Idle rises after riding?

[andrew361]

First off, bike is an 82 450SC. Beginning of the season it received a nice tuneup, oil, plugs, valves adjusted, etc. When I fire the bike up and get it to temp it idles happily at around 13-1400rpms. After maybe 15-20min of honest riding (not putting around or stoplight to stoplight), idle raises to around 18-1900rpm, where it will stay until the end of the ride. Is this normal for these bikes or should I be looking for any issues? The bike runs like a champ and I have over 1600 miles this season with no change in behavior, so I haven't worried about it. Just though I'd ask since it's better to be safe than sorry.

Thanks!

[82CB750SC]

Maybe silly question, but are you using the choke during start up?  I so are you turning it back after the bike is warmed up?

[Tryvelcro]

Was going to ask that same thing.

Also, have you tried to adjust your idle to lower it? 

[TDodge7]

1400 is very high as it is , next time go for a ride untill the idle is at it's highest , and then back it off till you're running at about 1100 or 1200 . When were the carbs last synced ?

[jerjohn]

mine does the same. Dont sweat it, the carbs just take along to warm up. Once the bike is warmed up lower your idle to around 1200-1300. When you start it cold the choke will just stay on a bit longer.

[Brittles]

Yea, idle should be adjusted at full operating temperature.

[gammer]

First off, bike is an 82 450SC. Beginning of the season it received a nice tuneup, oil, plugs, valves adjusted, etc. When I fire the bike up and get it to temp it idles happily at around 13-1400rpms. After maybe 15-20min of honest riding (not putting around or stoplight to stoplight), idle raises to around 18-1900rpm, where it will stay until the end of the ride. Is this normal for these bikes or should I be looking for any issues? The bike runs like a champ and I have over 1600 miles this season with no change in behavior, so I haven't worried about it. Just though I'd ask since it's better to be safe than sorry.

I agree that your idle is too high. It should be down around the 1000 RPM mark.

Now, it is normal for a bike to idle a tad higher after some long driving or after it warms up to running temp. For example, when my bike is cold, it idles around 900 RPM. Once its warmed up it idles around 1100 RPM. If I go on a long stretch with the revs up, it might idle around 1200 RPM for a while after. But eventual finds its way back to 1100 RPM.

If everything on your bike is tuned and working properly, then you should give it a good carb sync. This will help with erratic idling.

[jerjohn]

Not being nitpicky but the factory specified idle for a cb450sc is 1200+or- 100 rpm. 1000rpm is really too low for our little twins. Cheers

[andrew361]

Thanks for all the quick replies. I have not yet synced the carbs or adjust idle. I did see that the 450 idle speed should be about 1200 so it didn't seem that far off. The 500 or so rpm rise is what got me curious.  Sound's like it just takes longer to warm up than I was thinking. I'll try adjusting the idle down once it's up to full temp. I did make a carb synchronizer recently but haven't had time to check it yet. Maybe I can push off my yard work today and tinker with the bike. Doesn't sound bad, right? 

[Munkey]

Maybe you have an air leak that only happens when the bike's warmed up.

Starts sucking air. leaning out the mixture, idle goes up.

Just thinking out loud.

[gammer]

Maybe you have an air leak that only happens when the bike's warmed up.

True...
A good way to check for this, is to mist water around the intake boots when its running. If the mist gets sucked in, they you know you have an intake leak.

[TDodge7]

I always test for leaks with the engine running and a non lit (but turned on) propane torch , you can fit the nozzle in between the boots really well and if the engine revs you know you have a leak . But I think your problem is the sync and the idle setting , mine did the same thing until I synced and lowered the idle .