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Vintage Monark
P.O. Box 4116
Oceanside, CA 92054

Phone: 760-754-8177
E-Mail info@vintagemonark.com

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Technical Information Sheets (TIS)

    These are information sheets issued by the distributor, Inter-Trends, Inc., in order to keep our dealers aware of any problems, maintenance or other relevant information on the MONARK 125cc machines. These will be sent out to cover existing or new components and a sheet listing all technical information numbers will be sent out as it is updated.


    #1 125cc GS Model Changes
    #2 1251/6B Std and GS Technical Information
    #3 SACHS 6-speed G Box Adjustment
    #4 Clutch Chainwheel Problems
    #5 Moto Plat Electronic Ignition
    #6 Wheel Hub Front and Rear
    #7 GS Porting Specifications

    Geoff Burgess
    Product Development Engineer

    #1 125cc GS Model Changes

      MONARK 125cc Models 690012 MX - GS & 6901125 EN - GS

        Using SACHS 1251/6B Engines

        The above models are basically the same as standad 125cc 6-speed models, the difference being mainly with engine and carburetor changes.

        The GS models have the following special parts in place of standard components:

        Part No. Description Price (Dlr. Net)
        286-317-022 Selector Boss $17.30
        634-106-201 Selector Gear $12.10
        681-007-000 Carburetor - Bing No. 2/28/101 $29.17
        687-107-515 Crankcase $61.60
        687-108-260 Cylinder (Aluminum) $96.00
        645-102-000 Retaining Plate .10
        690-002-003 Gear Changing Device $28.50
        690-002-004 Gear Changing Device $28.50
        647-103-001 Bushing .99
        613-106-007 Cylinder Head $24.62
        686-207-035 Piston $18.20
        686-207-036 Piston 1st OS $18.20
        686-207-037 Piston 2nd OS $18.20
        686-207-038 Piston 3rd OS $18.20
        686-211-000 Bracket $3.50
        639-003-000 Spring .20

        Selector Boss #286-317-022
        This component is basically the same as std. 6-speed one, but has an additional quadrant for positive stop mechanism.

        Selector Gear #634-106-201
        This is a replacement for #634-106-101.

        Carburator #681-007-000 - 28mm Choice
        This carb. replaces the 27mm carb. on std. engine.

        Crankcase #687-107-515
        Various minor modifications are in this component.

        Cylinder #687-108-260
        This cylinder has more transfer port area. The ports being enlarged by 4mm in width. The inlet manifold is also enlarged to 35mm o/d to suit the 28mm carb. Exhaust and transfer timing is unaltered from std. cylinder.

        Cylinder Head #613-106-007
        The clearance volume in the head is reduced to give a compression ratio of approximately 15/1

        Piston #686-207-035
        The position of the lower ring location peg has been moved approximately 7.5mm to the rear of the piston in order that the ring does not become trapped in the enlarged transfer port. Also the inlet skirt has been shortened by 2mm to give a longer inlet timing, 164 degrees.

        Retaining Plate #645-102-000
        This is an additional plate for the retention of the positive stop spring.

        Gear Changing Device #690-002-003 or 004
        This is the rear change selector shaft assembly, complete with modified boxx #286-317-022

        Eccentric Bush #647-103-001
        This is a replacement for the standard bush.

        Bracket #686-211-000
        This is the positive stop locator.

        Spring #639-003-000
        Positive stop retaining spring.

      Monark 50cc Model Schoolboy Scrambler GS Eng. utilizing 50/6 GS Engines
      The following is a list of special parts used only on the 50cc GS Engine in place of the standard components.

      Part No. Description Price (Dlr. Net)
      213-134-105 Cylinder Head $20.60
      214-163-115 Piston #12.60
      234-123-001 Selector Gear 1st $7.80
      234-124-002 Selector Gear 2nd $8.30
      234-125-002 Selector Gear 3rd $6.35
      234-126-001 Selector Gear 4th $4.85
      234-126-002 Selector Gear 5th $4.20
      234-127-101 Selector Gear 6th $4.55
      247-110-001 Ring F. Sel. Gear .46
      247-110-002 Ring F. Sel. Gear .46
      247-110-003 Ring F. Sel. Gear .46
      267-115-000 Intake socket $8.12
      281-123-000 Carburetor - Bing No. 2/22/39 $20.00
      285-110-001 Layshaft $22.00
      286-341-001 Mainshaft $21.50
      287-105-407 Crankcase $44.00
      287-112-212 Cylinder (Alu) complete with piston $51.80
      288-104-206 Crankshaft $42.10
      290-112-102 Gear change w. lock $30.50
      290-118-000 Selector rod $3.80
      250-157-000 Gasket for intake socket .35
      647-103-001 Bush .97

      Geoff Burgess
      Product Development Engineer

    #2 1251/6B Std and GS Technical Information

      MONARK 125cc SACHS 1251/6B Engines

        1251/6B Std. 1251/6B GS
      Bore 54mm (2.126") dia. 54mm
      Stroke 54mm (2.126) 54mm
      Cubic Capacity 122cc (7.505 in3) 122cc (7.505 in3)
      HP@RPM 21 @ 8300 24 @ 8700
      Compression Ratio 12/1 15/1 (approx.)
      Carb. Choke Sz. 27mm (1.06") 28mm (1.10")
      Main Jet Size 120 145
      Needle Jet Size 2.73 2.80
      Idle Jet Size 40 or 45 45
      Ignition Timing 2.75-3.25mm (.108-.128") BTDC 2.5-2.7mm (.100-.108") BTDC
      Spark Plug Bosch W260tl or Champion L2 G (Street)
      Bosch W290tl6 or Champion L3 G (Racing)
      Bosch W290tl6 or Champion L3 G
      Port Timing
      Exhaust 188 degrees total opening 188 degrees
      Transfer 132 degrees total opening 132 degrees
      Inlet 155 degrees 163 degrees
      40 degrees fully open


      Geoff Burgess
      Product Development Engineer

    #3 SACHS 6-speed G Box Adjustment

      SACHS 6-Speed Gearbox

      Several queries have arisen as to the correct adjusting procedure for this gearbox. Outlined below is the correct checking procedure.

      First, remove the kickstart and gearshift levers. Remove the outer chain case cover #611-114-100, then remove the clutch by extracting the circlip #245-020-001 and lifting off the hub complete. Refit the gear change lever, loosen screws #1940-144-000 (no. 17 on illustration page 25 of parts book), rotate eccentrick lobes #31, part #647-103-000 away from motion of travel of pawl, it: anti-clock for L.H. lobe, clockwise for R.H. lobe. Retighten allen screws.

      Engage low gear (1st) ensuring that the selector rod part #619-004-001 (part no. 9 on illustration page 22) is fully home. No check to ensure that there is clearance between selector shoe part #248-135-000 and the slot in the selector rod. In other words, make sure that there is no end load on the selector rod. Next, engage high gear (6th) and repeat the above checking procedure.

      If any end load exists or if there is very little clearance, adjustment is made by loosening nut, part #642-108-001 and rotating eccentric screw, part #640-110-000 to give the correct adjustment.

      Now engage low gear again, ensuring the selector rod is fully home and allow gear change lever to return to neutral position. Then observe the position of the selector pawl, part #248-136-000, when the gear lever is depressed into low gear again, under the return spring pressure. A mental note of the clearance between the pawl and the engaging tooth on selector boss part #286-317-021 must be made as the lever is depressed from the neutral position. Engage high gear (6th) and repeat the above procedure.

      The clearance between the pawl and the corresponding tooth on the selector boss must be equal in both gears. If this is not the case, loosen allen screws no. 17 page 25, and move the selector adjusting plate, no. 16 page 25, part #248-133-001, to ensure that this is the case.

      Engage low gear once again, and rotate the eccentrick lobe so that it is just clear of the pawl when the lever is held fully down in the low gear position. Repeat in high gear position.

      The gearbox is now adjusted completely and should function satisfactorily.

      If the machine is a GS model and has been fitted with the gear locking device, the following additional points should be noted:

      Before attempting the above procedure the return spring #639-003-000 must be unhooked, allowing the pawl arm #686-211-000 to drop away from the positive stop locations.

      The above proceedure can then be carried out to correctly adjust the gearbox.

      To adjust the pawl arm to the correct position, third (3rd) gear must be located. The pawl arm should then locate in the 3rd gear notch on the positive stop quadrant. Select 2nd and 4th gears by lifting the gear changing pawl over the tooth on the selector boss in each case and depressing the gear change lever against the stop. Check to see that the pawl arm moves an equal distance each side of the 3rd gear notch on the positive stop arm. If this does not happen, then readjust the position of the pawl arm by loosening the L.H. allen screw and moving the same. Retighten the allen screw and hood the spring.

      Tightening torque to allen screws is 8.7 ... 10.8 ft lbs.

      Geoff Burgess
      Product Development Engineer

    #4 Clutch Chainwheel Problems

      Clutch Chainwheel #685-100-001

      Several problems have come to light with new machines being run in the first two or three hours, with this component.

      It is most important to check that the chainwheel is shimmed to the correct end clearance (.003-.005) on the layshaft, as several complaints have been made that excessive end play exists. This, of course, will damage the circlip groove and cause breakage of end of the layshaft. Four thicknesses of shim are available, #246-015-000, 001, 002 and 003 to take out all excessive play.

      Another problem that has shown up is the "seizure" or "freezing" of the layshaft wheel bushing #632-113-000, on the layshaft. It would be advisable to check that the wheel is free running and has sufficient clearance for good running; also ensure it is sufficiently lubricated. On the outer side of the chainwheel there is a slot cast into the hub, but after the bushing is pressed in, it obstructs the slot. It is advisable to machine or file away the bushing in this area to create a channel so oil can enter into the bearing area when the engine has stopped.

      Geoff Burgess
      Product Development Engineer

    #5 Moto Plat Electronic Ignition

      Moto-Plat Electronic Ignition

      Several failures of the generator system have been reported by various dealers. It must be emphasized to the customer that under no condition should the engine be rotated if the spart plug is not grounded out. Severe damage to the diade will result if this does happen. Enclosed is the service bulletin no. 6/1972 (Editor's Note: This is unavailable at this time) concerning the checking of the moto-plat system for condition, etc.

      Geoff Burgess
      Product Development Engineer

    #6 Wheel Hub Front and Rear

      Wheel Hub #870-174-002

      The above component has been known to fail in a number of cases. The customer should be instructed upon collection of the machine to ensure that the spokes are tightened up constantly after each race or run, as the spoke nipples "beds" itself into the rim. After the above procedure has been completed four or five times the nipple has completely bedded down and no further loosening will occur. If the spokes are allowed to loosen and the machine is run, hub damage will certainly occur. The normal lace pattern is of the cross 3 type, however, we will supply a heavy duty cross 4 lace pattern spoke for competitors who wish to build a stronger wheel to withstand severe going.

      Geoff Burgess
      Product Development Engineer

    #7 GS Porting Specifications

      MONARK 125cc 1251/6B

      Modification Sheet to GS Specifications

      Several modifications can be done on a standard 1251/6B alloy cylinder to achieve an improved horse-power output.

      As can be seen from the enclosed drawing, it is mainly modifications to port sizes and shapes.

      1. These are the basic port sizes after modifications. It must be stated that if the transfer ports are enlarged to 30mm chordal width, a GS Piston Part #686-207-035 must be used, as this part has a relocated 2nd ring stop peg, which prevents the ring end from protruding into the enlarged port. All port timings are as the standard engine, so no alteration in height is required.

        All ports should be smoothed out and polished, and blended into any matching areas.

        The rear of the transfer ports should have a 2mm lift so that the gas charge is released into the rear of the cylinder when the piston uncovers the port.

        2mm should be removed from the inlet skirt side of the piston to alter the inlet timing to 163 degrees total opening.

        Approximately 3mm should be machined from the cylinder head, and the hemispherical squish band should be remachined, to alter the compression ratio from 12/1 to 15/1 approximately.

      2. Exhaust Port
        Machine all excess aluminum away as shown on illus. #2. Blend and polish.

      3. Transfer Ports
        Remove aluminum from transfer port area to match up w/crankcases.

        All ports in the cylinder sleeve should have a 1/16" radius so that the piston rings do not snag on any sharp areas.

        A 28mm choke carburetor (Bing or Mikuni) can be used and, of course, the inlet tract will have to be opened up to suit.

        Ignition timing should be reset at 2.7mm (0.108").

        Geoff Burgess
        Product Development Engineer

    Source of Technical Information Sheets

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