Workshop: Suzuki GS750 rebuild part three

Suzuki GS750

The motor was in basically sound condition but will be going back together with a new camchain and a couple of second-hand gears to cure the problem Jason had had with it jumping out of gear. The piston rings measured well within factory tolerances so the new valve stem seals I’ve fitted into the head should cure the oil-burning problems.

These engines really are solidly designed and it’s a testament to the superb Suzuki engineering that this is all the engine needs after almost 25 years of service.

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Picture one: First job is to assemble the gear selectors into the lower crankcase. The GS motor has its three selector forks mounted on two separate shafts which slide into the crankcase either side of the selector drum. Pins on the forks engage into slots on the drum so, as it’s rotated by the gear selector shaft, the forks slide sideways along the drum. Note the detent roller which, on the GS, runs in grooves at the left-hand end of the drum and is retained by a spring hooked across from the rear selector fork shaft.

Picture two: Outboard of the selectors now, this is how the end of the drum looks in the back of the clutch housing. Suzuki use a twin pawl mechanism to rotate the drum instead of the more usual twin claw type found on Honda and Kawasaki engines. This is solid and compact and seems to be more or less bullet-proof. The two metal plates not only locate the drum laterally in the crankcase, they also blank off the two holes for the selector fork shafts and fix them in place. I always give the retaining screws a crack with the impact driver to make certain they don’t work loose with the engine running.

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Suzuki GS750 engine rebuildPicture 3: Now I can start to assemble the major parts of the engine into the crankcases, starting with the crankshaft. I’ve hooked the new camchain over it and fitted new oil seals at each end before offering it into place. On a roller-bearing crank like this one, the main bearing outer races each have a depression machined into their edge which must engage with a short dowel pin pressed into the upper crankcase. These dowels serve to line up the bearing races oil feeds to the drillings in the crankcase and, if they’re missed out, the main bearings could be starved of oil. With the crank in place it’s easy enough to gently rotate each bearing until you feel it drop into engagement with the dowel pin. Finally, I put lots of fresh engine oil down each feed hole and rotate the crank to make sure it will turn freely.

Picture four: I’ve engaged the kickstarter shaft into its housing and then dropped both gearbox shafts into place. Again, watch out for the dowel pin in the crankcase which engages with a recess in the kickstart plain bearing. The gearbox shaft bearings each have a groove on their outer race which slot into half-circular clips in the casing. These locate the shafts laterally and must be in place. The bearings also have small pins on their outer races which must fit into small recesses at the edge of each bearing housing.  If in doubt, gently rotate each bearing until it drops into its location. Note the new oil seals on the output shaft and for the clutch pushrod; these must be in place before the crankcases are joined.

Picture five: Now the crankcases can be joined and bolted up. I use a thin smear of Hylomar on the joint surfaces and always pause to check all the shafts rotate freely before bolting everything up. At the back of the clutch housing are two metal plates which span the upper and lower crankcases and these can now be added. The larger plate serves to connect an oilway from the lower casing to the end of the output shaft and should have a gasket behind it. The other plate wraps round the input shaft bearing; both are retained by short countersink screws which, again, I always seat firmly with a tap from the impact driver. The plate visible in the upper crankcase has not been disturbed for the rebuild. Now the oil pump assembly fits up against two new O rings and I can secure it with three crosshead screws.

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Picture six: At the rear of the clutch housing I’ve used a hefty pair of pliers to tension up the kickstart return spring and hook the end into its slot in the crankcase. A slotted alloy spacer fits into the kickstart spring to stop it twisting out of position. Now I can slide the gearchange shaft through the crankcase from the right and engage this gear quadrant with the ratchet mechanism on the end of the selector drum. The quadrant and ratchet shouldn’t be a problem to index together and merely need to line up centrally. Don’t forget to grease and fit a new gearchange shaft seal to the other side of the crankcase before sliding the shaft home. At this stage it’s a good idea to temporarily fit the gear lever and run through the gears on the bench: you should be able to select all five gears and neutral by operating the gear lever as you turn the input shaft by hand. Any problems with gear selection need addressing now before proceeding with the rebuild.

Picture seven: Before starting to assemble the clutch this oil pump drive gear fits on the end of the input shaft. The gear runs on its own needle roller bearing which I’ve oiled before fitting, and the whole plot sits on a machined boss which fits inside the bearing. Once the gear is in place and engaged with the oil pump drive gear, check the position of the two drive lugs on the gear face as these will have to engage with two recesses on the back of the clutch basket. Incidentally it’s possible to buy aftermarket gear sets for the oil pump drive which will increase the RPM of the oil pump relative to engine speed. Don’t make the mistake of thinking they will increase oil pressure, but they do increase the rate of oil flow and are a good idea if the engine is intended for racing.

Suzuki GS750 engine rebuildPicture eight: Now the clutch basket goes on the gearbox input shaft. The primary drive on the GS engines is taken directly from the gear on the crankshaft to the gear on the back of the clutch; no horrible primary chains to wear and rattle on these engines! However, this does mean the clutch has to fit over the input shaft and then engage forwards with the crankshaft; to make this easy Suzuki have mounted the whole clutch on this large-diameter boss. Fit the clutch basket first, then slide the boss into the clutch bearing. Doddle. The design leaves lots of space inside the clutch housing, almost enough for your flask and sarnies! Make sure the clutch basket is correctly engaged with the oil pump drive gear before moving on.

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Picture 9: The clutch follows conventional Japanese design. With the basket fitted, the inner drum pushes on the splined end of the input shaft and is bolted up with a hexagon nut and locktab. I use a Kawasaki clutch-holding tool to stop the clutch rotating while I bolt it up; at a push you can wrap a length of drive chain round the output sprocket and clamp it into a bench vice to hold it firm while you tighten it. I’ve fitted the clutch pushrod into the input shaft before fitting the clutch lifter mushroom and needle roller bearing, and now I can pop the clutch plates in, starting with a friction plate then alternating with plain steel plates until they’re all in place. Finally the outer pressure plate goes on and I can fit the springs and bolts to hold it all together. Finally the outer cover goes on with a new gasket; note that a boss in the back of the cover butts up against the end of the gearchange shaft and locates it laterally.

Picture 10: Turning to the other side of the engine now, here’s the switch which sends signals to the GS750’s bank of gear position indicator lamps on the dashboard. The copper contacts on the back of the switch body are earthed in turn by this tiny spring loaded plunger on the end of the selector drum. As the drum turns to each new gear position the plunger earths each circuit sequentially, lighting the relevant lamp. Note the switch has six gear position terminals and one for neutral, although the GS750 has only five gears. Obviously the same switch fits the six speed GS550 variant, too. An O ring on the switch body seals it against the crankcase and two crosshead screws hold it in place.

Picture 11: Up the top end now, I’ve replaced each piston on its original con rod and oiled the small ends before sliding the pins home. I always use new circlips for this job as a circlip coming loose on the rebuilt engine will do disastrous damage to the cylinder bores. Note that I removed only the outer circlip from each piston when I stripped it down, so I need to fit only one new clip to each piston now.

Picture 12: I’ve greased and fitted a new base gasket along with two new egg-shaped rubber O rings around the outer rear cylinder studs before fitting the barrels. Lots of oil on the bores and piston rings makes this a fairly straightforward job, though it takes some patience and care to make sure the rings are properly engaged with the bores before sliding the barrels home on the crankcase mouth. Also make sure the ring gaps are positioned at roughly 120-degree intervals to each other; if all the ring gaps line up it could reduce compression in the cylinder. At the top of the barrels I’ve fitted a new camchain tunnel seal and head gasket before fitting the head. Note this extra hole in the head gasket; early engines like this one could weep oil at the front after high mileages and Suzuki added an extra bolt on later engines. The late gasket is now the only one supplied but it will do the job fine. Note the rear camchain slipper blade I fitted before the barrels went on, and the two rubber seals on the outer cylinder studs.

Suzuki GS750 engine rebuildPicture 13: Fitting the head is simple enough, but do remember to torque down the head studs in sequence and in two stages to make sure it doesn’t warp. The nuts should be torqued to 27 lb/ft. Now I can slide in the front camchain slipper blade. As this engine has had the valves reground, the chances are I’ll have to shim it up from scratch but, to give me a starting-point to work from, I’m fitting the original shims back where they came from. Note the size etched on the shim face: this should always go face-down or the action of the spinning cam lobe will polish off the size and make life harder when you need to change a shim. Note also the tacho drive gear I’ve reassembled into the head before I fit the cams.

Picture 14: Timing the cams is easy enough if you have a manual or drawing to work from; I sketched the position of the timing marks before stripping the engine. As a general rule on any dohc four, with the engine set at 1&4 TDC one outer exhaust cam lobe should face directly forward and its counterpart on the inlet cam should face directly backward, but there’s no substitute for timing marks and the timing must be correct. I find as I’m bolting the exhaust cam caps down this engine has an annoying habit of jumping round a tooth, so eventually I refit the central idler sprocket first to take up slack in the camchain and keep it tensioned on the cam sprocket while I tighten it down. Finally I rotate the engine forward a couple of times and check and double-check the timing marks are correct. If it’s not right, go back and retime it now before moving on.

Picture 15: Now the long and painstaking process of shimming the engine can begin. Rotate the engine until each cam lobe faces away from the shim and measure the gap. Comparing the existing gap with the required gap allows you to figure out what size shim you’ll need to fit to get the correct clearance of 0.03mm to 0.08mm. The problems begin when the valves have been ground in and you have no clearance to start with – all you can do then is work with the thinnest shim you have and fit it into each tight valve until you get a starting clearance to work from. It’s easy to do, but can be long and tedious and requires careful note-taking. To change shims you can take the cams out, but life is easier if you have a tool to hold the valve partly open while you pop out the shim. This is actually a Kawasaki Z900 shim tool, but it works fine on the GS engine.

Picture 16: With the engine shimmed up I can now fit the camchain tensioner to the back of the cylinder block. The tensioner body has a pinch bolt and locknut on the side, so I’ve wound back the thumbwheel and pushed the main plunger back into the tensioner body as far as it will go, then nipped up the pinch bolt. This means the tensioner will not be under spring pressure while I bolt it up. Once it’s fitted I can loosen the pinch bolt again before rotating the engine a couple of times to set the chain tension correctly.

Picture 17: I’ve left the sump till almost the last job on this engine for two reasons. Firstly, it makes the engine much more stable on the bench and stops it wobbling around while I work on it; secondly, it means if I did inadvertently drop something down the camchain tunnel it would simply drop out on the bench. Before the sump goes on I’m refitting the sump strainer gauze which is held on with three M6 crosshead screws.

Suzuki GS750 engine rebuildPicture 18: This plate mounts on the top of the crankcase and carries the oil pressure switch. The switch works by earthing out against the crankcase to illuminate the warning lamp; when the engine is running oil pressure pushes the plunger upwards into the housing and breaks the circuit. As these crankcases have been painted silver to mimic the original finish, I’ve scraped some paint off from around the oil hole under the switch to make sure it works properly. If the paint succeeded in insulating the switch plunger the oil pressure light wouldn’t work.

Picture 19: The starter motor drops easily into the housing on top of the crankcase and slides into position. There’s an O ring on the starter motor body which must be in place to seal it against the crankcase. Two M6 bolts hold it down. I usually find it easier to fit the lead to the motor before fitting it on the engine as space is quite restricted to bolt it on later.

Picture 20: I’ve engaged the starter clutch into the back of its housing, which on the GS engines is bolted to the back of the generator rotor. This means I can then offer the whole rotor and starter clutch up to the crankshaft as an assembly. Two needle roller bearings give the starter drive gear a surface to spin on, and the bronze thrust washer controls sideways movement. The generator rotor fits on a taper on the crank end and is usually pretty bullet-proof – some bikes do suffer from the rotor coming loose on the taper and, if there’s any sign of problems here, I’d lap the tapers in with a little grinding paste before assembling.

Picture 21:The starter motor links up to the starter clutch with this reduction gear. There’s a plain steel thrust washer either side of the gear, and I’ve found the easiest way to assemble it is to stick one of the thrust washers on the engine casing, then offer the gear into place before sliding the freshly-oiled spindle in. Finally the outer thrust washer fits on the shaft.

Suzuki GS750 engine rebuildPicture 22: The generator stator is still bolted into the back of the outer engine cover, and there’s no need to remove it unless it needs replacement or rewinding like the spare one here. The GS series developed an unfortunate reputation for eating generators owing to the strange wiring circuits on the standard loom. One set of generator coils is  linked in only when the lights are switched on, which means that some parts of the stator can run cool and some hot, leading to cracked insulation in the wiring and burned-out stators. The temperatures are equalised by engine oil in the generator cover and, if the engine oil level drops too low, the generator suffers first. Moral: keep the engine oil at the correct level and ride with the lights on all the time. If rebuilding the bike, it’s possible to convert the wiring to the kinder Kawasaki system, but that’s another article!

Picture 23: With the outer engine sprocket cover bolted up I can now fit the clutch lifter mechanism. This follows a conventional ramp-and-ball design and rarely gives any problems. The three balls are captive in a pressed-steel cage, and Suzuki’s attention to design details has even provided three small rubber pads to seat the two halves of the lifter together. I’ll give this a good smear of grease before fitting it. Before fitting the outer inspection cover the clutch lifter arm fits on with the central adjuster pin. Clutch adjustment is set by winding the adjuster in until it just meets the end of the pushrod, then winding it out a half a turn and locking it up with the locknut.

Picture 24: Almost there now. The final job on Jason’s engine is to refit the auto-advance unit and points assembly and retime the ignition. The procedure is more or less the same as I described in Mechanics issue 193 using cigarette papers to static time the points backplate. With the engine fitted and connected to the bike’s electrical system, it’s possible to use an electrical testing lamp to set the timing, but this can’t be done with the engine on the bench. The timing will be accurate enough to provide an initial start-up, but I always recommend it’s fine-tuned with a strobe once the bike is running.

Picture 25: And here she is, cleaned, polished, sorted and ready for many more years’ solid service. Once again my thanks are due to John Kemp at Inox Fasteners for the superb stainless screw kit which really does add the finishing touch to the rebuild. bike

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