Topic: CB750 idling high & other concerns

[SurfinBird]

When the engine is cold it seems to idle low (about 900), but once it gets going its fine. Once the engine is good and hot, 20 mile + ride, it idles around 2000. Why? I know I need a good carb sync and there's a hair gap (just a sliver) on #2 from the airbox to the carb because the boot is shot.

Another thing i'm finding (and it could totally be user error) is when its hot, the clutch doesn't seem to catch at the same spot as when it's cooler.

A third thing, when it's hot it seems to suffer from fuel starvation (a problem I've had in the past), switching to reserve seemed to help, but there's plenty of fuel. My cap my not be venting at it's best and maybe heat makes it need more venting?

[HondaMan]

The clutch: that's the nature of the beast. The friction point often moves a little after warmup.
The carbs: if you have a bad boot between the carbs and head, you need new ones. Nothing else will fix it, nor the idle issue. You didn't mention which model 750 you have: the K0-K6 boots are available new from Z1 Enterprises. The later ons for cylinders 2,3,4 can be had from Honda, but #1 is no longer available. If they are shpaed OK but just real hard, you can soften them: search these forums for many ways to do that. The wintergreen and xylol is my favorite method.

Only after fixing the boot problem should you work on the "fuel starvation" issue, as it is more likely a problem from the leaking boots.

[SurfinBird]

Thanks, it's a 78 F Supersport. I shop around for the #2 boot.

[bollingball]

You did say from air box to carb they are still available from Honda part 18 750 f8

http://www.servicehondapsn.com/fiche_section_detail.asp?section=125305&category=MOTORCYCLES&make=HONDA&year=1978&fveh=2986

[bollingball]

Now the insulators between carb and head are a different story 1&3 are available but I can not find 2&4 But have had good luck with using 1&3 instead of 2&4 buy rotating them 180.

[Blitzburgh207]

I just ordered four new insulators from western honda. $21-23 apiece and yes, although 1, 2, 3, & 4 are listed, ed says they only have 1 & 3 for some reason... Just got two of each and will rotate them like bollingball says. ...Dont care to chase which is leaking because i figure another will crack as soon as I replace the afflicted one!
High idle: Arent you supposed to set your idle once its hot anyways? and just have to manually keep it revved at first w the later models? I have to turn it down a little once she warms up too if i adjust it when i first start up. (:
Good luck!

[HondaMan]

I just ordered four new insulators from western honda. $21-23 apiece and yes, although 1, 2, 3, & 4 are listed, ed says they only have 1 & 3 for some reason... Just got two of each and will rotate them like bollingball says. ...Dont care to chase which is leaking because i figure another will crack as soon as I replace the afflicted one!
High idle: Arent you supposed to set your idle once its hot anyways? and just have to manually keep it revved at first w the later models? I have to turn it down a little once she warms up too if i adjust it when i first start up. (:
Good luck!

The later models all run lean (for emissions). Their choke arrangement should be pulling the throttle rack open a little for you when it is engaged, then as you warm up and push the choke in (i.e., off) the speed drops back down. Check to see if your choke cable is working right?

[SurfinBird]

Thanks guys. I'm ordering a new set, can't hurt to replace them all, right? I think I had the idle screw backed all the way out, but we'll know better after I replace the boots. The choke cable seemed to be working fine when I cleaned the carbs, closed and opened completely and smoothly. Almost to smooth, i cleaned the actual choke knob and now it tends to slide open a little, the vibration of riding will slowly open the choke all the way when on the road. The choke is all the way off (butterflies are open).