Fuel level cb750k2

[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.

[Martin6]

Yep, you described the way an air screw works.

I wrote to Mark directly, to double check what he had written and he confirmed:
"The idle-air screw in those carbs (all of the 750 roundtop carbs) controls the amount of fuel-air mixture that gets pulled up for each intake stroke. It is already mixed as fuel & air inside the tiny tube on the top of the idle jet, where those holes are, when air from the little brass orifice at the back of the carb's bell met the rising fuel in the jet's metering hole. Then, the position of the screw admits more (or less) of this mixture during the intake suction."


I am testing this now. I had 2 lean plugs and 2 sooty. The lean I opened up 1/8th turn. The sooty, in 1/8th. I have found two of my lean showing plugs are tan and one of my sooty plugs is improved. The other is still sooty, so I am trying a further 1/8th in.

All on non-ethanol fuel, but general riding, not a strict plug chop, as I believe my needles are in the right position, stock jets, airbox, 341 exhaust etc. Points set, timing checked, ditto valve clearances. Carbs are cleaned, synch'd and float levels triple checked. Although I do find float levels a little tricky.

So, not conclusive, but the early signs for me is following this advice has given me a small improvement.

I apologise if this has taken us away from@Pauarc's original problem. It could be relevant. Not difficult to just give it a try, after adjusting the floats to spec. 👍

[Pauarc]

Adjusted floats all within 1mls now no leaks at carb rubber it was still popping check exhaust copper washer it was leaking now no popping and air screw set to 1 1/2 so I think it was exhaust washer

[Martin6]

  👍

[K2-K6]

Adjusted floats all within 1mls now no leaks at carb rubber it was still popping check exhaust copper washer it was leaking now no popping and air screw set to 1 1/2 so I think it was exhaust washer

Good to get to the bottom of it, probably beneficial to have that odd one out float chamber now matching the others too.

The idle airscrew adjustment does allow for difference in individual flow within Honda set-up routine, being the real icing on the cake in regards to carb balance and smooth low speed running.

This is exactly what modern fuel injection systems do for smooth idle rpm, modulation of cylinder mixture by monitoring of the cylinder speed individually through ecu. Honda did this way before by observation of rpm and singular adjustment of idle circuit to get them smooth.