High Idling

[AntCBX]

Hi Guys
First post from a sunny Cape Town. I have an 82 Supersport, which on roll off hangs onto 3000rpm for a few seconds and then drops to low idle, around 600rpm. Also, when cruising in urban areas, it seems to not emit that smooth distinctive exhaust note, rather it sounds a bit rough. The minute you ease on the gas, it sounds perfect. If you blip the throttle, it will not return to idle, it hangs onto 3000rpm for a few seconds.
As a kid in the early 80's the CBX was my ultimate bike, riding this piece of biking history is indescribable.

[daves79x]

Welcome!

Your high idle could be several things, but is most likely something to do with the carb's idle circuits being plugged and/or being way out of sync. This usually is from someone trying to sync the carbs with dirty idle circuits. That's where I'd start, after checking that the choke cable is returning fully open and that your advancer isn't sticking.

Dave

[oldschoolcarbs]

+1

Not knowing what's been done to the carbs or engine it's hard to pinpoint a definitive cause.

That said, sluggish return to idle is THE classic symptom of of synch issues but it could be attributable to other things as well. I virtually never find a carburetor set where one or more of the bodies aren't suffering from the throttle butterfly being a tad out of alignment. The result is anywhere from "balky" all the way to "won't close tight at all." It's one of the things we take care of as a matter of course but while it's an easy fix it can only be competently addressed when they're separated.

Another thing to keep in mind are the two general rules of carburetion: 1) if you have trouble when asking for horsepower, as in poor power when rolling on the throttle, you're rich; 2) If you hesitate while running at constant speed, you're lean. There are loads of exceptions but they give you a pointer as to where to start looking. And yes, dodgy idle circuits cause lean running.

You may well have more than one thing happening inside there but like I said with little to go on we can speculate forever. You could have something as obscure as funky exhaust gaskets or an air leak somewhere.

The one thing I preach to everyone who calls is to check your compression before you do anything else. Without it you're dead in the water until it gets resolved. With it, everything else is just details.

So get out the comp gauge and post up the results and we'll get you sorted or die trying.

Jack

[AntCBX]

The helpful and speedy responses are much appreciated. It gives me a few specific diagnostic areas to focus on. Certainly glad I joined. Will give some feedback once sorted

[NobleHops]

Welcome Ant!

N.