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Discussion Starter · #1 ·
Hi,
I have done a lot of work to my ‘71 CB250 K3 and have it almost running on the button, except for a flat spot when the throttle opens from idle, almost causing the bike to stall. I believe it is from a rich needle, allowing a lot of fuel through just as it lifts out of the emulsion tube, and wonder if there is a way to adjust the height or source a different Keihin needle or tube?

I have the original Keihin needles installed, but the same problem happens when I have used replacement Keyster needles from a CB250 rebuild kit.

The reasons I suspect that it is being caused by the rich needle setting and not a sudden starvation of fuel are...

1 - Using a Gunson Colortune I can see the flame turn bright orange when I open the throttle slightly.

2 - The flat spot only happens when the engine has warmed up.

3 - Once warm, the engine pops at around 3000rpms when decelerating with the throttle slightly open.

I have another set of used carbs I may rebuild and use as a test but was hoping for any suggestions as to what I can do to adjust the needle on a CV carb to at least identifying this is the reason?

Many thanks!
 

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You might try raising your float level a touch (lowering fuel level) before mucking with the needle position to confirm its rich. I think you can shim needles on these carbs, but not sure...
 

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Discussion Starter · #3 ·
You might try raising your float level a touch (lowering fuel level) before mucking with the needle position to confirm its rich. I think you can shim needles on these carbs, but not sure...
Thanks for the advice middletons.

I’ve had the float level at 19mm but the top of the floats have to hit the top of the chamber in order to be so high.

I’m currently running them at 26mm on the advice from the forum, but recently tried the clear tube method and found that to get the fuel level 3mm under the gasket level, 22mm seemed to be the ideal setting.

There’s no hesitation if I increase the throttle very very slowly. If I rev hard and quickly it rarely dies. But twist the throttle at a normal speed and it wants to stall/die as soon as the throttle opens from the idle position.

On closer inspection using the Gunson Colortune, it looks like there is a moment of black between the blue flame of the idle and the orange flame of the needle jet at higher revs. It’s a fraction of a second, but appears that on both cylinders there a split second where it goes from blue to black to orange. Not sure if this is standard and the engine can cope with a quick change from the idle mixture to a richer needle mixture?

How can I tell if it is either...

1 - too much fuel suddenly being drawn through as the needle lifts

Or

2 - a lack of fuel/too much air caused by the change in atmospheric pressure?
 

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Discussion Starter · #5 ·
what carbs are you running (stamped on side)?
They’re 725b’s. I know the 722a’s on the 350 of that year had a 26mm float level.

Additionally, the more I turn in the throttle screws to increase the idle the less problem with hesitation when opening the throttle.
 

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none of my carb info references 725's, so can't help there. shoot for middle of the float being level with body, then tweak from there i guess.

if the prob goes away when you get into higher revs, then that points to the idle circuit being the problem. as you get above idle, the idle circuit is not in play anymore. The needle circuit doesn’t come in til later. my guess is that it's a combination of float level and idle mix screw setting that is the prob.
 

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Morning, out of interest have you used a rebuild kit for the 725s?

I have the same bike and year but can not seem to find a rebuild kit �� for the 725s.

Cheers
 
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