SOHC.co.uk Forums > CB750
Jetting?
Arch stanton:
Never mind.
K2-K6:
Subtle changes you could use to edge it towards better response would be, close the idle jet screws down to approx half normal setting, raise float height a couple of mm if possible, and lift the main needle jets a notch (move circlip down) to give more fueling across the whole range.
It should at least let you see if any improvement is made to running.
Try it across its rev range, then look at plug colour to see what you're getting.
Arch stanton:
Thank you for your intelligent response to my question.
Trigger:
--- Quote from: Arch stanton on April 14, 2019, 09:21:21 PM ---Thank you for your intelligent response to my question.
--- End quote ---
My responce was a intelligent one :o If you think a F1 is the same as a K1 then, think again. Different high compression pistons, different cam and so on.
You will never get it to run perfect across the full range because it was never designed to run on pods and a straight through exhaust. You will have to sacrifice the 2000 rpm range to get good top rpm or vice versa and you will always get a flat spot mid range but, what do i know ;D
I have been asked this question a million times over the years and when i have met people who recon they have it spot on, i ask for the keys to the bike to test it for myself and surprisingly they decline ???
K2-K6:
I think generally over the years people just throw bigger jets at it, which heads towards the results described above.
To expand briefly, when Honda set it up it had a vacuum gradient (if that's the right description) with the airbox etc in place. This they matched with carb components to get fueling in the right range. Specifically the throttle slide cutaway and shape of the needle are designed to meter correctly with the prevailing vacuum.
When airbox is removed, the air flow now has a different shape of vacuum "mapping" for the same engine speed and airflow it had before.
Really what it needs is a throttle slide with a different cutaway, plus a change in form of needle to match it. As that's not going to happen in most situations you're left with trying to compensate with what you can change to try and bring back a parity of mixture over the engine response range.
Overriding is that the engine doesn't need to burn more fuel to work, it just pulls less fuel than it did because of altered vacuum. So it mostly runs leaner.
If you just put in bigger idle jets to correct low speed running it may fix that but shift the blend to main jets too rich, so you get a blubbery flat spot as you come up on the throttle, which you struggle to get rid of. If you up the mains, it may correct the top but not link to the lower, hence the steps in response.
You are in effect running into and out of ideal fuel mix ratio. Less than 10 parts air to 1 fuel will give you too rich, this generally gives you those "dead" flat spots as the plug won't reliably fire the mixture. So you can at least feel which way the error is.
More than 13.5 parts air (on conventional carbs) to 1 fuel will give you those lean "waffety" slow responses that just feel lethargic. This is usually because the combustion mix fires but fails to completely burn in its entirety. Mostly, you should be getting this, so trying to push it back towards correct range without overshooting straight out the other side to rich ratio, is what you're trying to get.
The answers usually don't conform to convention (as compared to book values) but by squeezing it with the adjustments you have, it can be reasonably set up.
Out of interest, what plugs and plug caps do you have in it?
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