Technical Service Information
Tyre Checks on XS750 This bulletin is about Dunlop TT100 tyres, which could apparently foul the mudguard (fender, in US English). These tyres had a strange sort of triangular cross-section and were really designed for British bikes. tt100.pdf 
Front wheel set-up How to fit the front wheel so that the left hand brake caliper doesn't rub on the disc. Dated 22 November 1976 and applicable to the 750D, but presumably also the same on later bikes. front_wheel_setup.pdf
Bearing Selection A guide to selecting main and big end (rod) bearings in the XS range bikes. This is in the manual anyway, but for some reason Yamaha wrote to dealers about it. mms_gen_bearing_selection.pdf
Rear Axles Grease the rear axle periodically, or else it seizes to the alloy caliper mount. mms_gen_rear_axle.pdf
Brake Pad Servicing The disc brake pads have slots for water clearance. These have to be cleaned out occasionally. mms_gen_disc_pad_slots.pdf
Shift Drum Locating Bolt New torque figure, fit a tab washer and use bearing fit Loctite. mms_gen_shift_drum_bolt.pdf
Console Mounting You have to mount the speedo / tacho housing under the yoke, or there is a risk of cable breakage. The same warning was issued more than once, hence multiple documents.

mms_gen_speedo_tach_mounting.pdf

mms_gen_9_speedometer_mounting.pdf

Dipstick and Detent Bolt This one must have been really urgent because they printed it in red. The shift cam detent bolt gained a new lock washer and the dipstick was modified. mms_gen_dipstick_and_detent_bolt.pdf
Primary Chain

A whole kit of parts, including a new design of primary chain, designed to cure the problems with excesive wear.

There is also a later note describing the differences and the new part number.

mms_gen_primary_chain_improvement.pdf

mc_0033_77_primary_chain_improvement.pdf

Clutch Push Rod Oil Seal

Oil seal failure required the crankcases to be split and the internal oil restrictor jet checked. Also a pinion had to be lined up to ensure decent oil feed.

Later, there was a revised oil seal and bush.

mms_gen_clutch_pushrod_seal.pdf

mc_0031_77_clutch_pushrod_seal_bush.pdf

77e_003_oil_leak_clutch_pushrod.pdf

emc_77031_clutch_pushrod_oil_seal.pdf

Urgent Gearbox Modification New nuts and a lock washer for the main gearbox bearing carrier. This applied to electronics ignition models with engine numbers between 1T5-10000[0] and 1T5-101110. mms_gen_2_urgent_gearbox_modification.pdf
Pilot Mixture Screws Some very handy instructions on how to adjust the pilot screws without using special equipment. Apparently there were complaints about poor starting and idling on "the smaller four stroke models", but the instructions also applied to the XS750 - which was the largest four stroke made by Yamaha at that time. mms_gen_3_pilot_mixture_screws.pdf
Excessive Oil Consumption  Yamaha issued modified piston rings to fix a problem with excessive oil use (500 miles per pint or worse). No details of part numbers though. mms_gen_11_excess_oil_consumption.pdf
Middle Gear Case & Final Drive Oil Levels Dealers were failing to fill the middle and final drive gears with oil, so Yamaha warned them that such failures were not going to be covered under warranty. 

mms_gen_16_middle_final_oil_levels.pdf

mms_77e_014_final_middle_oil_levels.pdf

Alternator Cover If the bike has been dropped on the right hand side then the dowels should be checked - otherwise there's a risk of seizing the alternator. mms_gen_17_alternator_cover.pdf
Modification Programme A summary of all the modifications that dealers had to carry out. The primary chain mod for 2D models, plus dipstick and lock washer and gearbox output shaft lock washer on the 750E.  mms_gen_23_modification_programme.pdf
Brake Dragging Design change to the disc pad springs, to reduce dragging. The caliper also changed slightly (the bleed nipple angle was altered).

mc_0020_77_brake_dragging.pdf

76024_brake_dragging.pdf

Fuel Tap The fuel taps were redesigned because the originals allowed you to empty the tank when they were in the "on" position (so that reserve was effectively useless).  mc_0024_77_fuel_tap.pdf 
Shift Cam Retaining Bolt This could potentially become loose. mc_0027_77_shift_cam_bolt.pdf
High Oil Consumption and Big End Bearing Failure

The definitive guide to oil grade. Yamaha were recommending 20W/40, or 10W30 in low temperatures. They suggest using the same oil as in aircooled VW car engines.

A later note described the improved oil ring design, and another note combined the oil recommendation and improved piston ring set.

mc_0029_77_oil_consumption_bearing_failure.pdf

emc_77027_oil_ring_improvement.pdf

emc_77029_excess_oil_consumption.pdf

Oil Leakage From Breather Hole Oil sometimes leaked from the rear axle breather in cold conditions. Use SAE80 oil if this is a problem.

mc_0030_77_oil_leak_breather_hole.pdf

emc_77023_shaft_drive_oil_grade.pdf

Battery Discharge Problem Early regulators caused the battery to drain. 77e_001_battery_discharge.pdf
Rear Light Installation Fit the nuts above the mudguard (fender) to avoid the tyre hitting them.  mms_77e_032_rear_light_installation.pdf
Rear Engine Mounting If the gap between engine and frame is too large, you need shims. mms_77e_033_rear_engine_mounting.pdf
Modifications to Some Component Revised clutch plate design to prevent dragging, modified gear selector cam for better selection of neutral and modified selector forks / 5th gear wheel to fix the infamous 2nd gear selection problem. Also an improved oil pump pickup and sump pan. 76027_some_components.pdf 
Mission Gear Splines Design change. 76029_mission_gear_splines.pdf
Interchangeability of Shaft Drive System Very detailed description of the parts differences when they moved from Getrag-made shaft drives to Yamaha-built systems. emc_77004_shaft_drive_interchange.pdf
Crankcase Assembly and Bearing The big middle gear drive bearing gained an extra couple of circlips for better location. emc_77013_crankcase_assy_bearing.pdf
Valve Clearance Adustment Pads Advice on how to stock the most common valve shims. emc_77015_stock_control_valve_pads.pdf
Oil Pump Check Valve Discontinued, because it didn't do anything useful and sometimes caused problems. emc_77017_oil_pump_check_valve.pdf
Rear Disc Pad Material Improved compound for reduced disc wear. emc_77021_rear_disc_pad_material.pdf
Bearing Housing Improved design to prevent the needle bearing coming out.

emc_77024_bearing_housing_2.pdf

mc_77053_bearing_housing.pdf

Parts List Corrections Errata for various parts lists. emc_77030_parts_list_corrections.pdf
Clutch and Related Parts Modifications to the clutch plates (already mentioned elsewhere), plus the clutch boss, damper assembly and primary chain. emc_77035_clutch_and_related_parts.pdf
Camshaft Bearing Cap No. 1 Revised slightly. mms_78004_cam_bearing_cap_1.pdf
Oil Delivery Pipe Revised slightly. mms_78036_oil_delivery_pipe.pdf