Last update I stripped Lorenzo's 1979 XS650 engine down for exploratory surgery. The bike had proved troublesome on the daily commute, and had been relegated to the back of a damp cellar for almost five years, neglected and forgotten. Despite the ravages of time the engine proved fairly straightforward to dismantle, and I can now turn my attention to examining the internals to see what parts need attention before the rebuild.
||1. With the cylinder head off the combustion
chambers show signs of heavy carbon deposits, particularly on the left
cylinder. This looks dry rather than oily, and leads me to wonder if the
bike had been running too rich. Before cleaning up the combustion chambers
I'll take a closer look at the pistons.
||2. The piston crowns also have heavy carbon
deposits, but otherwise look in fairly good shape. A few minutes work
with some steel wool and Solvol Autosol reveals shiny alloy beneath. The
pistons are standard, indicating that the bike has not yet been re-bored.
This gives plenty of scope for re-conditioning if required.
||3. The ring gap measurements are well within
factory tolerances. I always measure the gaps at both the top and the
bottom of the stroke. No wear ever occurs at the bottom of the cylinder
bore, so an excessive gap here gives a measure of piston ring wear. Any
wear on the bore itself will be at the top of the stroke; if the ring
gap is appreciably larger here a re-bore might be necessary to recondition
the bores. These simple measurements give a quick and accurate indication
of both bore and ring condition.
||4. Satisfied that the bores and piston rings
are serviceable, I can now turn my attention to the valve gear. Each valve
is retained in place by two split collets engaged into the top valve spring
collar. A valve spring compressor is needed to compress each spring far
enough to remove the collets, then the springs can be lifted off and the
valves slid downwards out of their guides.
||5. Once again, evidence of heavy carbon deposits.
The backs of all four valves need a good cleaning up with a rotary wire
brush before proceeding. The inlet valve on the left has still to be cleaned
off, the one on the right has been cleaned up. Carbon build up like this
won't significantly affect the running of the engine, but does nothing
to help gas flow and should be removed while the valves are out.
||6. The valves need to have a good compression
seal in the cylinder head. After many thousands of miles the valve sealing
surface, indicated here, will begin to develop small pits as the metal
suffers from constant wear and heat. Running the engine with incorrect
valve clearances will also contribute to rapid valve pitting.
||7. The valve seats in the cylinder head suffer
in exactly the same way. With the combustion chambers cleaned up some
pitting is evident in the seats. This is the area to watch carefully on
older engines designed to be run on leaded fuel. Modern unleaded petrols
do not have the additives required to protect these vulnerable areas,
and a professional unleaded conversion will include the fitting of new
valve seats and new valves made from higher grade hardened steel.
||8. In my experience most Japanese engines,
even quite old designs, will cope quite happily with unleaded petrol.
However the hardened steels are tough to re-condition by hand, and traditional
valve grinding methods don't usually make much of an impression on them.
Before sending the head out for re-conditioning I decide to try the
traditional method of valve grinding using a wooden stick fitted with
a rubber sucker. With a little grinding paste smeared on the valve seat
I can then rotate the valve by hand, lapping the valve head into its
||9. Lo and behold the traditional method
works a treat, producing a splendid matt grey ring around the valve
head and its matching seat. Two or three applications of grinding paste
are sufficient to remove all traces of pitting, and I can finish off
by carefully washing the valves and seats with petrol. However the ease
with which I lapped in these valves makes me suspicious of running this
engine on unleaded fuel, and I'll be recommending Lorenzo uses a fuel
additive like Wynns Valve Guard when running the rebuilt engine.
||10. The XS650 follows a traditional SOHC
design and has four rocker arms held captive in the rocker cover. With
the large diameter chromed plug removed the rocker pivot pin should
simply slide out and allow the rocker to pull free. In practise the
pivot pins are quite a tight fit in the rocker cover, but Yamaha have
thoughtfully drilled and tapped each one so that an M6 screw can be
wound in to help remove it. I found that by packing washers against
the side of the casing I could progressively wind an M6 allen screw
into each pin to withdraw it.
||11. The rocker arms have the valve clearance
adjuster at one end and the cam follower at the other. Each follower
rides over its respective cam lobe whilst the engine is running, and
is susceptible to wear, particularly if oil changes have been missed.
Some slight scuffing is evident here but doesn't warrant the price of
fitting a replacement. Heavy wear would mean replacement, and possibly
a new camshaft to boot. Check the valve clearance adjusters too, these
are prone to pitting on the XS650 motor and might need to be replaced.
||12. To remove the ravages of corrosion
and years of baked on dirt we have opted to have the head, rocker cover
and barrels bead blasted. This is a specialist job and needs to be done
with care. It's all to easy to get tiny amounts of grit into engine
oilways, leading to horrors once the rebuilt engine is started up. I
entrusted the parts to Paul Coward at Bikerworld, who once again carefully
masked up all the vulnerable surfaces before producing a splendid finish.
The next step is to thoroughly wash the parts in paraffin and blast
everything clean and dry with a compressed air jet. To make doubly sure
everything is spotlessly clean I then pop the bits in the dishwasher,
making sure the wife is well out of the way first!
||13. Before the valves go back in these
valve stem seals need to be replaced. They just pop over the tops of
the guides, and serve to stop excess oil running down the guides which
would cause the engine to smoke on the over-run. Replace them every
time they are disturbed.
||14. Now the valves can be re-fitted in
their original positions. The valve spring compressor allows me to hold
the valve springs down whilst popping the split collets into place on
the valve stem. A smear of grease holds them in place while I unwind
the compressor and remove it. Don't forget the steel washers under the
||15. For purely cosmetic reasons I've cleaned
up the crankcases and given them a coat of silver engine enamel. I'm
reluctant to opt for bead blasting on such complex castings due to the
risk of getting grit into the oilways. An hour spent with paraffin and
steel wool before another application of the dishwasher gives a good
enough surface to take the paint. Inside the top crankcase I noticed
the legend "Paul 1991" scratched into the alloy, evidence of a previous
re-build. I've now scratched "Rod 2002" alongside it!
||16. Last month I commented that the engine
can be stripped without splitting the camchain, and here's the evidence!
Unfortunately the chain does loop around the rear slipper blade mount,
so will have to be split if it is being replaced. The "soft link" can
be easily found as then end plate is a lighter colour.
||17. The front slipper blade is mounted
into the barrels and retained by two M6 screws. On this engine the rubber
face has become separated from the alloy blade and will have to be replaced.
Apparently this is not an uncommon problem on XS650s and is worth checking
at every top end strip.
||18. The oil pump follows conventional Japanese
design and consists of two concentric rotors running in a carefully
machined housing. Unusually the XS650 oil pump lives inside the clutch
cover, and the tacho drive will have to be removed first to access it.
Check carefully for score marks in the housing and rotor faces, but
if any damage is evident the clutch case itself may have to be replaced.
||19. While the tacho drive is apart check
for this tiny oil seal which stops engine oil escaping and leaking out
from the tacho cable. The threaded part of the cable mount screws out
and the oil seal presses into the housing beneath it.
||20. The gear cluster shouldn't usually
give much trouble, but older XS650's can sometimes develop a fondness
for jumping out of gear. Careful inspection of these dogs should reveal
any problems, rounding or wear on the pins indicating a problem. I don't
normally strip gears from their shafts unless strictly necessary, but
if replacing a single gear cog support the gear shaft upright in a vice
and carefully remove each gear, circlip and washer before replacing
in the same sequence.
||21. Any gear selector problems can usually
be traced to the selector forks. Watch for excessive wear or signs of
blueing due to excess heat on the fork tips here. Bent selector forks
will fail to engage the gears correctly and should be replaced.
||22. The kickstarter mechanism is a pretty
sturdy affair and is unlikely to give problems, though this is a good
time to replace the return spring if it has been showing signs of weakness.
The drive gear shows signs of polishing on the teeth but this doesn't
||23. The starter bendix gear looks to have
had a rather harder life, and is showing signs of wear on the edges
of the teeth which engage with the crankshaft. However looks can be
deceptive; the gear teeth are actually relieved as part of the manufacturing
process to help the starter engage. Though some wear is evident I consider
this gear still serviceable, and we'll be re-using it.
||24. The old sump filter (right)
proved to have a piece of gauze missing. Not only was this not filtering
the oil as it should, it also makes me wonder what happened to the missing
piece! However as no signs of foreign bodies were evident in the engine
I'll simply replace it with a new one (left).
||25. Final thing to check before the rebuild
are these two carbon brushes which fit onto the alternator stator and
bear against two copper tracks on the rotor. Excessive wear on these
brushes is a common cause of charging problems on these bikes, and is
easy to overlook. These are about half worn, and should be good for
a lot more miles.
And that's all the bits sorted out. The engine has proven to be in better condition that we'd expected, and I'm wondering if Lorenzo's original problem was down to a carburation fault as evidenced by the excessive carbon deposits in the top end. Reconditioning the valves should help it to run better, and we've decided to replace the camchain as a precaution while it's being rebuilt.
Next month I'll be clearing the workbench and bolting it all back together. Thanks to Paul Coward at Bikerworld (tel 01422 844681 or 07944 698793) for the bead blasting.