How to Tune the Stromberg CD Carburetor — Complete Manual

.. . , -· ·t ... 1: ,. '· Q• " I I ' .1 . ,, . ':· - I AID NO. :' 510 40 I· SUBJECT: CARBURETION .__ - MODEL: '· ALL i· • I r· I • .. .· '. • < :· . '\ / . ·. ... .. . :.• . .... ... ;. · .. MG .... ... http://www.triumphwedgeowners.org

_, I N DE X PREFACE ••• CAR.B URE'IION . . . . . . . TEMPERATURE COMPENSATOR • • IDLE/SLOW" RUN EMULSION CIRCUITS CD4 CARBURETOR BYPASS VALVE OR OVERRUN VALVE VENT VALVE MANUAL CHOKE AUTOMATIC CHOKE (BI-METAL SPRING TYPE) AUTOMATIC CHOKE CAPSULE TYPE) DIAGRAN - CDSEV CARB URE TOR RIGHT SIDE DIAGIVJ1 - CD4 CARBURETOR RIGHT SIDE DIAGRAM - CD4 CARBURETOR LEFT SIDE DIAGRAM - CD4T 150 CARBURETOR LEFT SIDE DIAGRAM - CDSEV CARBURETOR LEFT SIDE DIAGRAM - CD4VT 175 CARBURETOR LEFI' SIDE . . . DIAGMM - CD5T 17 5 CARBURETOR CROSS SECTION • DIAGRAM - CDSEV CARBURETOR EXPLODED VIEW DIAGR..t\M - CD4 CARBURETOR EXPLODED VIEW DIAGRAM - CDFEVX 175 CARBURETOR •••• DIAGRAM - CDFEVX 175 CARBURETOR EXPLODED VIEW • DIAGRAM - CD4VT CARBURETOR EXPLODED VIEW Sl040 i • :;·,, ' .. - .,.... ......... ----- ":'" . - - ... -·-· ' ... ' . , 1 2 6 7 7 8 9 9 9 12 ·.·.::·]·1. 15 ·:.i) 16 17 18 19 20 21 22 23 24 25 26 . i http://www.triumphwedgeowners.org

; 1 . I" I r .I i I r .;:J• T • ;. ' .. 1 N DE X (Cont.) ITEMS THAT MUST NOT BE CHANGED IN SERVICE • BYPASS VALVE ADJUSTMENT (Where Applicable) VEN'.! VALVE ADJUSTMENT . TUNING: Idle Adjustment . ·. • • • . . . . . . . Fine Mixture Adjustment • • . . . . Needle Adjustment . . . • . Fast Idle Adjustment. (Manual Choke) AUTOMATIC CHOKE (BI ... METALLIC SPRING 'r'LPE) • Sl040 Check and Adjust Remove and Refit 27 27 28 30 31 31 32 33 33 33 - ·- - --" ·: ...... . ............. ·•. :!) • . J; . '• r.\', • http://www.triumphwedgeowners.org

i ,, ?' .. \ ,. .. ·: · :·:i ··I :"•' :I ':1 :; I .,, '' ! .. , : . .. . . ·. ,, ,r-: ., ··' l . .. . ;:j ' ,'! : ' I ,. .. l! t} t ·:; 1 "· '1 .' l .. ... . .. .. ,. .. !; .. l ,I ,, ·.I ':.! I .· .. I ·· . ,. ·: ' I ... ., '' .. P R E F A C E The information contained in this booklet is provided as an easy refer- ence guide For technicians. More detailed information is available in the appropriate Workshop Manual. S1040 HIGH ALTITUDE VEHICLES VEHICLES SOLD IN HIGH ALTITUDE REGIONS MUST HAVE THE CO CHECKED WITH INFRARED EQUIPMENT AND, IF NECESSARY, THE MAIN METERING NEEDLE ADJUSTED TO BRING THE VEHICLE IN SPECIFICA- TION. THE JAGUAR XJ6L AND MGB REQUIRE FIT- MENT OF SPECIAL HIGH ALTITUDE NEEDLE (S) , NEEDLE NUMBERS AND 45M RESPECTIVELY, PRIOR TO THIS ADJUSTMENT. WHEN DESCENDING TO LOW ALTITUDES, THE CO MUST AGAIN BE CHECKED WITH WFRARED EQU IPMENT AND THE MAIN METERING NEEDLE ADJUSTED TO BRING THE VEHICLE INI'O SPECIFICA:.. TION. THE XJ6L AND t-1GB MUST BE REFITTED WITH LOW ALTITUDE NEEDLE(S) BEFORE THE AFOREMENTION- ED ADJUSTMENTS ARE CARRIED OUT. ALL ADJUST- MENrS MUST BE MADE BY MOVING TilE MAIN METERING NEEDLE • FOR DETAILS ON HOW TO ADJUST CARBURETORS, SEE PAGE 30 OF TH I S TRAINING AID • THESE INSTRUCTIONS APPLY TO FEDERAL SPECIFICA- TION CARS ONLY. Copyright British Leyland Motors Inc. - 1976 Page 1 of 37 -;:;.'l r -, - -- . . -· ....... --·- ---· - f '1 • .l • :·· · http://www.triumphwedgeowners.org

·" \ .. 51040 CARBURET ION The Zenith Stromberg carburetor is a constant velocity variable choke (venturi) carburetor. The variable choke (Venturi) an increase or decrease in choke size depending upon throttle plate position. This variation of choke (venturi) size maintains a constant air velocity over the jet hence allowing a constant depression over the fuel supply at any throttle plate position. To achieve a variable choke (venturi), a diaphragm is fitted to a piston and the pressure above the diaphragm is lowered by the depression created t in the carburetor low depression area by engine suction hence atmospheric pressure to lift the piston. The underside of the diaphragm is vented to CHOKE (VENTURI) INTAKE FUEL SUPPLY In theory, the diagram above depicts a simple carburetor suitable for running an engine at a fixed r.p.m. provided some auxiliary method for starting could sufficient air velocity to create a depression over the fuel supply to draw out the fuel to with the air. Unfor- tunately, this carburetor is unsuitable for automobile use where a wide range of revs. is desired. Page 2 of 37 -- .- ' "' . .. -- ---:-:--; .. -- .. •: ' .. .. , . , http://www.triumphwedgeowners.org

i ... ! 1 . !. I .j l ' ' I . ' CHOKE (VENTURI) FUEL SUPPLY INTAKE By controlling t he air velocity by means of a throttle plate, this carburetor could run an engine over a very limited r.p.m. range by varying the air velocity and thereby depression over the fuel supply. While an improvement, this is still not suitable for automobile use. CHOKE (VENTURI) FUEL S ul?l?LY INTAKE In addition to controlling the ai r velocity, we now contr ol the choke (venturi) size which will provide a constant depression over the fuel supply. However, as the fuel supply rela tive to choke size throttle plate position is not governed, the carburetor would be r ich at the lower end and lean at t he higher end a 1 though the r. p. m: range wi 11 be greatly ;: increased. Sl040 Page 3 of 37 ·-- ·- ... - .. - ·- '. .( \. · 1. .. .. .. . ··' .., .. : http://www.triumphwedgeowners.org

.. . ;· !· -. CHOKE (VENTURI). FUEL SUPPLY METERING NEEDU: By addition of a 'stepped tapered' fuel metering needle, the fuel can now be metered in relationship to the choke (venturi) size and throttle plate This, therefore, gives a basic carburetor with a wide range of r.p.m. suitable for automobile use. Piston control is further refined by the addition of a weight/return spring • In addition, a bridge is built under the piston to create turbulence and better atomization. CHOKE (VENTURI) INTAKE NEEDLE FUEL SUPPLY !here is one exception where a constant depression over the jet is net ?nge 4 of 37 - I " ., ,. '• . •i. .. . ,. http://www.triumphwedgeowners.org

rr - .. ·.:-;--·- _. .. )::!. . .. . .. desired; this is acceleration. In this case, the throttle would be wide open and the should also move to a corresponding position to main--- tain a constant depression of the jet. This, however, would create a too lean condition for rapid acceleration. Therefore, the piston must be held down to increase the air velocity over the jet and hence pick up more fuel necessary under accelerating conditions. This is achieved by a damper which is a hydraulic device which initial piston movement to approx• imately • 070" - .140" depending upon application. The piston then continues to move up for.cing t he oil in the piston rod co pass on the outsid e of the . damper valve. When the throttle is rele ased , the damper valve falls and allows rapid oil displacement and hence piston return. Also nece ssa ry is a cold starting normally refe rred to as a manual choke. This consist s mainly of a disc with a seri es of small drillings which, when rotated, allows progressively more drillings to be open to fuel supply which is dratm into the low depression area of the carburetor. Federal Law applicable to vehicles produced after 1968 requires further refinements to control exhaust emissions to closer tolerances at idle and deceleration, the conditions under wh ic h emissions are highest. SlO.:.O Page 5 of 37 -- ' -- -- -·---·- ., .. C\ ,• .. http://www.triumphwedgeowners.org

, TEMPERATURE COMPENSATOR To reduce emissions at idle, a temperature compensator is fitted. This is a capered valve controlle'd by a temperature sensitive bi-metal strip designed to allow AIR to bypass the bridge and enter the carburetor in the low depressi on area. This weakens the depression wh ich then allows the piston to fall slightly thereby cutting available fuel and creating a leaner mixture. Sl040 Page 6 of 3i 'I I ..• http://www.triumphwedgeowners.org

.. IDLE/SLOW RUN EMULSION CIRCUITS VENT VALVE EMULSION AIR PICK-UP IDLE AIR REGUlATOR CD4 CARBURETOR OWN STREAM DISCHARGE CIRCUIT The temperature compensator is replaced on some versions of the carburetor by a separate idle/slow speed <;ircuit which discharges mixture down- stream of the throttle plate - mixture flow, however, is still via the metering needle. Also, there is an emulsion circuit for both idle and main fuel feeds gives improved atomiza tion and a decrease in sensitivity to fuel viscosity changes due to temperature variations. This system replaces the temperature compensator on some applications. St040 Page 7 of 37 ' ,. . ' ... '• ;. http://www.triumphwedgeowners.org

How to Tune the Stromberg CD Carburetor — Complete Manual

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