TOPIC:

i searched and searched, but i can't find a answer.
im NOT talking about a hanging idle like when you got a leak (2500+ rpm and no idle). the engine idle, but after you close the gas it keeps 2 or 3 seconds to go down.
its a zx900a1 ninja, 1984.
i have to say im enough sure i have no leaks because:
-new intake boots
-propane torch test ,nothing change
-wd40 test, nothing change
new vacuum plugs
new carb diaphragm
new mix screws, and orings.
new vacuum line to the fuel tap
new fuel tap assy..
new ignition coils , and plugs , wires and caps
valve TIMING it's triple checked and it's ok.
valve clearance adjusted and rechecked
carb sync done.
i tried mix screw at 1 turn, 2 turn, 3 turn, all the way in all the way out...nothing change about this issue.
COLD start : perfect.
WARM start: perfect
under load: a rocket...
running like a champ.

i just want a steady LOW idle, and i want to hear the rpms drop fast when i shut the throttle off.

what is wrong? i have got a headache. please advices and ideas........

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Possible hanging advancer for whatever reason not instantly returning to fully closed position upon closure of throttle?

Good Fortune!

there is no advancer, no mechanical, two pickups and the cdi box . You can adjust only the air gap between pick up and rotor. Done. No change.
Can be a Cdi box issue?

What you describe is a lean condition at idle. Using propane and/or WD-40 is NOT a reliable method of finding small vacuum leaks.

Is the airbox installed w/stock exhaust? What kind of air filter? Did you fully break down the carbs and dip them? Are the stock Mikuni brand jets still in use or did someone install and aftermarket "carb kit" full of Chinese jets? Is the idle smoothness and speed affected by tweaking the pilot screws? If not that suggests the passages may not be fully clean.

shamal wrote: there is no advancer, no mechanical, two pickups and the cdi box . You can adjust only the air gap between pick up and rotor. Done. No change.
Can be a Cdi box issue?

Your bike has a CDI?

Stock airbox stock air filter new intake boots and manifolds. Oem kawasaki.
Stock jets, perfectly ultrsonically cleaned and new bowls gaskets, fuel line orings, lines.. all new.
The only thing not stock are the float needles and mix screws. They are from keyster.
Turning the mix screws all the way in make the engine stall. 3 or more turns out smell rich and idle bad, like toot toot toot toot .. toot . The hang idle its still here.

Nessism wrote:

shamal wrote: there is no advancer, no mechanical, two pickups and the cdi box . You can adjust only the air gap between pick up and rotor. Done. No change.
Can be a Cdi box issue?

I mean the stock cdi box or black box or igniter. I dont know the exact terminology. Anyway ignition is not adjustable. Its idiot proof ...

Your bike has a CDI?

shamal wrote: Stock airbox stock air filter new intake boots and manifolds. Oem kawasaki.
Stock jets, perfectly ultrsonically cleaned and new bowls gaskets, fuel line orings, lines.. all new.
The only thing not stock are the float needles and mix screws. They are from keyster.
Turning the mix screws all the way in make the engine stall. 3 or more turns out smell rich and idle bad, like toot toot toot toot .. toot . The hang idle its still here.

Well, a hanging idle like you describe typically occurs when the mixture is lean and/or the sync is bad. I'd set the pilot screws 2.5 turns open and then double check the sync. Be sure there are no open vacuum ports while syncing. I've heard of cases where the throttle shafts were sucking air through worn out seals/bushings but this a long shot. If your bike still has the air injection system I'd check to be sure all the vacuum lines are properly sealed. Another thing to check is the spark advance using a timing light to be sure everything is working like it should. Also, air leaks on the back side of the carbs, from the boots to the airbox and/or even the cover on the airbox itself, will cause mixture changes. On my bike removing the cover on the airbox caused the idle to go high for example. These old bikes have a very sensitive system toward air entering the system.

Boots from airbox are tight too. No vacuum ports open and no air system. Its a europe model so the pair system its not present from the factory.
Running with air filter or not does nkt change. Still hang 2 or 3 seconds before go to idle.
Can a blackbox go bad and change the electronic advance?

I dont know what else can i think. Maybe a sticky valve? Compression checked. I have 1234 175 180 175 180 psi

Check the throttle bracket and see if it operates and closes smoothly, it may be stiff on the way down.

I had a 1981 Suzuki GS550T that had that same hanging idle business. I changed the intake boots, airbox boots, fully rebuilt the carbs including soaking them in carb dip and replacing all the O-rings, vacuum sync, etc. I spent a lot of time on that bike and the only thing that helped was to open the pilot screws to about 3.5 turns open. Those screws max out at that point but thankfully it took care of the problem.

You said that opening up the pilot screws to 3+ turns open made the idle too rich and didn't fix the problem, which flies in the face of my thinking. Only other thing I can think of is the vacuum sync but you say that's been done too. I'm out of ideas.