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I've seen several posts online about issues with engine stalling at idle or having an erratic idle. I have a problem on my own Z125 with an idle-speed that "hunts"; changes rpm significantly faster and slower which sometimes causes the engine to stall (always at the worst possible moment). My configuration is; 143cc piston, V2 race head, Chimera intake, Yoshimura pipe, and Bazzaz fuel controller (no AFM). Can we talk about that?

To run, an engine needs compression, air/fuel mixture, and spark. What affects those things?

Not withstanding engine damage, the two main things affecting compression are the compression release mechanism on the Z125's stock cam and crankshaft momentum. I used to think that the compression release mechanism was intended to aid the starter motor by making the engine easier to turn over. I'm using a high-lift roller cam though (no compression release) and my starter motor doesn't seem affected. That makes me wonder now if the compression release operates at idle. It doesn't seem likely to me (it's centrifugal and should disengage at a very low RPM) but I'm not certain.

Some people have installed a MUCH lighter primary gear on their crankshaft but one wonders why Kawasaki used such a heavily weighted primary gear to begin with. A lighter primary gear gives the crankshaft less momentum to carry it through the compression stroke and so it need a higher idle-speed to prevent stalling. Sure your engine revs faster and feels more responsive with a lighter primary gear but does it then also require a ridiculously high idle-speed?

If you've made air/fuel mixture changes via intake, exhaust, throttle-body, injector, or fuel controller mods, then mixture that is too lean or too rich can affect engine idle. Without an exhaust gas analyzer, it would be difficult to tell how accurate your idle mixture is. The hunting idle I have is typical of a lean mixture but I've tried bumping up the fuel amount with my laptop connected to the fuel controller on a running engine and adding either a little more fuel or a lot more fuel doesn't improve my idle. I checked and can't find an air leak in the manifold gasket either.

Regarding spark, the spark plug needs to be "hot" enough to avoid fouling at low engine temperatures yet "cold" enough to avoid detonation at high engine temperature. Having the right plug gap is critical also; too little gap means a very small spark while too big of a gap means the spark could be erratic or absent. My plug gap is within spec so that’s not an issue.

One last thing; the stock bike ships with a flimsy aluminum heat shield between the head and the throttle-body. I took mine off because it looks like trash stuck in the engine and it didn't seem to me that it's an effective insulator. But what if it's true purpose is to block not heated air but heat radiated as infrared light? You can blow all the cool air you want between two objects but if one side is radiating infrared energy (like your cylinder), that heat energy will go straight through the air into the other object (like the intake manifold next to the cylinder). I wonder if adding that gimpy heat shield back would improve my idle any?
 

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Everything you said seems right on track to me. I was running similar setup except with a solid cam sprocket. I wonder if your decompression is working against you with the larger head and displacement? I can't think of why it would but it sounds like you have ruled out everything else I could think of. I even had the lightened primary gear. I was honestly expecting to have idle issues, but after about 30sec of warm up time the idle was rock steady. I did reinstall the heat shield but I can't imagine that would effect it to the extent your experiencing.
 

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I ended up putting the stock exhaust back on now it runs sweet?
I'm getting the Bazzaz auto tune so we can use the after market Zoom pipe.
 

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Just purchased a new z125 that seems to be suffering from the idle speed hunting issue. It has a pro-taper handle bar and a modified headlight but the exhaust looks stock, not sure about the intake. Is there anyway to check if a rich/lean fuel mix is causing this? What is normal idle rpm usually? I'm at 1600 rpm

I have the bike shut off / stall on me couple of times usually towards the end of the ride (thankfully). Now I'm getting a red light warning on the battery, and funny enough the modded headlight shuts off at times on me and i'm not sure why. I would appreciate any thoughts you guys have.
 

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It could be loose battery terminals?
Or your battery might be dying My bike idles at around 1800.
So I would check that first..
 

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idle hunts

I've seen several posts online about issues with engine stalling at idle or having an erratic idle. I have a problem on my own Z125 with an idle-speed that "hunts"; changes rpm significantly faster and slower which sometimes causes the engine to stall (always at the worst possible moment). My configuration is; 143cc piston, V2 race head, Chimera intake, Yoshimura pipe, and Bazzaz fuel controller (no AFM). Can we talk about that?

To run, an engine needs compression, air/fuel mixture, and spark. What affects those things?

Not withstanding engine damage, the two main things affecting compression are the compression release mechanism on the Z125's stock cam and crankshaft momentum. I used to think that the compression release mechanism was intended to aid the starter motor by making the engine easier to turn over. I'm using a high-lift roller cam though (no compression release) and my starter motor doesn't seem affected. That makes me wonder now if the compression release operates at idle. It doesn't seem likely to me (it's centrifugal and should disengage at a very low RPM) but I'm not certain.

Some people have installed a MUCH lighter primary gear on their crankshaft but one wonders why Kawasaki used such a heavily weighted primary gear to begin with. A lighter primary gear gives the crankshaft less momentum to carry it through the compression stroke and so it need a higher idle-speed to prevent stalling. Sure your engine revs faster and feels more responsive with a lighter primary gear but does it then also require a ridiculously high idle-speed?

If you've made air/fuel mixture changes via intake, exhaust, throttle-body, injector, or fuel controller mods, then mixture that is too lean or too rich can affect engine idle. Without an exhaust gas analyzer, it would be difficult to tell how accurate your idle mixture is. The hunting idle I have is typical of a lean mixture but I've tried bumping up the fuel amount with my laptop connected to the fuel controller on a running engine and adding either a little more fuel or a lot more fuel doesn't improve my idle. I checked and can't find an air leak in the manifold gasket either.

Regarding spark, the spark plug needs to be "hot" enough to avoid fouling at low engine temperatures yet "cold" enough to avoid detonation at high engine temperature. Having the right plug gap is critical also; too little gap means a very small spark while too big of a gap means the spark could be erratic or absent. My plug gap is within spec so that’s not an issue.

One last thing; the stock bike ships with a flimsy aluminum heat shield between the head and the throttle-body. I took mine off because it looks like trash stuck in the engine and it didn't seem to me that it's an effective insulator. But what if it's true purpose is to block not heated air but heat radiated as infrared light? You can blow all the cool air you want between two objects but if one side is radiating infrared energy (like your cylinder), that heat energy will go straight through the air into the other object (like the intake manifold next to the cylinder). I wonder if adding that gimpy heat shield back would improve my idle any?
Did you ever get this issue straightened out? I just added an m4 exhaust and a k&n ru-1130 and bazzaz zfi and with a map I downloaded I am having hunting issues at idle. I suspect a lean map so I might try the one you posted up on bazzaz's website, as I noticed you added some fuel in the 0% throttle area, while my current map has no added fuel down there. Any advice would be much appreciated!! Thanks!
 
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