The cylinder intake and exhaust port timing were changed for the 1975 M model year.
The lower edge of the Intake port was changed from 101.5 mm as measured from top of the cylinder, to be 106.5 mm. The exhaust port upper edge was changed from 42.5 mm to be 40.0 mm (so raised).
The combustion chamber volume was decreased from 30.6cc to 29.7 cc by shaving the cylinder head by 0.2 mm, and as well the exhaust gasket was reduced in thickness from 1.5 mm to 0.8 mm.
Note:
to the right are the three known head gasket styles used over the length of production (excluding the 'split head' variant sold only in Japan). The earliest is on the left (note the un-blanked areas at the outer edges), the thick version that replaced it during the J production, and the one on the right is the thinner version introduced at the start of the 1975 model year.
The net result was a change in compression ratio from 6.7 to 6.9:1 and a supposed power increase from 67 to 70 HP.
The exhausts couplers were removed, and also the sweep of the exhaust pipes were decreased in radius plus the diameter of the mufflers was also decreased from 29 mm to 20 mm to try and add additional ground clearance.
The clutch plates were changed from resin to be aluminium substrate.
The primary driven gear (clutch centre boss) was changed from being a bushing to be a dual needle roller.
The above changes were implemented, per service bulletin GT-7 at engine 57533 and frame 52823.
Drive ratio was changed - a 43 tooth driven sprocket with a 16 tooth drive sprocket as standard.
The carburettors were Mikuni BS40 constant velocity style, as were used in 1974, but note the tuning specifications in 1975 were quite different:
The throttle slide cut away was now a 110.
Pilot jet was a 45.
Spark plug changed to a B8ES.
Ignition timing changed to be 3.62mm for the R and L and 3.42mm for the centre.
Air screw setting was changed to be 3/4 turns in 1975.
The cooling fan was still available as an optional extra.
The radiator used the same full plastic grill that covered the front and sides of the radiator as used in 1974 - new for late 1975 was the addition of a cushioning o-ring to be used with a modified set of four mounting screws introduced at frame number 58021. This required larger mounting screw holes in the radiator grill which also changed part numbers. The photo shows stainless steel reproduction parts - the originals (09139-05012) had Phillips heads, rather than Allen heads.
Possibly as a cost cutting measure, one chrome radiator hose cover (part number 17856-31000) on the upper radiator hose was eliminated and only one was installed at the end nearest the radiator. The same is true for the A and the B models.
As previously offered, the kick starter was now labelled on the right side engine cover, and the gear shift pattern cast into the left side drive sprocket cover. Also worth noting is that the oil filler cap for the transmission oil is threaded in the early model years, and then changed over to a push fit (the original part number is 09259-20003 changing to 09259-19001 for the 1975 model year).
The kick start lever is fairly straight when compared to the post-1973 models, and the rubber cover had a Suzuki "S" embossed in the middle on one side.
