idle Speed Question

This is not directed specifically for me. But more of a topic I had a question on.

Owners Manual States that Stock Idle Speed Setting is 1000 RPM +/- 50.

I have seen some people with performance cams / race cams that need to Idle as high as 1800 RPM.

Question is simple - Besides adding Cams what other factors could contribute to a bike running better at a higher idle than stock?

Carb holder leaks and ignition timing are two issues that spring to mind, but I'm sure there are other factors.

Carb holder leaks have a much more profound effect at low rpm because the slides are pretty much closed down. With the slides down, any air leaking in through carb holder cracks is a higher percentage of all air entering the carb, and this can affect the engine idle. With the carb slides raised, a greater proportion of the total air coming into the carb is coming through the intake throat, so the higher the slides the less noticeable the carb holder air leaks. In fact, at wide open throttle carb leaks have no noticeable affect at all.

The ignition timing advances (up to a point) as the rpm level rises. If the timing is off a bit, the effect of the timing advancing can help the engine run smoother. Ed

That is some good info. I can understand both of those. I guess I was curious if higher compression could cause a need for an increased idle RPM..

Higher compression will not require a higher idle speed. If everything else stayed the same and you just raised the compression, chances are you could get away with a lower idle rpm if you wanted to.

Cool

Thanks for the tip. I enjoyed going through your build page before. Nice job.

M

Big carb's on a small motor can cause a need for a higher idle.
My motor seems to like around 1400-1500 and a lot of that is the VM29's.

With the smoothbore's on my small motor they get a weak idle signal
causing me to have to run a huge pilot jet.
Sudco lists for a 750 cc motor base line jetting of 25 pilot and a 125 main
Right now I'm running a 22.5 pilot and 125 main.

Un-synced carbs is a factor that could contribute to a bike running better at a higher idle than stock.
As poor carb sync is less influential at higher rpm, un-synced carbs might disallow running at 1000 rpm but permit running at higher rpm.

Carb pilot circuit issues are also a factor that could contribute to a bike running better at a higher idle than stock. An imperfect pilot circuit could disallow running at 1000 rpm but permit running at higher rpm.

A leaking valve, for whatever reason (such as for example due to insufficient valve clearance) may also contribute to a bike running better at a higher idle than stock. Failure to hold an idle after attaining normal operating temperature is a classic symptom of Insufficient valve clearance.

Good Fortune!