TOPIC:

Hi guys I'm having some tuning problems. This site has been invaluable but I still haven't quite solved the issue.

When the bike is warm it will idle at around 800-900 rpm nicely. But if i give the throttle a blip I will have a slow return to idle. Once the bike is proper warmed up after a ride the revs will hang at 2000-3000rpm for up to 10 seconds then slowly return. I sometimes need to adjust the idle screw to drop this. Or release the clutch to "pull" it back down.
Throttle snaps shut, I am confident the slides aren't hanging up.

Here is a list of things I have done to try rectify this.

Replaced old cracked carb boots after testing with carb cleaner around the mounts ect. This did help a bit but didn't solve.
Bench synced slides
Adjusted float levels
replaced throttle cables (nothing wrong with the old but may as well rule it out)
Replace pilot screw o-rings
Returned to original air-box from pods I tried out.

I have not yet Vacuum synced yet with the air box back on.

Bike is running 4 into 1 Vance and Hines currently. With airbox. I am unsure if the bike has been jetted to suit. But am trying to rule out any other issues first before messing with them.

Hope someone can help before I pay someone to do it! Thanks

Jim

Good to know what bike your are talking about?
Carbs air ports, main and pilot jets are clean?

Please introduce your bike with some pics.

Vacuum sync'ing and richening up the idle mixture will typically help a hanging idle. Also, check the advance mechanism cam to make sure it moves smoothly (twist the rotor against the springs and release to make sure it snaps back to the base position).

In addition to the usual possibility of a vacuum leak, if you are idling on the main circuit you will get behavior as you describe. Idling on the main circuit is usaully the result of a clogged pilot jet or pilot-air-fuel mixture system clog.

More often than not, when I come across a bike with that problem, it's idling on the main. And then after fixing that we can start actually troubleshooting vacuum leaks... so having both problems is common.

loudhvx wrote: In addition to the usual possibility of a vacuum leak, if you are idling on the main circuit you will get behavior as you describe. Idling on the main circuit is usaully the result of a clogged pilot jet or pilot-air-fuel mixture system clog.

More often than not, when I come across a bike with that problem, it's idling on the main. And then after fixing that we can start actually troubleshooting vacuum leaks... so having both problems is common.

More times than not when the pilot circuit is plugged the cylinder won't fire at idle. Checking to make sure the headers are getting hot from a cold start is a good check.

Hi Guys, thanks heaps for the replies.

The bike is a 76 KZ900. See attached.

I did give the carbs a clean a few months back when I had them out. And checked all the jet ports, were clear. with wire (not recommended i know).
I will check the advancer tonight. hopefully have some luck there.

After that if the Vac sync and richening the idle doesn't solve it, I'll have to strip them down and give another clean to rule out the pilot circuit being blocked as mentioned.

Cheers!

Nessism wrote:

loudhvx wrote: In addition to the usual possibility of a vacuum leak, if you are idling on the main circuit you will get behavior as you describe. Idling on the main circuit is usaully the result of a clogged pilot jet or pilot-air-fuel mixture system clog.

More often than not, when I come across a bike with that problem, it's idling on the main. And then after fixing that we can start actually troubleshooting vacuum leaks... so having both problems is common.

More times than not when the pilot circuit is plugged the cylinder won't fire at idle. Checking to make sure the headers are getting hot from a cold start is a good check.

But it will run when the throttle is held open because it's able to run on the main. Then people turn the idle speed adjustment until the throttle is held open far enough to run on the main. The mixture is so far off, the bike seems to idle even though the throttle is held open much farther than it should be. This is a meta-stable condition, unlike a normal idle which is stable. So, at first it leads to the appearance that the bike is idling, but if they blip the throttle the overly rich mixture clears out and the proper mixture for the now far-open throttle takes over and the RPMs run away... losing the meta-stable condition. You can restore the meta-stable condition by loading down the motor to force the RPMs down or reducing the idle adjustment and increasing it just before the motor dies.

I see this most commonly with bikes that use white tank creme coating. The bike I'm currently working on has this problem. It runs fine after you clean the jets, but just a few hours later the pilots get gummed with white goo again. So the plan is to get a new tank, but I've been toying with the idea of cutting the bottom out of the cremed tank, cleaning it out, and tigging it back together. With any luck, only the underside of the tank will need painting... in order to preserve the factory paint job which is still in good condition.

Just putting this out there since this is something that messed with me on my 440 since I got it (And I only just got figured out last weekend).

If your 900 has points, double check that both sets of points have the correct gap and that they aren't pitted. I had been having issues that looked a ton like a lean condition, even after cleaning out the carbs and ensuring proper back-pressure on the exhaust. Until we took off the points cover and saw that they were barely opening at all. Setting the points made everything get completely better and the bike now runs like a dream.

It should be pretty easy to check the gap, so might not be a bad idea if you're not fond of pulling the carbs.

I see this most commonly with bikes that use white tank creme coating. The bike I'm currently working on has this problem. It runs fine after you clean the jets, but just a few hours later the pilots get gummed with white goo again. So the plan is to get a new tank, but I've been toying with the idea of cutting the bottom out of the cremed tank, cleaning it out, and tigging it back together. With any luck, only the underside of the tank will need painting... in order to preserve the factory paint job which is still in good condition.

No experience with this but it looks worth a shot: POR-15
If it works, it's certainly easier than cutting & tigging.

That's the type of stuff I'm trying to get rid of.

I've stripped failed liners out of two different tanks. It's doable but not for the faint of heart. I used heavy duty paint stripper and uncoated aquarium rocks to scrub the metal. If you can get real MEK in your area it's reputed to cut Kreen. I'd try that before the stripper if possible because it's less harsh.

Nessism wrote: I've stripped failed liners out of two different tanks. It's doable but not for the faint of heart. I used heavy duty paint stripper and uncoated aquarium rocks to scrub the metal. If you can get real MEK in your area it's reputed to cut Kreen. I'd try that before the stripper if possible because it's less harsh.

I used MEK to remove kreme liner in my tank. AKA, liquid death. Nasty stuff, but a better choice than stripper for sure. MEK, will eat the kreme, rubber gloves, your paint, your fingers, and anything else it gets on. ::

Remove anything in the tank and make up block off plates for all of the openings.



I found an expanding "freeze plug" at a parts store fit my fill hole perfectly.