Funny CV carb problem

Bike suddenly developed a stumble just above idle, in
the 2500-4000 RPM range. Higher RPMs and idle were
fine.

Having just refueled, I figured bad gas. Dumped the tank,
cleaned the filters, refilled with some carb cleaner and
found the problem unchanged.

Checked the compression and spark then pulled the
carbs to clean them. Jets were all fine, but on the second
carb, (naturally it had to be the second) when I pulled the
slide, I discovered that the needle was just bouncing up
and down loosely.

For reasons I don't understand, the screw that retained
the needle had suddenly decided to come loose, meaning
that the needle was moving up and down totally out
of sync with the carb slide.

Put the whole mess back together and it's now running
fine. Passing this along because it came as such a total
surprise to me. Something else to think about when
rough running leaves you baffled.

On Aug 8, 7:22 pm, bob prohaska's usenet account <b... RemoveThis @www.zefox.net>
wrote:
> In rec.motorcycles.tech Rob Kleinschmidt <Rkleinsch1216... RemoveThis @aol.com> wrote:> Bike suddenly developed a stumble just above idle, in
> > the 2500-4000 RPM range. Higher RPMs and idle were
> > fine.
>
> [snipped details]
>
> This wasn't by any chance on an SV650 was it?
>
> Mine developed exactly the same problems a while ago
> and I'm finally getting round to investigating. Hard
> for me to believe the loose needle would do just what
> you describe, but it's worth investigating.
>
> How much free motion did you detect in the needle
> relative to the slide? I'd like to do the measurement
> without pulling the carb. Do you think it possible?

BMW R100GS airhead twin.

Probably 1/4" motion or better in the needle.

Maybe if you held the throttle open to pull the slide
up, you could then wiggle the needle or hold it
with your finger as you backed off the throttle.

Myself, I think I'd probably open the carb(s) up so I could
have a look at the diaphragm and make sure the needle
was on the right notch.

A friend says he had exactly the same problem on a
brit sports car (Healy ?) with a CV carb.

> Thanks for posting!

Please forgive the motorcycle content.