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CB550 - full trottle performance
Laverda Dave:
--- Quote from: K2-K6 on May 27, 2021, 11:37:55 AM ---Octane in absolute terms is associated primarily with compression ratio and wouldn't change the mixture. It's mixed in with the causes of "pinking" with lean mixtures but the two reasons (too low octane induced, overheat conditions) need to be considered separately.
The overall experience from plugs and riding description are saying too lean, although not by much, which need to be addressed to bring it into more normal combustion performance.
The idle jets won't affect the upper ranges of rpm from adjustment of the air screw, but only if you changed their size. Once the idle jet reaches maximum flow it can't contribute more however the screw is set. The adjustment alters the slope of gain for idle jet flow and not the maximum.
Air screw further out will bring maximum flow at higher speed, in comparison to turned inwards when the idle circuit will reach maximum flow at lower rpm.
Worthwhile for some contingency protection while running is to raise the plug number by one to mitigate against plug tips going over temperature potentially causing pre-ignition, which you really don't want.
--- End quote ---
Thank Nigel. I have just learnt something new. Everyday is a school day :)
Erny:
Just measured Keyster needle (declared for carbs 022A) - it has 2.520mm that somehow corresponds to original Keyster 2.515mm.
Replacing them will (to me) make mix even more lean.
K2-K6:
Unsure I understand which way you are saying that, but lower number in diameter will give richer mixture as it blocks less of the main jet hole.
That for the needle itself.
Erny:
Exactly this is what I'm saying (sorry for not being clear).
According to Gerben (who did rebuild) now I shall have in carbs genuine Keihin needle (shall have 2.515mm according to HSM or bit less as it can be worn to some extend) in combination with new Keyster tubes.
Keyster needles I have available have 2.520mm so installing them into carbs will make mix even leaner.
K2-K6:
Yes I see which way round you've described it, and agree with you about the change it would make.
Possibly worth describing the problem as I understand it;- it's running almost perfectly as regards mixture for steady state (not accelerating) combustion.
With slide type carburettor this "perfect" mixture is reluctant to increase speed due to characteristic within the system in that as you start to open the throttle the immediate change is to reduce the vacuum in the carb throat, and force the mixture to move toward lean just at that point. This prevents the engine from gaining speed in this circumstance ordinarily.
The way this is compensated for is the manufacturer sets the mixture effectively too rich for technically ideal running to make sure it can cover this inherent error. It's one of their limitations on a road engine as it will always have to run richer than perfect to make sure you can easily accelerate it. We also get used to seeing plug colours that are effectively showing slightly rich and accepting those as normal, but they really show the error we have to run to describe them fully accurately.
It's this that CV type carbs and accelerator pump equipped slide carbs are designed to eliminate.
Back to this engine and carbs though, and you can judge where it is mixture wise from your riding description. As you open the throttle fully it makes it too lean to gain rpm but backing off a little just brings the vacuum and so the mixture back into competent range to get the increase.
You can see from this that it's running with just enough excess fuel to cover light to medium acceleration, but not fully wide open.
What it needs is just that final extra percentage to completely cover all throttle application.
It's very close but with that obvios error. Decision is how you make that final adjustment to get it just right.
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