Dual Carburetor Synchronization

Rolls Royce Silver Cloud and Bentley S I

July 20, 2002 - Saratoga, CA           (click for event Pix)

Jim Weager Presiding Technically

Hosts - Barbara & Austin Kilburn

                                                              click on the title for PAGES:

INTRODUCTION……………………………………………..    2 

MATERIALS  NEEDED………………………………………    2 

TOOL  SUGGESTIONS…………………………………….…   







G- IDLE  SPEED  ADJUSTMENT……………………………  10 



I- FINAL  CHECK……………………………………………..11 


K- FINAL ASSEMBLY……………………………………….. 12 

THE  STICKING  PISTON…………………………………….. 12


Prior to attempting the following procedure, it is assumed that the engine and the ignition system, including the fuel pump, fuel filter and air filter are all in good working order. 

After the removal of the Air Cleaner and Cold Air Runner, if you are absolutely certain that the needle position is the same on both carburetors and positively sure that both throttle plates are in synch with each other, you may begin at Step E- Setting Throttle Lever Clearance.  

Materials needed: 

Gasket (1) Choke Body to Cold Air Runner

Gaskets (2) Cold Air Runner to Carburetor 

Note: The use of the old gaskets is certainly permissible.  The key here is that they are in good condition.   Small air leaks due to poor gaskets are not fatal because it represents an air leak prior to the introduction of the fuel mixture.  A major cold air leak could create an audible hissing sound, weaken the effect of the air cleaner, and most certainly implies maintenance practices inconsistent with your PMC. 

Tool Suggestions: 

Note: The short length of the tools, while not essential, will assist in the application of the tools in tight places. 

                        Screw Driver

                        7/16th inch box and open end wrench (suggest 4 inch length)

1/2 inch box and open end wrench (suggest 5 inch length)

13/16th inch box, open end or socket

9/32nd inch box and open end wrench (suggest 4 inch length)

Feeler Gauge (.0187 in.-.020 in. & .098 in.)

Steel Ruler, 6 in.

Putty Knife 1 inch or narrower

UNI-SYN ™ (flow gauge)*


Two (2) Strips of Typing Paper 1/4 in. x 6 in.

Fender Cover

Strong Box or Stool to stand on

*UNI-SYN ™ may be purchased from Performance Products 1.800.423.3173.  www.performanceproducts.com  $28.95

or other performance or sports car supply source. 

Note: It is better to begin the following steps with a cold engine.  This will make disassembly more comfortable.


1.  Remove the air cleaner assembly from the Choke Body (Fig. A)


Fig. A          1. Choke Body              2. Cold Air Runner

 Caution: Be careful in the following steps not to drop any nuts, lock washers or screws.


            2.      Remove the 4, 1/2 in. nuts and lock washers from the studs of the Cold Air

      Runner attached to the Carburetor Intake Throats. 

3.      Remove the 4, 7/16th in. nuts and lock washers from the studs of the Cold Air

Runner attached to the Choke Body. 

4.      Remove the Cold Air Runner from the engine. 

Note: Be careful not to kink the vacuum tube or bend the linkage.


1.  With a 13/16th in. wrench loosen and remove the Hydraulic Dampers from the

     Piston Chambers.  Be careful to note which damper belongs to which chamber. (Fig. B)

Fig. B          Damper removed

  2.      Place a finger into each carburetor throat and move the piston up and down,

checking to see that it moves freely and bottoms to the seat without binding.  This will serve as a benchmark when performing step D-1.     (Fig. C) 

If binding is present, go to THE STICKING PISTON found on the last page of this manual.


Fig. C       Checking for free movement

3.  Remove the 4 screws which attach the Suction Chambers to the body of the Carburetor.

Be careful when removing the chamber from the Carburetor Body.  There is a long spring inside the chamber at the end of which is a perforated conical washer. (Fig. D)    

Keep the correct chamber and spring/washer with the correct damper and remember to which carburetor body they belong.

Fig. D

             4.  Being careful not to apply side pressure, lift the Piston straight up out of the

Needle Jet.  (Any side motion during this procedure could bend the Needle or damage the Needle Jet.) 

5.  With the Steel Ruler, check to see that each needle extends the same distance

     from the piston skirt.  If not, adjust one of the needles to equal the other.  This

     can be achieved by loosening the set screw located on the side of the piston

     skirt.   Hint: In most cases there is a ring groove cut into the shaft of the needle that acts as a

      visual assurance of proper positioning of the needle.  (Fig. E) 

6.  Check to be sure the Piston and the inner walls of the Suction Chamber are

free from heavy carbon deposits.   If cleaning is in order, use carburetor cleaner or lacquer thinner and a soft rag.  Under no circumstance should you use steel wool or sandpaper on these surfaces. Proper tolerance between piston and chamber is critical.

Fig. E      Confirming length of needle extension

1. Ring Groove location   2. Needle Set Screw


  1. Back off the throttle lever set screw until it no longer touches the stop.
  2. Pull up on the throttle lever so that the throttle plates in each intake bore begin to open.  While keeping the plates in the open position, place a long 1/4 in. wide strip of thin paper in the intake bores.  Let the throttle lever close so that the throttle plates close on the paper strips.  (Fig. F)    A gentle tug on each strip of paper should yield no reaction.  The papers should remain in place. 

Fig. F      Testing the Throttle Plate position

1.  Throttle Plate   2.  Paper Strip    3.  Lift Pin

  Note:  If one or both throttle plates fail the previous test, proceed to Step 3.   Otherwise, continue on to

          Section D, Carburetor Suction Chamber Assembly.  

3.  Loosen the Pinch Bolts that are adjacent to the flexible couplings.  Using your finger, gently push the throttle plate in each carburetor until it is closed.  Retighten the Pinch Bolts. (Fig. G)      Again try the test in Step 2.

Fig. G     Throttle Shaft Pinch Bolts


  1. Re-assemble all of the components back in their proper place.  Do not tighten the

four screws which secure the Suction Chambers to the body of the Carburetor.     With the screws still loose, place a finger into the carburetor throat and move the piston up and down (Fig. C), checking to see that it moves freely and bottoms to the seat with out binding.  While repeatedly moving the piston up and down, snug up one screw and then the screw across from the first screw, continue the  tightening in a crisscross fashion until all are snug.  Check one more time for free piston movement.  Then repeat the same procedure on the second carburetor.   If binding occurs, wiggling the chambers around the axis of the mounting screws, with the screws softly seated, should fix the problem. Note: If the pistons moved freely in step B-2, the lack of free movement at this point is likely caused by play between the mounting screws and the mounting holes in the flanges at the base of the Suction Chambers.  Should the wiggling of the Suction Chambers around the axis of the mounting screws not solve the problem, you will have to check to see if you might have accidentally bent the Needles in the handling process.

    1. Remove the offending Needle from it’s mounting bore.
    2. Place the Needle on a flat surface and roll it.  If the needle is bent, it will roll in an irregular fashion.
    3. Straighten the Needle  


      1.   With the Throttle Stop Adjusting screw backed off from the Throttle Stop, place a

            .0187 in.- .020 in. feeler gauge between the Throttle Stop and the tip of the

Throttle Stop Adjusting Screw.  Turn the Throttle Stop Adjusting Screw in until it touches the feeler gauge.  (Fig. H) 

Fig. H      1.  Adjusting Screw   2. Feeler Gauge

The following procedures employ the use of the UNI-SYN ™ (flow gauge) 


1.      Check to be certain that the Fast Idle Cam is disengaged from the Fast Idle Set Screw.  (Fig. I) 

2.      Turn in both of the Slow Running Adjusting Screws until they stop.  Now   back the Slow Running Adjusting Screws out 2 turns. (Fig. J) 

3.      Start the engine and hold the UNI-SYN ™ (flow gauge) against the intake bore of one carburetor.  Note the position of the “pea” in the site glass of the flow gauge. Now place the flow gauge against the intake bore of the other carburetor and turn its Slow Running Screw in or out until the “pea” in the site glass is in the same position as on the previous carburetor.  The carburetors are now synchronized. (Fig. K)


Fig. I          1. Set Screw  2. Fast Idle Cam






Fig. J            Slow Running Adjustment





Fig. K

Applying the UNI-SYN ™

The Indicator Pea


Hint: If the engine refuses to idle smoothly, engage the Fast Idle Cam against the Set Screw one or two steps until the engine is running smoothly.  Let the engine continue to run until it has reached normal operating temperature.  Then once again disengage the Fast Idle Cam from the Set Screw.        

          4.   Attach the Tachometer.


  1. Proper idle speed is achieved by turning in the Slow Running Adjustment

Screw (to reduce idle speed), or turning out the Slow Running Adjustment Screw (to increase the idle speed).  Idle Speed for Silver Cloud I and S 1 is 400 to 425 r.p.m. Remember, you are dealing with two carburetors that both contribute to the idle speed on an individual basis.  When you turn one Slow Running Screw in or out, you must turn the other Carburetor’s Slow Running Screw the same amount.  Make your adjustments in small increments. (Fig. J) 

Note: If you set the idle speed too high, by turning out the Slow Running Screws too far, the carburetors will whistle at speed. 


Fig. L    Mixture Adjustment Screw


1.      With the Fast Idle Cam still off the Set Screw (disengaged) and the Tachometer still functioning, turn one of the Mixture Adjustment Screws out until there is a change in the idle speed (it should drop in r.p.m.).  This is a lean mixture. (Fig. L) 

2.      Raise the engine speed for a few moments by lifting the Throttle Lever.  This will

      help to keep the spark plugs from fouling.  This should be done periodically while

      you are executing the Mixture Adjustment. 

3.      Turn the Mixture Adjustment Screw in a half turn at a time until the engine is

      running smoother. 

4.   Repeat the same process on the second carburetor.

Note: At this point the engine may be running nicely but the mixture must be tested to be sure it is correct for each carburetor.


5.  Lift the Mixture Adjusting Screw with your fingers about 1/16th inch and hold that position off the stop. If the mixture for that carburetor is correct, the r.p.m. will increase slightly and then settle to the original speed.  If the r.p.m. drops and stays there the mixture is too lean.  If the r.p.m. increases and stays at that level, the mixture is too rich.  

Alternate Methode: Under each Suction Chamber is a Lift Pin.  It is located on the right side of each carburetor. (Fig. F No. 2)  Push the Lift Pin up about 1/16th inch and hold as in Step 4.  The Lift Pin on the rear carburetor is easy to push with your finger. The Lift Pin on the front carburetor is very difficult to push because of the Float Bowl location.  But it can be done.     


  1. With the engine still running be certain that the r.p.m. is within specification;

Idle Speed for Silver Cloud I and S I  is 400 to 425 r.p.m.  For vehicles fitted with air conditioning, you may wish to increase the idle speed to 550-600 r.p.m. 

  1. With the UNI-SYN ™, once again insure that both carburetors are operating at an equal volume.  If not, each Slow Running Screw must be adjusted until both carburetors are balanced and the engine is operating at the desired r.p.m.

 Note: With the carburetors properly in synch, slow acceleration should yield smooth response from the engine.  No shudder should be present.     


  1. With the Throttle Plates still in the closed position, back the Set Screw away from

 the Fast Idle Cam.  Place a .098 in. feeler gauge between the Cam and the

Set Screw.  Turn the Set Screw in until it touches the feeler gauge. (Fig. M)

Fig. M


1.      Place a liberal coating of petroleum jelly or spray dry silicon on both sides of the new gaskets.  This will allow disassembly at a later time with no damage to the gasket material. 

2.      Replace the Cold Air Runner. 

3.      Replace the Air Cleaner Assembly. 


A sticking piston may be indicated by one or more of the following:           

A.     Stalling

B.     Poor Slow Running

C.     Lack of Power

D.     Abnormally High Fuel Consumption 

A sticking piston, one which does not move freely up and down, may be caused by one or more of the following maladies: 

1.      Carbon build up on the sides of the Piston Assembly

a.       Remove the Suction Chamber as described in B- 3.

b.      Follow the steps B- 4. and B- 6.

c.       If no carbon build is noted go to Step 2. 

2.      Fowled Needle

a.       Lower the Jet by pulling up on the Mixture Lever which is attached to

the Mixture Adjusting Screw (Fig.  L).  If the binding goes away, the problem lies between the needle and the seat.

b.      Centralize the Main Jet.  The procedure is too involved to describe here.  Refer to “Work Shop Manual for the Rolls-Royce Silver Cloud”, page K14,  “Main Jet- to centralize”           

3.      Bent Needle

a.       Loosen the Set Screw as described in B- 5.

b.      Follow the steps as described in the “Note” in D- 1

  The Event   (click on any picture to enlarge it)

WhyThatQuestion_.jpg (65359 bytes) TrustmeAustin.jpg (75421 bytes) Question.jpg (78168 bytes)
(Jim) MAN! That's a good Question. Austin....TRUST ME! (Jim) Questions?...Naw-Swapping Lies
D.Rogers,I.Gallo&R.Murray.jpg (88639 bytes) Austinindeepthought.jpg (61388 bytes) MarkBaroni,th.jpg (85452 bytes)
(l to r) Rogers, Gallo, Murray Well....back when I.... (Austin) Can you imagine a garage AND a shade tree?  Austin must have fallen into it!

Other PMC Attending   (click on any picture to enlarge it)

SallyMarden'sSI.jpg (67971 bytes) IvanGallo'sMkVI.jpg (92845 bytes) DaveRoger'sShadow.jpg (85039 bytes)  
Sally Marden's SI Ivan Gallo's MK VI Dave Roger's Shadow