Acceleration Misfire

[Saesneg Shaun]

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.

Thanks it is a K6, good to know as I thought it was the other way around.

[K2-K6]

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.

Thanks it is a K6, good to know as I thought it was the other way around.

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.

[Martin6]

I would love to get to the bottom of this. Being used to an Amal's airscrew, and also having always understood if the screw is before the slide (on the airbox side of the carb), then that's air adjustment, if after (cylinder inlet side), then it is fuel, I was expecting the screw to work consistent with K2-K6's comment.

The advice seemed so odd, I wrote separately to Mark (Hondaman), to check this advice. He was certain. He explained the screw goes into a section of the carb which already has air and fuel mixed, so opening the screw richens the mix entering the cylinder. As he has perhaps 50 years experience on these Hondas, I took this as good.

This is such a fundamental for tuning these bikes. I'm sorry if I have caused confusion. Does anyone have a diagram of these carbs which might put this to bed?

Ted, I can't really tell any difference in temperature at the silencer. I guess one of those snazzy electronic sensors would, but my hand isn't sensitive enough.

[Martin6]

I asked this:
"Is this true for my K6, I should be adjusting the air screw clockwise (inwards) to make the idle mix leaner?"

Here is the relevant part of Mark's reply to me:
"Yep, turning inwards on all the SOHC4 carbs will make it 'leaner' by restricting how MUCH of the fuel-air
mix is pulled up the tiny jets during the intake stroke. It is already aerated, sitting inside the tiny chamber around the emulsifier tips of these little jets (for about 1.5 seconds before the air gets away from the little drop of it). "

[McCabe-Thiele (Ted)]

I thought it's called an air screw for a reason - it lets in air to reduce the flow of the petrol from the jet in the carb bowl. Interestingly the 500  and 550 have a completely different design of air screw. My 500 now has the screw with all the holes in it not the solid brass type.

When I had a cooler front  pipe on the 500 I found that screwing in the air screw for the relevant carb made the pipe as hot as the others. I'm talking front pipe being hot here not exit gas at the tail pipe.

I think Bryan will know the answer from his workshop experience.

[K2-K6]

If the "header" pipe was cool from not firing (insufficient fuel mix to ignite) because of impaired idle jet circuit, then it will pick up to operating temperature by forcing it rich with a closed air passage. 

Basically as you close the air screw down it will just force it to suck what it can get through partly blocked fuel jet. Obviously a weird set point but it can bring regular firing to that cylinder instead of none.

[Saesneg Shaun]

OK so to be clear I need to turn the air screw IN to richen it a bit?
I'm thinking of getting a cheap Amazon temperature gun for checking the header temps. Anyone used one for this?

[McCabe-Thiele (Ted)]

I'm convinced that on my 500 air screw turning out makes it leaner.
I can't comment with any authority on your model of 750.

We need Bryan or  Graham/Julie to say with certainty it might be useful to include your carb model type number that is cast on the flange end near the inlet manifold.

[K2-K6]

OK so to be clear I need to turn the air screw IN to richen it a bit?
I'm thinking of getting a cheap Amazon temperature gun for checking the header temps. Anyone used one for this?

Yes, turn it IN for richer.

If you wanted to "guinea-pig" it .... then turn then to only 1/2 out total from closed and run it, then to 2 1/2 turns out and run it like that.  Should answer the mixture question by showing what happens.

[Nurse Julie]

I'm convinced that on my 500 air screw turning out makes it leaner.
I can't comment with any authority on your model of 750.

We need Bryan or  Graham/Julie to say with certainty it might be useful to include your carb model type number that is cast on the flange end near the inlet manifold.

It's an air screw on all Keihin CB SOHC 4'S carbs, except the Keihin PD carbs. Turn the screw in for less air = richer, turn the screw out for more air = leaner.

[K2-K6]

As Julie notes ... they are called AIR SCREW ADJUSTMENT in original CB750 workshop manual.

Further ... the fault finding table in assisting diagnosis of idle problems is consistent in description of faults and detection with how we understand it here, that its adjustment of air in modulation to effect a change in mixture.

[Martin6]

OK. I will follow that advice from now on. I'm sorry I took us off at a tangent. Thank you.