Contributed by Richard Nowson
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Flange mounted carbs on finned intakes and a short port divider/bridge with oil injection nozzles mounted directly behind the short divider.
SRIS returns are all on the front of the block with two to the center cylinder and one to the right cylinder.
Exhaust port is 42.5mm down from the top with a negative 0.5mm deck height to yield a port 42.0mm below TDC.
Intake port floor is at 101.5mm below the top of the block – 101mm below TDC.
The early block is arguably the best looking block and is easiest to fit different carbs by using different Mikuni rubber flange mounts.
Late 1972 J model
Spigot mounts, introduced at frame #21134, instead of the flange mounts on early motors. All other details appear to remain the same. Port timing similar to all J/K/L models.
Intake port divider remains short and with the same port timing as the earlier flange block.
SRIS lines same as early model J with all three lines to the front of the block, two to the center cylinder and one to the right side. All lines eject into the transfer ports.
1973 K model
Similar to the late J but different SRIS lines….
The third SRIS line now goes to a position below the left intake port
Externally almost identical to the K but with indents on the sides of the cylinders to take LIQUID COOLED metal badges.
Port sizes and timing remained the same as J and K, but the intake port divider changed to full length of the intake port. That change dictated a change in SRISnozzles. On early motors, the brass nozzles were pressed into the port almost vertically and in line with the short divider.
From L model onwards, the nozzles were changed to angle them to be centered on one of the two intake ports and that required a small change to the outside surface of the intake side of the casting.
On the front (exhaust) side of the block, the cast boss where the two SRIS ports entered changed slightly to eliminate the third port.
1975 M, 1976 A and 1977 B
M, A and B models all appear to be identical to each other and are outwardly similar to the L block but with minor external and significant internal differences.
Exhaust port was raised to 40mm from TDC for earlier port opening and larger port area which was designed to improve top end horsepower.
Intake port floor dropped to 104mm for more intake time and area. That also helps top end power but on a stock motor, the loss in power below mid range was much more significant than the minor gain at the peak. The combination of more aggressive intake and exhaust port timing made for faster road test ¼ mile times, but a bike that needed to be revved harder to get it moving.
The casting was changed to allow for that lower port and the lower outside surface of the intake port is also lower than early models. The difference in casting is subtle but Suzuki managed to lower the whole port floor to yield larger ports.
Consequently the M-A-B barrel can be modified for much longer inlet port timing than the J-K-L barrels.