The starting serial number is 'GT750-10001', and the 1972 models are unique in all having the engine serial numbers located on the right side of the upper engine case underneath the right carburettor. Early production K engines were the same.
The J series engine internally differs from later versions in several respects:
The sealing area of the water bypass line in the original crankcase design (part 11300-31845) is quite small and therefore prone to leakage if not carefully sealed - later models broadened the flange area and added an o-ring at frame number 36198 which is about halfway through the 'K' production.
The original starter clutch uses a single large needle roller bearing - later models used two beginning with engine 57533.
The clutch uses a sleeve bearing and resin clutch plates - later models used a needle roller bearing and alloy plates beginning with engine 57533.
The lower SRIS valves are not threaded, and did not use a filter which was added at engine 39084.
The original crankshaft design used a non-flanged oil seal on the right inboard oil seal (next to the drive gear) - these should all have been changed over under warranty repair to the new oil seal style having a flange to prevent the seal from moving, or more probably just had a 'c-clip' installed to achieve the same thing to avoid having to disassemble the crankshaft. This is detailed in Suzuki service bulletin GT-6, and the changes were factory installed starting at engine 49895.
The original design also included fully floating small ends on the connecting rods, with fixed big ends - in fact the crankshaft, connecting rods, and pistons are quite different from the later models starting late 1973 and onwards. This table to the right shows the changes in parts numbers.
The images below show the difference in the early and later crankshafts - the wheels on the early style shown on the left are each 1mm wider than on the later design and the gap for the conrod is noticably narrower. The later design can be seen on the right - the gap is noticeably wider.
Early model "J" crankcases had a low breather vent in the upper crankcase casting which was matched by a longer stack in the oil pump cover. I'm not sure when this changed at this point, although I suspect it may have been as early as engine 17000. The photo on the left shows the cover, the middle photo shows the early upper crankcase and the photo on the right shows the later style upper crankcase
The alternator brushes in early models were different. The brush lead was soldered to the brush spring, rather than terminating on a post as seen later.
The starter clutch housing (at engine 27667), starter clutch gear bearing (at engine 26030), starter clutch gear (between engine 26030 and 29045) and the left hand oil seal (as stock ran out) were changed.
Early model year owners must also have been smarter than later ones as the kick starter was not labelled on the right side engine cover, nor was 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 1972 and 1973 model years, and then changes over to a push fit (the original part number is 09259-20003 changing to 09259-19001 for the 1975 model year).
Still looking at the right side cover, there were actually two different early covers, and at least three clutch adjuster covers that I have been able to identify to date. The earliest covers didn't have an oil seal. and splashed oil drained back into the crankcase via drains cast in the lower part of the clutch adjuster housing. The photo on the upper right shows the inside of an early cover. On the lower right is a photo of the three adjuster covers I've found to date.
The rear right side engine mounting plate is specific to the 'J' and 'K' engines.
The early 1972J was unique in having the company name cast in the alternator and points covers rather than being a label as was used in succeeding model years. Shaun in the UK has offered exceptionally high quality reproduction J points covers in the past, a sample of which are shown to the upper right, with an unrestored alternator cover shown below them.
Both the 1972 and 1973 models had polished, smooth engine barrel exterior faces - the 'Liquid Cooled' badges and indents in the barrels were introduced in 1974. Seen to the right is a somewhat unpolished 'J'/'K' example.
A cooling fan was installed as standard equipment.
The shift rod extending the width of the crankcase was shorter in the 1972 and 1973 builds than in later model years, and as a result the shift lever for both 1972 and 1973 has a pronounced sharper bend outwards than do later models. The rubber protector on the shift lever is a capped tube, rather than just an open ended sleeve, and there is a Suzuki 'S' in the end.
The water pump impeller was keyed to the shaft (later models used a pin).
The oil pump used in the 1972/1973 models had three plungers, rather than two as was used from 1974 onwards. For an excellent overview on GT750 oil pumps, see Richard's explanation on his Pink Possum web site , which is also available in the Appendices section of this site.
Early 'J' and at least some 'K' models had red coloured oil lines - as yet it is unclear whether this was due to Suzuki having several suppiers, or due to an actual design change.
Carburettors were 'low hat' AMAL style Mikuni VM32 SC's - these are 'handed' in that they are marked as being R, M and L (in very small print !) and the centre carburettor is jetted differently from the outer two.
The carburettor mount was bolted to the engine during the early production and then switched to a spigot mount as was used on the 1973 models, starting at frame number 21134. The photo to the right is of the earlier bolted flange mount.
There were two different choke perches - the early one (item 15 in the photo and part 58710-31600) had a flat lower plate, and the later one (item 16 and part 58710-31010) had a bent lip.
Around frame 22134 a change was made to the cylinder head as seen in the upper photo to the right.
On the left is an early head having the full 'water-fall' front finning, but also having an extra small fin just by the thermostat housing.
In the middle is a transitional head lacking the small fin.
On the right is the stepped front fin head in use from roughly frame 22134 onwards till the end of production.
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.
There appear to have been two types of exhaust retainer flanges used: early models had retainers with larger fins (14182-31000) seen in the upper photo to the right, which changed to smaller finned units (14182-31001) prior to the 'K' model release. A third retainer flange having slightly smaller fins and a 'rounder' look was introduced in 1974. In the lower photo to the right, the early J style is on the left, the later J style is in the middle and the style used from the L onwards is on the right side.
