Idle Speed Increases. Why??

My bike starts and runs really well. It's not cold natured like a lot of the bikes I read about on here, BUT, after I have run a while, the idle speed starts to creep up, until it gets to about 2000RPM. When I load it down with bike in gear and release the clutch a bit, it slows down and holds a low idle at 1100-1200, but eventually creeps back up, or is slow to return to normal. I know it it's not a cable bind, because I can slow it down with the idle stop thumbscrew, but then the idle becomes too low. It's an intermittent problem.
Any ideas?
BTW, it's a '78 KZ 650

Post edited by: TLH101, at: 2007/11/05 16:01

sounds like a vacum problem, spry some wd 40 or carb cleaner around the intake boots and see if it changes the way it runs

Yeah, I tried that, with WD, and carb cleaner. Niether changed anything.

Still sounds like a vacuum leak with a slow-returning idle. Does adding choke make a difference? How is top end? You might have one carb with a float needle not closing, or a vacuum fitting leaking, that WD-40 wouldn't catch.

Have you done anything to that area recently?

I found a little pice of something blocking one carbs pilot jet. removed it, and rechecked all orifices. Man what a difference.

I had problems with my 650 doing the same thing after it warmed up. Don't know what it was, but about 1000 miles on the odometer cleared it right up!

My guess would be that the centrifigual advance mechanism in the ignition system is advancing prematurely. It should not start to advance until the normal idle speed is exceeded. For example, a timing light should show the timing to be on the mark until the engine speed reaches, say about 1300 to 1500, and continue to advance until reaching a maximum at about 3000 or so.
If your advance kicks in at say 500 RPM, then the idle speed will go up as the advance takes effect. Then when slowing the idle speed with the idle screw, then it slows way down, since the timing is being retarded. It is like trying to hit a moving target.
I recommend checking to make sure the springs in the centrifugal advance are in place. If one has come loose, then the advance would begin too soon.

Post edited by: WABBMW, at: 2007/11/07 12:51

Hello all,

I'm brand new to the forum today and super happy that I've found you all. I've been having the exact same problem with my '76 Kz400 except it will vary even higher when idling--from 3500 down to about 900 rpm. But it always starts on the first kick. There is a little modified knob I can turn that I think is for the carb sync. If I turn it forward it will bring the idle down. Basically, I'm driving around town and turning it down at every other light.

Now, I'm a total newbie as this is my first bike and was hoping one of the above posters could maybe elaborate a bit more. I live in Austin and just got my carbs rejetted for way too much money by a guy that seems to have done a little half-assed of a job. The bottom of my left carbs seems to leak a little gas. Could this be it?

Where exactly should I try to spray WD40?

If it is the advance timing, is this something I could do on my own? What tools would I need?

I'm sorry for the cluelessness. Thanks for the help.

Aaron

Spray the WD40 around the carb holders (engine side) Also if idle changes a bunch see if turning the handle bars affects it, if so then check your throttle cable routing.

On my other bike, Ninja 250, has carbies and that happened after I competed the valve adjustment. After a quick ride to warmup the engine. I performed a carb sync, needed a slight adjust, and it was good to go.

Post edited by: love2ride, at: 2007/12/17 20:26

There's way too many possibilities. I'd go with a full carb clean making sure to check float height specs, needle and seats, clean and/or replace ALL jets and definately check your air/fuel mixture screw settings. Check the diaphrams if applicable. A tiny pin hole there will cause you lots of grief!

Another newbie here, evenin' all!
I've got to ask this because I'm not too hot at bike mechanics and it's puzzling me.
Why would anyone want to replace a jet? To my knowledge it's a turned piece of brass with a hole in it!
I can't see how the hole could enlarge in normal use and it can always be unblocked, so why replace?
I'm putting my fire jacket on as I speak...

"There is a little modified knob I can turn that I think is for the carb sync. If I turn it forward it will bring the idle down."
Smockron, are you shure thats not the idle level adjustment screw? Basically you cannot turn carb sync screws without tools. Carb sync is done once a year or when needed and should not be tampered with everyday. Syncronising the carbs means balansing the operation of individual carbs to make engine run more effectively and smoother.

Sparky: Jets do wear in time and especially needle+needle jet. And no fire jacket needed. Everybody is here to learn, nobody knows it all:P

IMHO I´d 1) check for air leaks 2) sych carbs 3) adjust idle level and ride a hundred miles to see if it´s OK.