle (Right) 

MISTAKES MADE:  So far (fingers crossed on 25 Apr), the only mistake made was in the purchase of a carburetor stand.  The one I bought accepts two different sized carburetor bases.  Size selection is accomplished merely by flipping it over.  The problem, neither size matched the base of the Rochester 4GC.  While it was still able to be used, it did not work as well as a proper one would.

After proselytizing on the subject of determining idle adjusting needles/air mixture screw settings, I proceeded to remove one without first counting the number of needed turns to seat it!  So, I had to default to a generic setting.  Not good!

When I bent my fuel line tubing, I did not pay attention the location of the fittings.  When I made my bend both fittings were at the same end of the tubing.  Needless to say, that makes it impossible to slide the one fitting around the curve you have just created in the tubing!

LESSONS LEARNED:  The taking of "before" pictures turned out to be more than helpful.  I had to resort to one photo to properly reconnect the accelerator linkage.

If you are lucky, you will not need to replace any metal fuel lines.  Unless one is experienced and adept at bending and flaring tubing, have someone perform this service for you.  My initial attempt in trying to install the glass bowl in-line fuel filter was a disaster.  It leaked like a sieve at every possible connection!  The first flaring tool I bought/used simply did not work.  It would not grip the tubing tight enough to permit the flaring head to do its thing.  The second one worked much better.  The problem with the second tool is that the tube tended to crack/split at the flare during the procedure.

Bending of tubing is a bit of an esoteric art.  It does require the use of a tubing bending tool.  My particular requirement called for two 90 bends.  That in itself is not that difficult.  The problem arises from the fact that what is really required on one is a compound bend.  The 90 bend at the filter end is routine, and no problem.  However, to get the tube fitting to line up with the fuel filter fitting, the tubing needs to also be bent on its axis.  That is no easy task!  Since it is not, I am now looking at using an angled fitting (elbow) on the fuel pump to attach the fuel filter in such a manner so that it will line up properly with the line running out of the fuel filter and into the carburetor.  It is hard to explain, but when one is under the hood trying to make all the necessary connections, it becomes painfully obvious what is required:
|Carb  |--------------------
|_____|                            /
                                 (o) Fuel Filter
    Fuel Pump  O ----- |

Much to my surprise and chagrin, even though I took pains to be sure I did have gas in the carburetor fuel bowl when I remounted it, the engine to date (2 May 2010) has yet to fire?  I am getting explosions, but not sustained firing?

[9 May 2010]  I have removed the fuel filter entirely, and have reverted (temporarily) to using the original line, sans filter, that ran from the fuel pump to the carburetor.

As I indicated above, when I was finished I had a check ball left over.  That I did made no sense.  So, I sent off an email to the Daytona Parts Co. folks asking what it was, and where it went.  To their credit, I did receive a very prompt reply.  The leftover check ball (#74) goes in the bottom of the accelerator pump well/bore.  A spring sits on top of it, and the accelerator pump sits on top of the spring.  Depending on the position of the accelerator pump during operation, the left over check ball controls the flow of fuel to either the carb fuel bowl or the primary venturi cluster.  Without it being installed, all fuel entering the carb went to the fuel bowl.  Without it being installed, pumping the gas pedal only yielded exercise of the ankle/foot muscles!  Duh!  No wonder the car would not start?  I never saw, or found, the original check ball that was in this location.  Perhaps it flew into orbit when I blasted the accelerator pump well with compressed air?  I don't know?  It is a mystery that shall continue to go unexplained, or at least until I clean out my parts washer tank.

[16 May 2010]  The carb is again removed from the engine and opened up to install the left over check ball in the accelerator pump well/bore.  After installing it, closing the carb back up and remounting it on the engine, with a little cranking she fired and ran!  I will admit, at first she sounded a bit like a concrete mixer, but once I got the engine warmed and the carb able to idle on her own I was able to make the necessary idle adjusting needle/air mixture screw adjustments to simulate normal operation.  Then I checked operation on acceleration.  She accepts the gas feed and has very good response.

I also have my fuel line leak situation rectified.  It took an hour of searching through parts cabinets and catalogs with my local NAPA parts guy to find the right 90 elbows, male-to-male fittings, etc., but the fuel pump now sends the gas from the tank into the filter before going onto the carburetor, and it does it without leaking!  There now is only a single tube running from the fuel filter-out port to the carburetor, and it has a single bend of 80 or so.  The fuel pump is now directly connected to the fuel filter-in port via fittings, eliminating the need of any tubing there.  This part of the project was better suited for the skills of a plumber, than a mechanic.  Nor am I at all ashamed to acknowledge that not only is plumbing not my forte, but that also, I hate plumbing! 

BOTTOM LINE:  Carburetors are not as complicated as they appear.  It does not take a specialist to do a rebuild.  An individual who regularly works on/maintains cars (by that I mean more than merely changing oil and anti-freeze) is more than up to the task.  What it does take is a solid base of knowledge of engine mechanics, a bit of patience, attention to detail and clean working conditions.  If you are contemplating taking on this task, I hope you find the detailing of my experience as a first time, novice, carb rebuilder to be of help.  Don't be afraid to give it a shot.  My advice, go for it!            

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Created 17 May 2010 - 11:44:59 Hrs.

 Copyright 2010 - 2014, G. David Yaros.  All rights reserved.