SOHC.co.uk Forums > CB500/550
CB550K - engine rebuild (partial or full)
K2-K6:
It backs the engagement dogs into one another which prevents you moving the lever easily. It's supposed to do this to prevent it spitting out of gear when you close the throttle, but when torque is released (as in changing gear) the changing is helped by the chain not still fully loading the gearbox. It's a subtle but significant contribution to how it changes.
But certainly there's one way to find out, if you are going to inspect and assess the cush drive anyway it's just the order you'd do that in which may help you to evaluate the gearbox condition prior to deciding to dismantle.
K2-K6:
To comment further on some of the checks " I’m thinking about:
- leak test (to estimate rings / head status to be confirmed when removed)
- redo compression test to have reference for comparison after rebuild. Last measurement was: dry: 120/120/110/130 psi, wet (teaspoon oil into each cylinder) : 150/150/145/145 psi
- detailed inspection of clutch and shifting mechanism under basket – maybe after that case splitting not necessary (?)
Any thoughts / ideas / advices what else I should do before pulling engine / stripping?"
I feel that around 130 psi dry and warm is realistic for these engines to require no urgent intervention in correcting it via rebuild. Get down below 125psi and it's getting closer to making that decision. They run remarkably well below that with no real risk or issues, but unlikely to make full power. So it's not a drastic mechanical condition, more an overall understanding of where it is.
Saying that, the numbers and noises you've posted just seem very typical of one of these, not much wrong really but desirable to plan longer term to eventually correct with rebuild.
Why would the psi readings be like that? Likely from bore glazing and perhaps light cylinder corrosion during storage with the possibility of one or more valves being left open. Again, it's not drastic, but clearly it's present. Oil to test normally seals the rings during the test and confirm imperfections in sealing pressure.
Rattle that dissapears when loaded, more or less confirms extended wear of primary drive chain along with primary drive rubber being stiff through ageing.
Backfire and rough running, suspected leak from HT circuit (your observations about being like this after getting wet) and needs looking into to prevent. Possibly the plug caps or water penetration where it attaches to lead.
With this ignition they all fire ALL of the time, so it won't missfire from crossover HT leaks. But as two plugs are on one coil (all these sohc systems) then if only one plug cap leaks HT, if that route to earth is low resistance it also can stop it's "paired" plug from sparking. That leads to excess fuel in the exhaust of both cylinders, which will then backfire when spark is re-established.
It seems a characteristic of the fully electronic Boyer that at low rpm the risk of igniting in exhaust is raised IF there is unburnt fuel in the pipe, but the original fault is that HT leakage that causes the fuel to get there in the first place.
Concentration on fixing HT leak should remove this problem, and help with rough running.
Erny:
Thanks for feedback
I checked HT leads and plugs - measured both are bit above 24kOHMs on both coils (with caps on leads), so should be w/o issue.
Additionally, I replaced carbs by 069A set with 40/100 jets (rebuilt by Gerben), issue with dark plug No2 dissapeaded. Even engine is now smoother, pull nicely from 2000 till 9000+ rpms linearly (better than before). But I have to say I spent quite some tim syncing them, and still did not reach satisfactory (to me) result, each time I snap trottle, it behaves bit different. Sometime idle rpms are lower sometime higher. Strange
Of course, rattling is still there and I still need to fix that IN rocker issue on cyl 1.
So, next I plan leak test and open valve cover with engine still in frame. Will keep reporting progress
K2-K6:
Some thoughts on syncing that you could consider as it's one of sequentially imposed conditions that I believe can lead the wrong way.
The cylinder is drawing vacuum against the carburettor slide as restriction/resistance, this is indicated to you on the dial as to how much the piston going down the bore can lower the pressure below atmospheric when measured against a fixed constant (probably a spring and plunger in the gauge) and so give you a number to aim for and adjust the slides to give parity across the cylinders.
BUT, you've got one cylinder that aggregated measures lower than the other 3 because it doesn't seal correctly. By extension, that cylinder will most likely impart LESS vacuum to be measured through it's stroke as it has effectively less restrictions placed on it (carburettor slide as restriction/resistance PLUS it's leaking) which in pure vacuum measurements forces you to lower the slide to observe parity on the gauge. Which of course is correct for absolute vacuum numbers, but not for combustion. It effectively causes you to set that throttle slide lower than the others to get the readings correct.
This is wrong for the combustion equalization of the cylinders (some of the rattling is because each cylinder doesn't input the same energy into the crankshaft so that it's rotation is more staccato) what it needs is the throttle slide to be more open than the others to make that cylinder equal the other three.
Worse than that, the error as i'd view it is doubled. The cylinder with low compression already inputs less energy into the crankshaft, setting the throttle slide lower (to achieve parity of vacuum) also adds to that error making the idle more erratic, not less.
To compensate you'd need to set the other three to parity on the gauges, and LESS (this is what modern sequentially fired ecu injection systems do to maintain smooth idle) vacuum on this compromised cylinder, which should improve idle smoothness, but could compromise higher rpm performance.
An aggregated carb setting for synchronisation would be accurate bench synchronisation and not use the gauges so as to avoid the prompted wrong settings of absolute vacuum readings. It wouldn't fully compensate at idle, but would leave more accurate higher rpm performance alone.
K2-K6:
Going on to the noises made while idling.
When new, the primary drive chain has fairly limited slack which keeps the driven components (clutch and gearbox internals) at more or less the same speed.
They are however a "remote" flywheel as they impart their own own constraint on speeding up the crankshaft when unloaded (idling and not driving the bike) and by nature fairly unsophisticated for noise insulation.
If the crank speed oscillation frequency arrives through a worn primary chain then these oscillations effectively keep transferring their difference in inertia by variously putting the slack in the opposing chain runs to keep then exiting the whole system which we hear obviously as unwanted noise.
If the engine running doesn't deliver equal pulses for each firing stroke it simply gets heard as more noise as it ricochets through the mechanical system.
As you raise crankshaft speed toward 1500rpm the primary chain centrifugal forces (effectively tensioning the chain) overtake the inertia from firing pulses, in addition the the firing pulses frequency rising above a point at which the flywheel natural frequency is exceeded cause the whole system to go quite. None of the faults disappear but just become harmonious.
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