Carburettor Operation and Maintenance

Carburetor AdjustingOperation.

 Two separate carburetors are used on left and right cylinders (one for each cylinder). The carburettors are virtually identical to the GRAETZIN  carburettors fitted to the original BMW R71, (although the throttle size is different for the CJ750 OHV engine).  The carburettors are simple and effective. The operation of the carburettor is shown in the schematic below.

(With grateful acknowledgements to CAJUN MIKE and WWW.CHANGJIANGUNLIMITED.COM)

To adjust the carburettors, start the engine and warm it up for 3-5 minutes.

1) Make sure that the throttle is fully in the idle position.

2) Screw in the idling adjusting screw (Item 6 in Fig 11) clockwise to raise the throttle valve, and slightly increase engine RPM. Ground the high-tension cable from the other cylinder and so that the engine is running only on one cylinder at a stable idle speed.

3) Adjust idling mixture adjusting screw (Item 5 in Fig 11) to increase the engine RPM to maximum; and to allow stable engine operation at that speed.

4) Gradually screw out the idling adjusting screw to reduce the throttle valve opening to the minimum engine RPM possible, and to keep engine operating stably and continuously at that speed

Repeat steps 1 - 4  above with the second carburettor.

 5) After each carburetor has been synchronised at idle, reconnect both high tension leads and start engine. Adjust each carb throttle speed screw individually by screwing outwards so that engine speed with both cylinders running is 750rpm -1000rpm. Tighten lock nuts (Item 4 in Fig.11) of the two screws.

6)  Adjusting at intermediate speed.

7)Start the engine and turn the twist grip open to about 1/8 to ¼ open. (Approx 2400RPM). Remove remove HT lead from one cylinder.Note the engine speed with the engine running on only one cylinder. Replace the HT lead.With the throttle still held in exactly the same position remove the HT lead from the other cylinder. The idea is to achieve a "balance" by adjusting the carburettors so that the engine runs at the same rpm on each cylinder. If the engine speed when running on only one cylinder is noticeably higher or lower when running with a constant throttle opening, it is necessary to synchronizes the carburetors.

8)Remove rubber cover from carburetor upper cover, adjust the steel cable connector on the carburetors to obtain even cylinder operating rpm at a constant throttle opening. (Item 2 in Fig11).Tighten steel cable connector nut

To check the carburetor operation, turn the throttle twist grip rapidly to center position. At this moment, engine RPM should increase evenly. For the well-adjusted engine, the surface of spark plug insulator skirt should be dry and brown.

Note: If the carburetors are correctly synchronized, and the throttle response is still poor it is possible that the fuel/air mixture needs to be adjusted to suit your climatic/altitude conditions. If the engine falters when opening the throttle suddenly this could mean that the mixture is too lean. This can be corrected by unscrewing the carburetor top, removing the throttle slide and RAISING the throttle needle by one or two notches. If when opening the throttle response is good, but black smoke is noted from the exhaust, this can be corrected by DROPPING the throttle needle by 1 or two notches.

Before any adjustments are made to the throttle needles it is important to make sure that the problem is not caused by incorrect ignition timing, points setting or fuel blockage. If the throttle response was good and has suddenly deteriorated it is unlikely that the adjustment to needle position is required and the fault must lie somewhere else.