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
