SOHC.co.uk Forums > CB750
Acceleration Misfire
Saesneg Shaun:
--- Quote from: Martin6 on September 05, 2024, 11:07:27 PM ---If it's a 750K6, @Hondaman on the sister US SOHC4 forum, has explained a few times that turning in the idle screw weakens the mixture, as it isn't an air screw, it is metering the flow of already mixed air/fuel. So unscrew to richen.
It still feels alien to me (I'm more used to Amal carbs). But, that's his advice.
--- End quote ---
Thanks it is a K6, good to know as I thought it was the other way around.
K2-K6:
--- Quote from: Saesneg Shaun on September 06, 2024, 08:18:21 AM ---
--- Quote from: Martin6 on September 05, 2024, 11:07:27 PM ---If it's a 750K6, @Hondaman on the sister US SOHC4 forum, has explained a few times that turning in the idle screw weakens the mixture, as it isn't an air screw, it is metering the flow of already mixed air/fuel. So unscrew to richen.
It still feels alien to me (I'm more used to Amal carbs). But, that's his advice.
--- End quote ---
Thanks it is a K6, good to know as I thought it was the other way around.
--- End quote ---
I'd be careful with that "information" as the carburettor layout shows nothing to differentiate it's idle circuit from any of the others with, what we understand as, conventional response.
Its been debated on here and US site without a verified conclusion. No logically observable working through has been forwarded, as far as I can see, to say that statement is correct.
Clearly, the PD type later carbs, along with CV type fitted after SOHC fours has the opposite response to turning the screws. This is very clearly differentiated by having the adjustment screw on the engine side of venturi as it's location. That's logical and clear in it's layout.
There's nothing on these K6 carbs to give that effect. Unless someone can adequately demonstrate it with HC and CO meter during adjustment, then it seems false.
McCabe-Thiele (Ted):
If your bike has the four silencers then I would be guided by the change in exhaust temperature when you make the adjustments.
Just using your hand at the tail pipe when the bike is up to running temperature will give you an indication of how the screw turn affects the mixture.
If you open the screw by a quarter turn it will norally result in an increse in exit gas temperature as it runs leaner.
Turning the screw in should richen the mixture resulting in a cooler exit gas temerature.
If the reverse happens then that would indicate it's not an air flow screw but a neat fuel one. Sorry if I'm stating the obvious.
This US video is basic but it has helped me get my head around adjusting these screws.https://www.youtube.com/watch?v=q9A2TL9RvwQ&t=338s
I'm not advising this a way to set your mixture but to help you work out what turning the screw does on your bike.
K2-K6:
The missing part for me though Ted is that these two design are fundamental in their differences, not just an interpretation of what it does.
Most FUEL adjustment screw type have a fixed air intake size in the carb entrance, with no modulation, then the adjustment happens on engine side of carb venturi by attenuation of the fuel passage on its way to entering the carb throat.
The air adjustment type (usually on these SOHC) up until fitment of PD carbs, are all AIR adjustment type. They are different, by design, in their application. Its not really an interpretation topic, they are designed as different system type.
The contrary claims are just that. With no (obvious and available) evidenced material to show why they should be considered differently when their design says otherwise.
If there's something available to show this, then that would be of benefit to the forum.
McCabe-Thiele (Ted):
I agree with you completely Nigel - my logic with the post about the temperature of the tail pipe gases is that it will confirm what most of us know is true & discredit the US idea that its a neat petrol flow screw rather than an air screw that reduces the draw of petrol via the jet. It makes you wonder if they meant to refer to PD carbs.
I have not been on the US sister sohc site much though I have been on some US websites for old Mercs & Nissan 4x4's. I have found the US sites occasionally full of inaccurate information re-inforcing my mental Redneck Sterotype.
I would add that when our old Merc W124 TE 4-matic broke down as Wendy was returning home one dark winter night a few hundred yards from home I turned to the US site for a diagnosis. I was in bed at gone midnight on my laptop looking for the likely cause. (the engine cranked over but would not fire - I knew it was not the OVP relay as that was new) At 1.30 am I got dressed, walked down to the vehicle, lifted the hood, removed a plastic cover on the bulkhead, with a spanner I tapped the relay as per the post advice. The car started straight away as per the US blogg advice. I repaired the relay as per the site advice to keep us mobile. Purchased a new MB relay for about £25-30.
My experience undermined my prejudices no end.
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