1. This looks like a challenge. We've decided to strip the engine completely and see what we find, though after standing for so long I anticipate spending some time with seized bolts and sheared off studs. The fuel tank and side panels come away easily, and I'm surprised to find the exhaust unbolts from the cylinder head without problems. Yamaha used big 10mm studs and dome nuts for the exhaust headers, so even after years of corrosion a good heave on a spanner slackens them off without shearing. So far so good. At this point I remove the sump drain screws and let the engine oil drain as work continues.
2. Removing the carbs on these bikes can be quite tricky as there is very little room between the rear of the cylinder head and the frame. I started by removing the air filters to make as much space as possible.
3. Even with the filters removed there's insufficient space to pull the carbs off, and I found it easiest to unbolt one of the inlet rubbers first. The carbs can then be waggled clear as an assembly, leaving one inlet rubber on the head. I unhooked the throttle and choke cables, then laid the carbs aside for attention later. The heavy smell of stale fuel confirms that the carbs will have to be completely stripped and rebuilt before we can even think of starting the engine later on.
4. Next, the upper engine mountings. The head steady brackets themselves are quite heavily rusted but with a little persuasion all the bolts eventually come free without shearing. All bikes will suffer from some cosmetic corrosion in this area under the tank, as condensation accumulates here when the engine is warming up.
5. At the front of the engine this large diameter oil feed pipe runs upwards from the crankcase and branches into two seperate feeds for the rocker gear. A gland nut secures the pipe onto an adaptor screwed into the crankcase, and first attempts at unscrewing the pipe resulted in the adaptor loosening off with the gland nut securely rusted to it. Eventually I managed to use two 22mm open ended spanners to free it off, but it's easy to damage the pipe in the process.
6. I continued the strip by removing the ignition components from each side of the head. The XS650 design is unique in that it carries the points on one side of the engine and the auto advance unit on the other, a shaft running through the centre of the hollow camshaft to link the two together. I marked the points backplate with a screwdriver before removal so I can replace it in the same position later on.
7. On the other side of the head the auto advance unit needs to be carefully dismantled for removal from the crossover shaft. A large flat sleeve nut holds the auto advance backplate in place, once unscrewed the crossover shaft can be slid out of the camshaft and stored away. This whole mechanism may look complex at first glance but it's actually quite straightforward to work on, albeit a bit fiddly.
8. With the points assembly, auto advance and crossover shaft removed the two housings can be removed from each side of the head. Each housing carries an oil seal which runs on the camshaft itself. These oil seals will be replaced during the rebuild along with the gaskets and "O" rings which seal the housings against the cylinder head.
9. I'm not going to disturb the rest of the top end until the engine is out of the bike and on the workbench, so at this stage I turn my attention to the side covers. The more I dismantle, the lighter the engine will be when I come to lift it out of the frame. On the left side the engine cover lifts away to reveal the generator unit and drive sprocket. The generator stator is retained in place with two long phillips screws, with these removed and the electrical connector unplugged the stator can be lifted away and carefully stored. I'll leave the drive chain on the gearbox sprocket at this stage as it will allow me to lock the engine with the rear brake, useful when unfastening the clutch centre nut and primary drive gear.
10. On the engines right side now, the first step is to remove the oil filter cover and filter. The filter is a washable gauze type and can be cleaned in paraffin and dried off with compressed air before refitting later. The brake pedal and right hand footrest come off next, which allows room to remove all the screws and lift the cover away. I had to give the front of the cover a couple of light taps with a hammer to break the gasket seal. The tacho cable should also be disconnected at this stage.
11. Removing the right hand cover reveals the clutch and primary drive assembly. Unusually the XS650 engine carries its oil pump mounted inside the engine cover, driven off a gear from the primary drive. This gear also drives the tacho cable. I'll take a close look at the oil pump later. The clutch assembly will have to be removed to gain access to the gear selector mechanism behind it.
12. With the clutch springs, pressure plate and friction plates removed the whole clutch assembly is held on the end of the input shaft with a single nut retained with a locktab. This nut is tight, and some means of stopping the clutch drum from rotating is useful whilst undoing it. Not for the first time I'm resorting to my trusty Kawasaki service tool, a rather crude device fabricated from two steel claws welded onto a pair of Mole grips. Not a difficult thing to copy if you wanted to make your own. Alternatively a couple of old clutch plates could be drilled and bolted together, effectively allowing you to lock the inner and outer drums together. A foot on the rear brake pedal with the bike in gear should then hold everything steady.
13. With the clutch removed the kickstarter mechanism can be lifted away, giving full access to the gear selector. The selector claw unhooks from the end of the drum and then slides out of the crankcase. Watch for the small springs that hold it in contact with the drum. Now is a good time to remove the detent roller and spring that clicks the selector drum into position. Watch also for any shims and washers on the input shaft, these will need to re-fitted in the same order during the rebuild.
14. Releasing this nut from the end of the crankshaft permits removal of the primary drive and oil pump drive gears, which should simply slide off. The primary drive gear is located onto the crank with a woodruffe key, collect it and store it safely for later.
15. The starter bendix assembly can now be released and lifted off its shaft. On the XS650 the starter motor is bolted underneath the rear of the crankcase and drive is transmitted to the bendix by a crossover shaft, seen here just to the left of the bendix. The crossover shaft has a splined gear at each end which simply push on to the shaft, the whole thing being located by the small catch plate and single screw seen here. Be careful if you're not intending to strip the engine completely; if you remove the catch plate and slide out the crossover shaft it will be almost impossible to slide it back into engagement.
16. Now at last I'm ready to remove the engine from the frame. First I make final check round the bike to make sure I've disconnected all electrical connections and control cables. Surprisingly all the engine bolts loosened off without too much trouble, and with the engine plates removed the engine lifts up and clear from the right side of the frame. I was working alone at this point but the assistance of another pair of hands is always useful when lifting an engine out.
17. With the engine supported on the workbench I can now proceed to strip the top end. Firstly the cam cover unbolts and lifts clear, revealing the single overhead cam and two-valve-per-cylinder engine layout. The cam followers remain captive in the cover, at this stage I'll put the whole assembly aside for examination later. The outer four cylinder head nuts sit atop rubber washers, these need to be replaced on the same studs on re-assembly to prevent oil leaks.
18. According to my workshop manuals it is necessary to split the camchain at this stage, and rivet it up again during re-assembly. I have an aversion to riveting camchains, preferring to use an endless chain when possible, so I decided to see how far I could strip the engine without splitting the camchain. The camshaft is supported on two pairs of hefty roller bearings, one pair at each side, and with the camchain tensioner removed there's enough slack in the chain to slide the bearings off the ends of the camshaft. With the bearings clear the camchain easily unhooks from the cam drive sprocket, and the camshaft can be lifted free.
19. Now the remaining three small bolts holding the head to the barrels can be removed, and the cylinder head lifted clear. Note the big screwdriver I've slid into place to stop the camchain dropping into the crankcase as I lift the head clear.
20. Removing the barrels is simply a matter of sliding them upwards and off the pistons. Be careful as the pistons emerge as they can rattle against the crankcase mouth. Again the camchain will need something to stop it falling into the crankcase. I've tied a piece of electrical wire around it and hooked the wire around a casing screw while I lift the barrels clear.
21. Now I can remove the pistons. Delicate work with a set of small pliers gets the circlips out, and the gudgeon pins slide out to release the pistons from their respective small ends. I always discard old piston circlips to avoid the temptation of re-using them, and I've marked each piston by scribing "L" or "R" inside the skirt to make sure they go back on the same rods on re-assembly. If in doubt it's also worth scribing a mark to indicate the front of each piston, though on this engine I've already noticed Yamaha have cast direction arrows on top of the piston crowns.
22. The rear camchain slipper blade is fixed into the top of the crankcase mouth with four phillips head screws which can now be released. And here I finally discover why it's necessary to split the camchain to totally strip one of these engines; the camchain loops around a small tubular brace on the slipper blade mounting. However I have learned that it's possible to strip the engine's top end with splitting the camchain, which should save a few headaches if you just want to do a rebore or valve grinding job.
23. Turning the engine over now, I can remove the bolts which retain the sump and lift it clear. The sump filter is a gauze strainer which can be cleaned off in paraffin and dried off. A small magnetic plate is bonded along the gauze and will also need careful cleaning before re-fitting.
24. A quick check round to make sure I've removed all the ancillaries and it's time to split the cases. Like all Japanese engines, there are crankcase retaining bolts fitted from both the top and the bottom half, and it's important to find them all and remove them before tapping the cases apart with a rubber mallet. With the joint split, the lower casing just lifts away and leaves the crankshaft and gearbox shafts in the upper casing.
25. And finally the crank and gear shafts lift out. Just to prove a point I've left the camchain in place while I lift out the crankshaft. So it's even possible to remove the crank without splitting the camchain, should you so desire.

Next update I'll be taking a good look at the parts from the stripped engine to see what needs replacing for the rebuild.

Dr.Rod


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