OH NO!!! HIGH IDLE!!!

[Honzilla]

Hi everyone,

I'm still a noob when it comes to my 91 CB 750.

Lately I have been noticing a idle speed of about 1500 to 1600 rpm when the bike is at running temps.

The service manual PDF thingy says that the idle should be 1000 +/- 100 rpm.

Since this is my first carburated motorcycle I have no idea what to check first.

Is there an "idle screw?"  If so, where the heck is it???  How much do I turn it???  Which direction???  Is the factory tach accurate enough to use as a reference???  

Please help!!!  I'm drowning in questions, and I don't even know if they are the right ones.

This is pathetic!  I've been riding for 15 years, and 8 of those with NO car...  Why do I feel so stupid when it comes to this bike?

[LukeM]

There should be an idle adjustment screw.  Look at the underside of the carb bank, and there should be a thumb screw device that acts on all the linkages for the carbs.  Start the bike, and slowly move the screw in and out.  I've noticed that my bike idles a couple hundred RPM higher when it's thoroughly warmed up.

A couple hundred won't be a big deal.  A thousand or so might be.  You might also look for some airbox leaks around the carb boots.  Should be a pretty easy fix.  Good luck, and keep us informed.

Luke M

[Honzilla]

Hey Luke,

Thanks a bunch for the info.

After my ride home from work today I got off the bike and left it running on the center stand.

I found the little knob you are talking about.  These things are definitely made for little Japanese hands not big Americans!  After burning the crap out of my hands and experimenting which way to turn it, I seem to have brought the idle down to about 1100 - 1200 rpms.

I know it was not much but I got back on it and took a bit of a spirited cruise.  It felt GREAT!  Also when shifting down into 1st gear from neutral it's not nearly as harsh and clunky.

Thanks again, Luke.  I feel like such a noob...

[Honzilla]

Hey Luke,

Thanks a bunch for the info.

After my ride home from work today I got off the bike and left it running on the center stand.

I found the little knob you are talking about.  These things are definitely made for little Japanese hands not big Americans!  After burning the crap out of my hands and experimenting which way to turn it, I seem to have brought the idle down to about 1100 - 1200 rpms.

I know it was not much but I got back on it and took a bit of a spirited cruise.  It felt GREAT!  Also when shifting down into 1st gear from neutral it's not nearly as harsh and clunky.

Thanks again, Luke.  I feel like such a noob...

[LukeM]

I'm glad I could help.  There's no need to feel like a noob.  We've all been there.  My son has to fiddle with the idle speed knob all the time on his bike, as it's notoriously cold blooded.  

If you want to get specific about it, you could hook up an accurate tachometer to your bike and set the idle per the manual spec.  Some would say the mechanical tach is a bit less accurate.  I'll leave that to you.  As long as it's working, and you're happy with it, I'd say leave it alone.  Generally speaking, the lower the idle, the smoother the takeoff in 1st gear, and the easier it is on the clutch and drivetrain.

So, what's next?  I'm on a roll here!
Luke M