Although the barrels have the later style of inset 'Liquid Cooled' markers used on the M, A and B models, the porting and the compression ratio (6.7:1) was the same as that on the J and K models.
The original inset 'Liquid Cooled' labels (middle in the photo) have a 'barred Q' and the replacements (seen at the top) which are still available from Suzuki do not have this feature. The Kettle Club in the UK has produced a number of replica originals having the barred 'Q' as seen in the bottom of the photo and while the folks handling the sales for the Kettle Club can be difficult to deal with if you are not a member, the quality is quite good if you can get them at a sensible price.
The eleven large 10mm cylinder head bolts changed to having a flatter, more 'industrial' head, although the part number didn't change (part number 09159-10010) Below you see the old style on the left as used in 1972 and 1973, and the newer style as used in 1974 onwards through to
the end of production in 1977.
The exhausts retained the crossover tubes of the earlier models, but were restyled. The black exhaust pipe tips were gone, and the pipes swept back to end in a more modern integrated 'reverse cone' outlet presenting an uninterrupted expanse of chrome from front to back. The rear hanger mounts were also changed to separate the passenger foot peg from the exhaust hanger bolt. Note that the internal fixed bulkhead baffle has a smaller centre hole than later 'M' pipes. While the outside pipes are easy to tell apart ('L' has couplers) the inside ones are less obvious. 'L' inside pipes are the only ones to not have GT750 stamped on them at the rear.
The head bolts now included as standard a sealing washer (part number 09168-14008) which the earlier models had lacked. This change limited
seepage of water into the stud opening causing extensive corrosion and making the barrels very difficult to remove. This change was fully
implemented at engine number GT750-51822, frame number GT750-45213 toward the end of the L model year
The connecting rods were now fully switched over to floating big-ends, non-floating small ends (at engine number 38060) Crank oil seal changed to replace C clip with an integral steel locating lip to ensure that the seal does not get displaced at engine number GT750-54214 and frame GT750-49895. Reference service bulletin GT-6 which is found at this link. Suzuki also included instructions with replacement crankshaft assemblies for use when fitting into earlier model crankcases as seen to the right. The information was similar to that included in GT-6.
The carburettors were Mikuni BS40 constant velocity style, as are used in the later model years, mounted on a rack and with idle speed adjusted by a single screw instead of at each carburettor. Fuel economy supposedly went from high 30's to mid to high 40's (in MPG) although I seem to recall getting better numbers than that. The oil pump was now controlled by a carburettor linkage rod, rather than by a cable from the throttle.
While this same style of carburettors continued to be used as standard equipment through the remainder of production, the 1974 tuning specifications were unique to that model year. Reference service bulletin Specification-6 which is found at this link.
The throttle slide cut away is a 120, which changed to 110 in 1975
Pilot jet is a 47.5 which changed to a 45 in 1975
Spark plug was a B6ES, which changed to a B8ES in 1975
Ignition timing is 3.64mm BTC for the R and L and 3.42mm for the centre which changes to 3.62mm for the R and L and 3.42mm for the centre
Air screw setting is 1/4 turn open, changing to 3/4 turns open in 1975
The stock choke lever actuator originally had a rubber cover seen to the lower left - the replacement unit on the lower right lacked this if you lost one, although the part number (13413-31210) didn't change
The BS40 carburettors were made under license from Solex, and in the owner's manuals you can clearly see 'SOLEX' cast on the caps. This was changed either before actual production started, or shortly afterwards, or perhaps even was only seen on Japanese domestic versions of the early BS40 fitted engines used in the JDM B4. As yet, I have not been able to find an original example.
The oil injector pump was now controlled by a rod that was connected to the linkage of the right hand carburettor, unlike the previous cable operated design. The original pump actuator rods had a bellows style boot (16765-31200) to keep water etc., from entering the oil injector
pump housing. This boot was replaced by a simpler design (16765-31201) being just a rubber cover in the GT750B model year.
The cooling fan was now an optional extra item. The upper thermostat housing (17661-31000) now had a plug installed (09246-16001) into the
threaded hole where the fan sensor formerly was located, but the wiring harness retained the fan and fan sensor connections, with the extra unused leads being tucked up underneath fuel tank.
The two plastic radiator end caps used in 1972 and 1973 were replaced by a full plastic grill (17761-31200-126) that covered the front and sides of the radiator and was painted Pearl Metallic (Suzuki paint code 126). This plastic grill was originally mounted with large-headed screws having no shoulder (02141-05127) and using a small diameter mounting hole. As 1974 and early 1975 grills often cracked in the area of the mount mounting holes a change was made at frame number 58021 when the mounting screws were changed to the larger shouldered design with larger diameter mounting holes in the plastic grill.
To accommodate the new radiator grill, the radiator crash bar was changed to remove the two vertical bars inset from the left and right sides and this new style of crash bar (94970-31200 changing to 94970-31201) was used through till the end of production.
The early 1974 'L' radiator bars appear to have been left over J/K bars without the two vertical bars, and cen be identified by the bend at the side shown by the arrow in the upper photo. For late 1974 and all following models, the radiator crash bar is straight at the sides as seen in the lower photo. The part number did change from 94970-31200 to 94970-31201 in 1975 (as per the Suzuki 'Model Applications Guide' manual).
