Hanging idle

Ok guys this one has me completely stumped. I just cleaned and rejetted my carbs for my pipe, got the jetting sorted out and it runs good all the way through the RPMs. Idles perfectly happy at 1100. It won't do this at first but once it's warm if I free Rev it past about 2500 it jumps up to about 4500 and just hangs. If I rev it past that point it will come back down and again just hang at 4500. I checked to make sure the throttle isn't binding on anything and sprayed carb cleaner all around the carbs and rubber mounts to see if I could find a vacuum leak but came up with nothing. I can't think of anything else it could be. Usually, but not always I can pull the choke out and it will fight but eventually drop down to normal idle, or if I'm riding I can let the clutch out and force it down. And like I said once down there it is perfectly happy running at 1100. I have no idea what the problem is.

You dint say what model of year or what carbs but my inexperienced mind suggests sticking needle.

Stuck spark advance?
Steve

Pinhole in a diaphragm maybe? I'm currently suspecting that with my 1983 GPz; brand new boots and passes the propane test, but idle shoots up to 4000 RPM randomly.

A hanging idle is typically an indicator that the mixture is too lean, often due to an air leak, but not always. Spraying crap around the carbs only shows gross leaks, not slight ones. Another thing that can cause a hanging idle is poorly synced carbs. You may want to double check the carbs and airbox boots, set the pilot screws at 3 turns open, vacuum sync the carbs, and then see if the hanging idle improves any.

ayeckley wrote: Pinhole in a diaphragm maybe? I'm currently suspecting that with my 1983 GPz; brand new boots and passes the propane test, but idle shoots up to 4000 RPM randomly.

That would bog it down. If you have a buggered up or bent needle it might not be dropping down into the jet fully. The slides might be sticking as well.

Or like Steve said the advancer isn't returning to idle position.

Sorry I forgot to mention what bike, same as my signature, it's an 83 GPz 750. Stock Mikuni BS34 carbs. I'm pretty positive my diaphragms are not damaged and the needles don't seem to be, I did drop one when I was shimming them but it didn't appear to have been damaged in any way. Also I don't really think I emphasized that it doesn't do this when it's cold, only after its warmed up. I haven't synced the carbs yet, I don't have ready access to the tool to do it (manometer?)
Personally I'm still leaning towards it being a vacuum leak I'm not picking up. I know one of my clamps is kinda fouled up. Thanks for the information, if anyone has anymore ideas keep them coming.
Oh and I don't think I have any kind of mechanical timing advance, I'm pretty sure it's all digital on the unitrack bikes.

On I also forgot to mention I'm not running the air box, I have pods on it. Don't really want to get into this debate, they are working just fine for me. And my jet kit said to set the screws 3.5 turns out, it seemed lean at that setting so I currently have them at 3.75 turns out.

I'm running brand new airbox ducts on mine as well as the new carb holders. It's possible that the holder clamps could be my issue - I bought four new ones from Z1E but two were ovalled while still in the factory packaging. I debated sending them back but didn't - maybe I should have...

For what it's worth, the 1983 GPz used an electronic ignition advance.

Did it do this prior to the working on the carbs and have you ever had it running good?

baldy110 wrote: Did it do this prior to the working on the carbs and have you ever had it running good?

When I got it last summer it ran but never really wanted to idle. Turned out 2 of the vacuum ports were just open on the carbs. I spent all winter taking it apart and cleaning it up. Now that I have it back together it runs much better overall except for this hanging rpm issue