Harley Davidson XR-750 (1978 to 1980). Service manual

GENERAL INFORMATION

The engine has been assembled with Bel-Ray MC-5 Engine Oil (SAE 40 weight). THIS IS DIFFERENT THAN IN THE PAST. We do not advise mixing any Castor base oil with Bel-Ray lubricant unless the engine oil tank and crankcase has been drained and washed of old oil.

The engine has been fitted with warm-up plugs, Champion No. N60R. These plugs are not to be used for racing or full throttle operation. The recommended spark plug numbers are specified in the SPECIFICATIONS section of this manual.

The XR-750 engine as shipped from the factory has an approximate maximum operating RPM of 8000.

— CAUTION —

Constant operation of the engine above this 8000 RPM will severely shorten the life of critical engine components and will require extra maintenance to keep the engine In peak condition. Also, full throttle operation before the engine has reached operating temperature will further shorten the life of the engine.

The XR-750 has been fitted at the factory with an air cleaner for each carburetor. These filters must be oiled with a straight viscosity non-detergent engine oil before using the motorcycle. Allow excess oil to drain off over night if possible. When the filters become dirty, wash them in solvent and re-oil. For best performance, you are advised to clean the filters after each event.

The XR-750 engine has two Mikuni carburetors, one for each of the cylinders. To start the motorcycle, lift the choke buttons. After the engine has started, return the choke buttons to the down position. The carburetors have been fitted with the correct low and mid-range jetting, but you will have to determine the correct high-

speed jetting, and this will depend upon the atmospheric conditions prevailing at the time of calibration. Always check main jetting for the race track and atmospheric conditions prevailing at each race meet as follows:

Engine must be at operating temperature and fitted with Champion N57G or equivalent heat range spark plugs. Starting with a *240 Main jet, conduct a full throttle run terminated with a simultaneous clutch disengagemeni and kill button application. Remove the plugs and inspect for proper mixture. Change main jets and repeat process until proper readings are obtained. It will not be uncommon that the main jet for the front cylinder will be differeni than the mam jet forthe rear cylinder. Needle position may also have to be varied front to rear to obtain a clear running engine.

The XR-750 engine has a high compression ratio requiring the use of a high grade premium fuel. Spark timing has been set at the factory at 33°BTDC. The flywheels have been marked in the proper location for the above timing for ease of adjustment. Refer to the magneto section for further information on these timing marks.

The XR-750 motorcycle requires a break-in period before any racing event. A new or rebuilt engine should be run-in for approximately 20 miles to allow the components to seat themselves. We recommend that the vehicle be run at nearly constant speed with intermittent full throttle application for short periods of time. Do not exceed 6000 RPM during this break-in period. After completion of break-in, change the engine oil. Check valve lash and ignition timing and adjust as necessary. At this time also check all chassis fasteners for tightness, paying particular attention to the suspension attachment points such as steering head bearing races and rear fork attachment.

SPECIFICATIONS

CRANKSHAFT SERVICE PROCEDURE

1. Measure width of assembled roller bearing set.

2. Measure pocket depth in each flywheel where retainer indexes into flywheel.

3. Measure total width of female rod. This should be within a few thousandths of the width of the bearing set minus the depth of the pockets. Material may be removed from either the connecting rod or the bearing retainers if necessary to adjust the width to obtain the proper tolerance.

4. Align oil hole in the crank pin with oil feed hole in the right flywheel. Press crank pin into the right side flywheel until it is flush with the outside of the flywheel.

5. Liberally coat the crank pin and the rod bearing set with oil. Assemble the rod set onto the crank pin.

Press left side flywheel onto crank pin until proper female rod side clearance is obtained of .020 in. to .025 in. It is OK for the crank pin to protrude through the sprocket side of the flywheel.

The sketches on the following two pages describe the above procedure. Drawings of the press plates and press pins shown may be obtained from the Racing Dept, by special request. Be advised that we do not manufacture these parts and that you will have to obtain the services of a qualified machine shop to make the parts for you.

8. To disassemble the flywheel assembly, it is mandatory that the plugs be removed from the ends of the crank pin. This is done by drilling oneside of theplugs out with a 13/32 drill and then pressing the opposite plug out with a long 3/8 bolt. The drilled plug may then be pressed out from the opposite side with the head of the 3/8 bolt.

NOTE: A change in design of the crank pin and plug will be apparent. The latest configuration will be a smaller hole through the crankpin ana smaller steel plugs will be supplied as spares.

CRANKSHAFT END-PLAY SHIMMING PROCEDURE

To facilitate shimming the crankshaft having barrel roller bearings, special spacer collars P/N 96680-72R have been made available to ease this procedure. Two spacer collars are required.

Assemble one of the spacer collars into the right side crankcase. Place the crankshaft assembly into the right side case. Be sure the collar fits flush onto the crank face. Assemble a .105 in. thick thrust washer onto the sprocket side of the crankshaft, and place the other spacer collar onto the crank. Assemble the left side crankcase onto the above assembly, and draw down tight.
Using a dial indicator, determine the exact amount of end-play the crankshaft now has in the crankcase. This measurement will determine which thrust washer size to use on the right side or gear side of the flywheel. If the measurement is more than .060 in., then a thicker sprocket side thrust washer will have to be selected, the correct selection of thrust washer thickness on the left and right sides of the flywheel will give you a correct end-play of .001 to .005 in. with the spacer collars.

You may now proceed to assemble the crankcases with the roller bearings. After assembly, the crankshaft end play may be checked again, at which time the end play may be as high as .035 in. due to the clearances within the bearing itself.