Topic: Throttle issue

[luvmy71]

Hope someone can help me with this.

When my bike is cold it idles just fine at about 1300 rpm.  Once it gets warmed up the throttle wants to open up when idling and the idle speed goes up to about 2100 rpm.  I can be sitting at a light and close the throttle manually but as soon as I let go it opens back up back several degrees and idle speeds up.   I've adjusted the freeplay so it has a nice snapback when engine not running.

Any thoughts?

[Rio_CB750]

I'm assuming this is with your CB750 K1?  I have a 1972, so first:
1. There should be no way you should be able to close the throttle manually sitting at a light, it should snap back and close completely, with strength and soundly with a strong snap, so check your adjustment and spring.
2. Check your rubber boots coming from the carb to the intake, make sure they are tight and in good condition, leaks there cause all kinds of issues

[flybox1]

@ Rio - The PUSH cable will allow you to further close the slides.  They only close all the way down if your idle set screw is turned all the way out from the stop.

When was your last vacuum sync?

[luvmy71]

Carbs were just synced when top end was rebuilt.  Less than 300 miles.

As far as throttle closing with force and a snap, when bike is not running it closes with a nice pop.  Its when bike is running and nicely warmed up.

[flybox1]

Clear tube test the bowls?
Idle plug chop insulator color?

[01Thomas]

You haven't told us anything about the choke so I'm assuming the choke is inactive/closed/unused?

If your bike runs well when cold and poorly when hot then it is too rich at idle/closed throttle. That could be due to your pilot screws being adjusted incorrectly (too little air or too much fuel) or your idle jets are too big.

I don't follow what you're saying about the throttle cable, but when it's adjusted properly there's about 3 mm freeplay at the throttle.

[harisuluv]

So you've adjusted the idle screw (singular) yes?

[luvmy71]

Choke is off both cold and hot.  Bike runs great when warmed up, its just that at idle the idle speed rises to about 2100 rpm when stopped. I can literally watch the throttle turning back counter-clockwise slowly if I let completely off when stopped.  I just think it may be something in the throttle body on the handle bars.

[harisuluv]

So you've adjusted the idle screw (singular) yes?

[Rio_CB750]

@flybox, if he is able to close the throttle with the grip while idle at a light, there is something very wrong with his adjustment, the throttle should completely snap back, I'm not talking about adjusting the push cable.

[luvmy71]

Rio - your point about the throttle snapping back is why this is so mystifying to me.  As I've mentioned when the bike isn't running I can twist the throttle back and when I release it snaps back with a nice pop.  I know what that's supposed to sound like.  Its something to do with the bike warming up or running.  Because when its warming up I can rev the engine let go of the throttle and it will return to its normal idle speed of about 1400 rpm.

[harisuluv]

Ok, I'm exiting the thread.

[luvmy71]

Sorry Haris, I'm very new at this carburetor system.  Which idle (singular) screw are you referring to?

[Rio_CB750]

@luv It's time you buy a SHOP manual and read it, understand it, find the parts that folks are referring to.  The idle screw is motorcycle 101, jump on Amazon and get that book sent overnight.  Attached is the OWNERS manual (not to be mistaken for the SHOP manual) check pages 17, 24,  and 43-45.

[BomberMann650]

Having read thru this thread (briefly)

I'm not convinced your issue has anything to do with throttle cable.

The inconsistent return to idle and climbing revs without cracking the throttle are indicative of lean/vacuum leak issues.

Which was mentioned before so I'm just making a very long "+1" post.

[luvmy71]

Did a vacuum leak test, no leaks.

[BomberMann650]

Did a vacuum leak test, no leaks.

Then check that fuel mix - by the book settings don't always work

Timing could be suspect too.

[Rio_CB750]

And you're back to adjusting...

1. Make sure your cables are properly set (see the owners manual I posted, page 43)
2. Start the thing cold, then warm it up and set the idle to 1000 using the idle stop screw (page procedure on page 44-45)
3. "After performing the adjustment in section 1 and 2 above, if the proper idle speed cannot be obtained or if the exhaust back pressures from the cylinders are not uniform, have the carburetors readjusted..."