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Fuel level cb750k2

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Pauarc:
As a matter of interest has anyone ever drained each carb and measured how much fuel in each bowl mine are no1 38mls, no2 31mls, no3 32mls, no4 31mls
I have a slight popping on number one at idle could this be the reason? If I set the mixture at 2 3/4 it disappears could it be because it’s running rich there’s no popping above Idle  and when running

McCabe-Thiele (Ted):
Would a difference in float height explain this?

Pauarc:
Look like carbs are coming off again to doublecheck float level on number one

Martin6:
In my experience on other bikes, popping is due to lean running from something like a leaky carb rubber, or a leaky exhaust gasket. However the float bowl difference and the mixture screw setting you're running is suspect.

It's counter to other carbs I've worked on, but the US SOHC/4 forum ('Hondaman') is clear that turning the screw out further is richening the idle. His experience is, the screw comes into play after the fuel is atomised from the slow jet, so it meters the amount of atomised fuel that gets through the carb. Unscrew it and you allow more fuel in and it richens the idle / slow speed running. If true, it is not a pilot air screw, which like you, is how I had been approaching them. Hondaman ('Mark') was a Honda technician and has written a book on CB750 restoration, so I'm inclined to listen to him.

I think your K5 is like my K6: A standard carb, with standard exhausts and airbox should be at 1 turn out, give or take 1/8th turn.  So over 2 turns out is a lot! Possibly at the point the screw no longer has any affect. Mark made another observation, that old carbs = wear and he finds then, that 7/8ths out is more like the central screw position, +/- 1/8th.

You could well have a rich float level, made more rich by the screw adjustment. Maybe take the screw down to 7/8ths out and see if it reduces the popping? But the float levels should also be set to close to recommended and be the same across all 4.

How are you setting the float levels?

K2-K6:
Agree that it would be ideal to reset the float height to get equal volume, within reasonable scope, for all the carbs.  Certainly it should remove suspicion at least for further evaluation.

I just can't see the view given by that scource about the direction of the airscrew in mixture volume though. Highly suspicious that's wrong and obviously contentious.

There's a good comparison too, with the F2 PD carbs, having the "mixture" adjustment screw on engine side of throttle slide, they emphatically work to change fuel volume rather than air. This type is turned out to increase fuel volume at low speed circuit.

These carb, K series, with screw on airbox side of throttle slide, change the air volume to alter mix by giving more air bypass to the circuit when moved outward (anticlockwise) such that the venturi vacuum can then exert less "pull" on the installed idle jet. Close them right inward, then it's pure fuel and no air they suck. Out makes it leaner, and sounds so too.

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