SOHC.co.uk Forums > CB750

Acceleration Misfire

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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?

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