Need help with a varying RPM problem

[flashback]

Maybe with all the experience here you can help me with a minor problem.

Scoot: Bandit UC-250z

Problem: The RPM wants to vary on its own. Once it has warmed up the engine RPM will vary from fast to slow. I have adjusted the idle speed up to prevent stalling when it is at the lower RPM. I have replaced the spark plug, changed types of gas, replaced the carb, vacuum lines, fuel filter, added an electric fuel pump, removed the intake air filter temporarily, all to no avail.
The scoot accelerates just fine, and runs good at high speeds but I have a little trouble maintaining a constant speed.

Any ideas?

Thanks in advance for any help.
Dave

[Jacine]

If the surging stops when throttle is applied, the throttle cable nut has probably loosened on the carb or at the bar.

[swampsniper]

Look for air leaks.

[fangerman]

Dave,
I am just noticing something similar with my scooter. I will be cruising about 25-30 mph and will feel the engine lag as if I had released the throttle. I haven't been paying too close attention to it when it happens and I can certainly re-create the effect by slacking off on the throttle, but want to see if it is just my hand relaxing or if it is something with the engine. I am taking my scooter in for a cleaning and tuneup this weekend and if I find something that looks like what your problem is, I'll let you know.

[flashback]

fangerman,

maybe we have the same problem, it will be interesting to see what is causing your problem, pls keep me posted.

thanks

[wireburn]

RPM will vary when it's running lean. I would definitely check for air leaks.

-Mike